 Cathedral Ceilings & Un-vented Hot Roof Roof Cavities
- Ice & Moisture Prevention Guide
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Hot roof designs, aka "dense-packed" insulated sloped roofs: this article describes various solutions for un-vented cathedral ceilings and similar under-roof spaces, offering advice on how to avoid condensation, leaks, attic mold, & structural damage when roof venting is not possible. This article series about roof and ceiling ventilation describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Un-Vented Roof Solutions: how to avoid condensation, leaks, damage when roof venting is not possible
Also see CATHEDRAL CEILING INSULATION and ICE DAM PREVENTION. See HEAT TAPES & CABLES on Roofs for Ice Dams. And see ROOF VENTING ENERGY SAVING DETAILS.
Also see COOLING LOAD REDUCTION by ROOF VENTS.
Here and at Correcting Roof Ventilation we argue that while some experts like the "hot roof" design that omits attic or under-roof ventilation entirely, and while there is good science behind that approach, risks of mistakes in construction, variations in building occupancy use and moisture levels, and wear and tear can lead to very costly surprise rot, mold, or insect damage on buildings where leaks and moisture are trapped in building cavities and remain un-noticed.
Some buildings, by their shape or design, simply don't make it easy to install continuous intake venting at the eaves or lower roof edge, or continuous outlet venting along a ridge.
For example, a house which has no roof overhang at all makes intake venting at the eaves difficult.
A house with a pyramid roof shape or complex roof shapes makes outlet venting at a ridge difficult.
Sketch (above left) is courtesy of Carson Dunlop Associates shows the two basic strategies for insulating cathedral ceilings and flat roofs.
Problems with Partial Roof Venting
On these roofs, partial venting can be worse than no venting. For example, adding a ridge vent, or several roof "spot vents" or roof turbine vents on a few roof slopes, typically mid-slope or in the upper third of the slope on roof surfaces not visible from the front of the building, may please the installer, but they are worse than ineffective.
Placing an outlet vent on a roof without adequate inlet venting (see Roof Vent Area Ratios) works against the
interests of the building and its occupants. As convection currents and heat loss into the roof space or attic vent out through these vents, the intake air needed to satisfy the exhausted air leaving the building will be drawn from the building interior - increasing building heating costs and possibly increasing particle movement from basements or crawl spaces
(if there is a mold concern in the building).
If you can't provide enough intake venting it is probably better to not vent at all in these conditions.
Solutions for un-vented roofs: Avoid cold-climate ice dam leaks & reduce cooling costs in hot climates
Ice Dams on Un-Vented Roofs in Cold Climates
Grange and Hendricks (1976) recommended a combination of eaves and ridge venting to avoid ice dams on buildings. Other authors found that ice dams seldom occur at temperatures above 22 degF. or when attic temperatures are below freezing.[7] Our photo (below left) shows an ice-dam prone roof on a tall building with a slate roof.
- Roof Edge Sheathing Intake Vent: There is a product called Hicks Starter Vent™ and similar products such as the SmartVent™ distributed by DCI products that replace the first few inches of roof sheathing under
the shingles or slates by a louvered vent so that air can sneak into the roof cavity by that path. It's cost-appropriate to install this when re-roofing but probably too costly to do so otherwise.
- Half-Ridge Vent: a half-ridge vent, basically a conventional ridge vent but cut in half lengthwise, can be installed at the up-roof edge where a lower roof abuts a higher building wall, such as where a
roof slopes up to butt against the wall of a raised dormer. Combined with soffit intake venting this roof vent design works well to cool and dry roof sections with this shape.
- Ice and Water Shield: On roofs that are too difficult to vent, a second-best solution is to remove the shingles (or slates) from the lower 3 feet
of those slopes where leaks and ice dams have been recurrent, install a waterproof but nail-able membrane such as WR Grace's Ice and Water Shield (other product names from other manufacturers) which will prevent any ice dam
backup leaks from entering the building.
This is basically a sticky membrane that is applied to the roof decking and through which shingle or slate nails can be nailed back onto the roof; the membrane seals around the nails so
that those penetrations do not form leaks during a water or ice backup.
- Adding Attic Insulation to Avoid Ice Dam Leaks: Indoors, unfinished attic: if we add as much insulation as we can fit into the attic floor of an unfinished attic space, paying close attention to insulating under the eaves
at the lower roof edges, and making sure that the insulation blanket is absolutely complete with no missing areas or holes or leaks, we can reduce the heat loss into the attic space and thus reduce the warming of the roof underside and thus reduce future ice dam formation and its related leaks.
It's better to place insulation in the attic floor than under the roof, since in the latter location ventilation and drying of the roof sheathing are prevented and there is a greater chance of future mold growth or rot caused by trapped moisture there.
- Un-Vented, Hot Roof Designs Indoors, finished attic: Where the attic space is finished with drywall or other ceiling materials installed against the underside of the roof rafters, while I prefer in-floor insulation,
here we'll have to insulate the roof cavity between the rafters.
In cases where there is no under-roof venting system (no soffit intake vents, no ridge vents), a "hot roof" design is followed: the roof cavity between rafters can be filled with insulation, followed by installation of a perfect vapor barrier, followed by finish surface of drywall or whatever else.
The vapor barrier and air sealing in particular, need to be perfectly installed to prevent warm moisture-laden air from entering the un-vented roof cavity. But even so, see Worries about the "hot roof" un-vented Cathedral Ceiling Designs, discussed below.
- Tips for insulating a cathedral ceiling, take care to seal ceiling penetrations such as around light fixtures or ceiling-mounted hard-wired smoke detector. More moisture enters building cavities through these cuts in the ceiling (or wall) drywall than permeates through the drywall itself.
While fiberglass insulation is an excellent and effective product for insulating most building cavities, in areas where there is extra risk of trapping moisture (and thus rot or mold infections) such as crawl spaces and cathedral ceilings where roof venting may be absent or minimal, we prefer to use closed-cell foam insulation products or spray-in icynene foam insulation: these products can seal the cavity against drafts and they do not as readily pick up moisture nor do they readily form hidden mold reservoirs.
See CATHEDRAL CEILING INSULATION and INSULATION LOCATION & QUANTITY for ATTICS. See Mold in Fiberglass Insulation and MOLD RESISTANT CONSTRUCTION for details.
- Use of roof de-icing cables or heat tapes to avoid ice dam leaks is described at HEAT TAPES & CABLES on Roofs for Ice Dams.
While we prefer to avoid ice dam leaks by good building design and good under-roof ventilation, where conditions require stopping ice dam leaks on an existing structure, proper installation of heating cables may be the fastest and cheapest solution.
Design Suggestions for Unvented “Hot” Roof Designs Where Venting is Difficult
As explained in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES and as discussed at COOLING LOAD REDUCTION by ROOF VENTS,
In cathedral ceiling configurations where it is difficult to
provide ventilation, some builders have eliminated the
vent space, relying instead on careful sealing of the ceiling
plane to prevent moisture problems. While experts concede
that this should work in theory, most caution that it is
difficult to build a truly airtight ceiling assembly.
Also,
cathedral ceilings are slow to dry out if moisture problems
do occur, whether from condensation or roofing leaks. If a
hot roof is the only option for a section of roof, take the
following precautions:
- Install a continuous air and vapor retarder, such as
6-mil poly, carefully sealed at all junctures. See the vapor barrier and air barrier articles listed at VAPOR BARRIERS & CONDENSATION in buildings.
- Do not use recessed lights or other details that penetrate
the ceiling plane.
- Carefully seal all penetrations in the ceiling assembly,
including top plates of partitions, with durable
materials. See these articles:
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK MINIMIZATION
AIR LEAK SEALING PROCEDURE
AIR SEALING STRATEGIES
- Use a nonfibrous insulation, such as plastic foam,
and install it without voids where moisture could
collect. Insulation choices are listed at INSULATION INSPECTION & IMPROVEMENT.
While fiberglass insulation is an excellent and effective product for insulating most building cavities, in areas where there is extra risk of trapping moisture (and thus rot or mold infections) such as crawl spaces and cathedral ceilings where roof venting may be absent or minimal, we prefer to use closed-cell foam insulation products or spray-in icynene foam insulation: these products can seal the cavity against drafts and they do not as readily pick up moisture nor do they readily form hidden mold reservoirs. See Mold in Fiberglass Insulation and MOLD RESISTANT CONSTRUCTION for details.
- In regions prone to ice dams, use enough insulation to
maintain a cold roof—preferably R-38 or greater. See ICE DAM PREVENTION and Ice Dams: Comparing Two Houses
- Ice and Water Shield: On roofs that are too difficult to vent, a second-best solution is to remove the shingles (or slates) from the lower 3 feet
of those slopes where leaks and ice dams have been recurrent, install a waterproof but nail-able membrane such as WR Grace's Ice and Water Shield (other product names from other manufacturers) which will prevent any ice dam
backup leaks from entering the building.
This is basically a sticky membrane that is applied to the roof decking and through which shingle or slate nails can be nailed back onto the roof; the membrane seals around the nails so
that those penetrations do not form leaks during a water or ice backup.
- Use of roof de-icing cables or heat tapes to avoid ice dam leaks is described at HEAT TAPES & CABLES on Roofs for Ice Dams.
While we prefer to avoid ice dam leaks by good building design and good under-roof ventilation, where conditions require stopping ice dam leaks on an existing structure, proper installation of heating cables may be the fastest and cheapest solution.
- Eliminate all sources of excess moisture in the home
(wet basements, uncovered crawlspaces, unvented
bathrooms). See ATTIC CONDENSATION CAUSE & CURE.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Worries about the "hot roof" un-vented Cathedral Ceiling & Hot Roof Designs
As explained in our "hot roof" discussion at ROOF VENTILATION SPECIFICATIONS and at HOT ROOF DESIGNS: Un-Vented Roof Solutions, we don't have confidence in the long term durability of "hot roof designs" because any future roof leak into this cavity produces trapped moisture and rot. We call this a "hot roof" design because failing to vent the roof from below not only misses a chance to avoid ice dam leaks and condensation damage in cold climates. In hot climates the roof temperature will be much higher on an un-vented roof, resulting in much shorter shingle life. This is less of a concern for slate and similar product roofs.
Also see INSULATION LOCATION for CATHEDRAL CEILINGS for more information.
In buildings where there is no roof venting anyway, an un-vented, well insulated "hot roof" is a second-best alternative to preventing ice dam related leaks in cold climates. Be sure to inspect the roof surface from outside for leaks and damage every year and to fix any damage quickly.
Frequently Asked Questions (FAQs) about un-vented cathedral ceiling under-roof spaces
Question: Some older buildings with no roof venting seem to be ok anyway - is there a concern?
Our house was built in 1920 it has never been vented. The second story is finished with the insulation directly on the roof sheeting. This year, after 30 years, we had the house reshingled the roof deck was in perfect condition no problems anywhere.
If we had the the choice I would lean toward venting but because of the way the home was built it's impossible but their has been no negative affect being unvented. - Jim
Reply: Some un-vented homes seem not to suffer, but not all: explanation and some warnings about un-vented cathedral ceilings on older homes
Jim,
I agree that many older homes were often drafty-enough that combined with the good luck of no unusual indoor moisture source (like a recurrent wet basement or crawl space) that they fared pretty well without more aggressive attic venting.
Building ventilation and moisture entry patterns change over the life of a structure
But one needs to be careful in drawing conclusions from those examples. The way buildings are used, heated, ventilated, insulated, and sealed changes over time. I have inspected homes that were more than 100 years old that had been in good shape as far as moisture problems were concerned, until energy costs led new owners to change the way the house worked.
What was in 1900 a cold, drafty house, was still inexpensive to heat with coal or oil in 1935 (at today's prices), but beginning in the U.S. in the 1970's (the oil embargo and energy crisis) people no longer wanted stunning monthly heating bills.
The addition of layers of siding, storm windows, caulking, and perhaps blown-in insulation in walls and attic floors significantly changed how such houses worked in handling moisture. Originally water ran into a basement through stone foundation walls, coursed across a dirt or stone floor to a hole where it exited, and moisture that passed into the building easily vented outdoors through many air leak points that created a high air exchange rate (ACH).
But after adding all those energy improvements, moisture that previously leaked right outside found itself trapped in some homes - water still leaked in but moisture had trouble leaving (like checking out of the Hotel California, you could check in but leaving was another matter). One approach to that problem is to reduce excessive indoor moisture levels (MOISTURE CONTROL in buildings).
Even in a 1920 home that has not had a noticeable moisture problem in its attic (sometimes we find a wet, rotted surprise in unvented cathedral ceilings), we might want to improve ventilation in order to keep the roof and attic cooler, extending the shingle life and reducing cooling costs for the home.
Finally, one decides to go ahead and ventilate such a home, as might be done during major renovations that happen to make it easier to provider an air path from soffit to ridge, adequate intake venting is critical lest the outlet venting simply suck heat out of the home in winter.
Reader comment: Replacing a wood shingle roof with asphalt over plywood changes how the building works
Lot of older homes vented naturally thru their wood shingle roof ...... When you tear off and replace with plywood and an asphalt shingle you are changing the design and could be asking for problems if not properly insulated and ventilated . Ansel 9/4/2011
Reply:
Good point, Ansel, and we agree. Your comment is an example of how changes to a structure over it's life also change how a building works and thus can cause new problems to arise, in this case higher moisture. We might add that a replacement wood shingle roof installed over solid plywood decking won't last as long as wood shingles over ventilated spaced nailers.
Question: We Can't Afford Spray Foam - Can we Just Cut Rigid Foam to Fit?
We have TJI rafters in a very weird roof area. no venting at all. I need to insulate and cannot afford the $750 minimum in our area for spray in foam. since the TJI has a flange top and bottom, what would be the best combo of rigid foam sizes? the flanges are 1.25" high and 1" deep and the remaining rafter depth between them is 7". the distance between the flanges within the bay is 21.5". the rafters are 24" OC . Each bay is 5' long for a distance across the roof is 21' please help me calculate. Claire - 9/16/12
in the interim, I figured out how I will do it. I will fill the space between the flanges with 7" strips of 1" foam. then I will cut and place 2" foam boards and 1" foam boards to fill the bays. tape and vapor barrier to block air
Our photo (above and below left) illustrate before and after spraying foam between the TJI rafters in our own project in 2011. The "R" value of the roof with the cavity filled will exceed what you can conveniently jam between the TJI's in your own roof and more, it's an air-leak-proof ceiling in the "hot roof" design we discuss below. To reduce our anxiety about future leaks into this roof structure (the foam and ceiling would hold water from a future roof leak until the roof rotted or the ceiling collapsed) we also opted for a durable standing-seam metal roof above. Photos courtesy Galow Homes.
Reply:
Claire
There is no technical reason why you couldn't cut solid Hi-R foam into slabs and custom fit them between your TJI roof truss bottom chords. And it's true that a spray foam job may cost twice that of installing fiberglass batt insulation in the same space.
But having done some cut-and-fit foam board insulation jobs myself, I have some reservations about the DIY approach using cut sections of foam board insulation in the case you describe.
Not only is the cut-and-paste effort labor intensive, but I worry about a leaky design that makes the insulation job ineffective. An advantage of sprayed foam under-roof insulation is that it seals perfectly against air leaks. Depending on the foam type used you'll still need a vapor barrier.
Insulating with cut sections of solid foam board
Our photo (left) illustrates a project using 2-inch high density styrofoam insulating board under the floor over a (dry) crawl space.
We used cleats to support the foam, not having the convenience of the I-joists that you have in your ceiling. You can see that even working with some care, it was difficult to keep the foam slabs cut for a perfect fit (green arrow).
A second problem with retrofit foam insulation using cut-foam boards is shown by our photograph of a feeble attempt to insulate around the waste pipe penetrating the floor. (Next photo, below left).
A combination of cutting solid foam board insluating slabs followed by judicious use of a few cans of spray insulating foam can improve the performance of a do-it-yourself cut and paste insulation job.
You may need to use fire-block foam at building floor, wall, or ceiling penetrations, depending on your local building code requirements however.
The R-values of sprayed foam depend on the foam type and weight-rating. Two common versions are Icynene® half-pound open celled foam and CertainTeed CertaSpray® two pound closed cell foam. Open-celled Icynene LD-C-50 type foam has an R-value of about 3.6/inch.
Properties of Spray Foams
In comparison, any properly-installed spray foam insulating job fits building cavity and mechanical system variations and penetrations perfectly.
Open celled foams are a bit less costly (per inch of finished thickness) than closed cell foams, are sprayed and trimmed, and are lower in R-value. Closed cell foams (two pound foam) are more dense and heavy, are usually sprayed with little trimming (there may be a concern about the care with which ceiling or wall or floor cavities are filled to an adequate depth) and this foam job may cost 20 to 30% more, gains in both air and moisture barrier properties, adds structural rigidity to the buidling roof or wall where it is installed, and has an R-value of about R 6.5/inch - almost twice as much. More details are at INSULATION R-Values & Properties and at INSULATION CHOICES.
You'll want to compare the R-values per inch of the foam board you planned to use and figure how much thickness you'd need to fit into place to get even close to the R-values of the spray approach. 1-inch Dow Tuff-R board has an R-value of 6.5/iinch for one-inch thick boards. That's not making any allowance for heat losses because of looseness of fit or cutting errors.
Once you've installed a makeshift insulation job, you'll be tempted to install the finish ceiling. And once the ceiling is in place you're not going to want to disturb that work for a long time, meaning you may be living with a poor or leaky insulation job and high energy costs.
Take a look at the "hot roof" or "packed" roof insulation deiscussion above and at the end of this article before making up your mind.
Question: dense packing sloped roofs, residential - Is there Data Supporting Dense-Packed Un-Vented Sloped Roofs?
Is there any data out there on condensation issues when dense packing a sloped roof. Cape cods and other dormer type construction have ceilings that traditionally have ventilation to the upper attic area and out the ridge vent. Sometimes these sloped roof areas have battens of fiberglass insulation.
We are running a residential program in Wisconsin and the contractors have been dense packing the sloped roof/ceiling in these areas.
Their anecdotal evidence is that they have had no problem with condensation or moisture. Are there any resources out there on data for zones 4 and 5.
By the way, I want to thank you again for joining us on the radio program, Constructive Solutions, in Wilmington NC and answering listeners questions.
- J.A., Madison WI, 5/15/2012
Our sketch (above left) shows the traditional 1940's - 1960's design for insulating and venting a cape cod roof. Current design, among those who are venting the roof at all, provides continuous soffit or eaves intake, continuous ridge outlet venting, and we eliminate the gable end louvered vents. - Ed.
Reply: there is science supporting un-vented roofs provided construction is perfect and no roof leaks occur
Also see the notes in our hot roof article (above) and also at
ROOF VENTING NEEDED?.
Roof venting for cathedral ceilings remains a contested issue, though as we elaborate below, there has been some expert work and plenty written about the topic. Roof venting for buildings with attics is also discussed below.
Our photo at left is not a Cape Cod style home, but this New York house illustrates differential snow melting indicating where heat is being lost into the roof structure.
Even in climates not subject to freezing temperatures and heavy snowfall, the snow-melt patterns on these Northern homes can inform us about how heat and air can move from the occupied space into the under-roof space of a building.
Special Venting (or "packed" hot roof) Problems for Cape Cod Roofs
Cape cod roofs present a special problem in that in the usual cape design there is a lower attic knee-wall enclosing what amounts to a "low attic" followed by the equivalent of a cathedral ceiling over the middle section of the roof, often penetrated by dormers when the second floor of the Cape Cod is designed to be occupied, followed again by an attic design for the upper third of the roof.
When I examine Cape Cods built in the Northeastern U.S. in winter, homes built in the 1930's through 1960's especially show ice dams on the lower roof, snow on the center roof, and all snow melted off of the upper third of the roof.
Those conditions mean leaks into the building walls, unnecessary heat losses (higher heating costs), and too often roof sheathing and insulation contaminated with several genera of mold.
And where we have removed insulation from between the rafters of such roofs we often find moisture has been trapped against the roof sheathing. Sometimes that has led to mold, delaminating plywood sheathing, or even rot.
In new construction we have an opportunity for better building design, implementation, and energy performance.
The bottom line (in my opinion) is that roofers and builders who like the "hot roof" approach trust their implementations of the hot roof approach and assert that it works fine but water or moisture leaks from either outside or from within the building into an un[-vented building cavity (like a cathedral ceiling roof) cause more damage than leaks into a vented roof.
Hot Roofs vs Vented Roofs in Hot Humid Southern Climates - Building Heat Gain
Our photo (left) shows an extensive Penicillium sp. mold contamination in an attic where moisture condensation had gone unnoticed. [The identification of tihs mold as Penicillium happened in the lab, of course, not based on just the visual inspectinon.]
TenWolde and Rose pointed out in 1999 that
No scientific claims have ever been made that attic ventilation is needed for moisture control in warm, humid climates. In these climates, the outside air is much more humid than the inside air which is cooled and dehumidified by air conditioning. In such climates, attic venting tends to increase rather than decrease moistur4e levels in the attic .... [and] may therefore increase the danger of condensation on [the] HVAC ducts [as well]. [7]
Our photo (left) shows stains on the upper building walls under an inadeqruately-vented soffit. This New York Home suffered recurrent ice dam leaks sending moisture through the building walls, with the expected problems that result.
Lstiburek's view is that particularly in the hot humid South (of the U.S. - and not the local ofthe buildings we are illustrating here) there is not much gained in venting a roof (2-3% reduction in heat transfer in a vented attic) and quite properly he continues to point out that once A/C ducts are placed into the vented attic heat transfer to the occupied space goes up to 5-7% (for tight insulated ducts) compared to routing those ducts through conditioned space of the home, and a much worse 25% heat gain to the home if the HVAC ducts are leaky. And his argument that air movement through the attic under the roof will not flush heat being radiated towards the ceiling below is compelling too.
Joe also argues that the moisture from warm humid outside air run through an attic to ventilate it can move through the attic insulation where it can condense on the cool building ceilings below. [5]
Those data are compelling, though I'm not entirely convinced that that nasty warm southern air my respected acquaintance describes actually moves "down" to the cool ceiling in the attic. In roofs I've worked-on and examined with air tests and smoke guns, warm air rising in the hot attic went zooming out at the ridge vent, drawing cooler (nasty moist Southern) air in at the eaves. The incoming air followed the underside of the roof upwards on the air currents exiting at the ridge. without however, recapping construction costs, or considerations for an existing home rather than new construction.
Joe pegs the impact of venting on roof temperatures at 5%[5][6]. I measured the temperature drop in a hot, un-vented attic in New York from over 145 degrees to 95 degrees after we cut in soffit and ridge venting. I was measuring air temperatures in the attic, not roof surface temperatures, and not radiated heat effects. That was a 35% drop in air temperature in the attic.
Hot Roof vs Vented Roofs in Cooler Northern Climates
The arguments above were aimed at homes in the hot humid South, not your area, in much cooler Wisconsin, and perhaps not in the hot but arid climates of the Southwest. [We need to ask Joe his opinion on that climate.]
Our image of a New York 1940's Cape Cod home (left) illustrates a snow-melt pattern telling us where the roof was insulated, where heat is flowing from the occupied space below, and where to expect ice dams.
TenWolde and Rose, ASHRAE experts from the Forest Products Laboratory and the Building Research Council respectively addressed these issues articulately in 1999, citing research on attic ventilation in the U.S. dating back to 1939 (Rowley et als) who recommended controlling indoor moisture and venting the attic.
The first explicit quantitative attic venting recommendations came from FHA in 1942 when I was just (-1) years old.[7] Rowley et al. (1939) documented that attic ventilation could reduce condensation on roof sheathing during cold weather, though Tenwold/Rose point out shortcomings of their study and cite further research by experts in the 1940's suggesting that condensation occurred only in homes with high indoor humidity.
TenWolde and Rose also point out that
In cold climates, cathedral ceiling construction is inherently more prone to moisture damage than is attic construction because isolated conditions are created in each rafter cavity. While providing effective ventilation to attics with simple geometries is relatively easy and inexpensive, providing effective soffit and ridge venting to each individual cavity in a cathedral ceiling is far more difficult and the advantages of roof vents over normal soffit air leakage are slight (Rose 1995). [7]
I concur also with the view of those authors that a cathedral ceiling (or for that matter any roof) with ridge vents but without sufficient soffit intake venting acts as a chimney, not only admitting harmful humid indoor air into the cavity but also increasing heating costs. They conclude that,
On balance, the case for vents in a cathedral ceiling is much weaker than that for attic vents.
I agree with this view as well, based on the science, but I do not entirely agree about the long-term durability of un-vented roof designs subject to the vagaries of leaks and time, as I have elaborated here, and even those authors pointed out that
... while humidity control can usually be accomplished by providing adequate building ventilation, an airtight ceiling may be much more difficult to obtain in practice. Rose (1995) demonstrated that the use of foam air chutes between the sheathing and the top of the insulation can be beneficial for moisture control in cathedral ceilings.
Roof Temperatures of Roofs over Attics Compared with Roofs over Cathedral Ceilings
... the temperature of shingles over unvented attics and cathedral ceilings is more than 160 degF. for a significantly longer time compared to that of shingles over vented roof. However, currently [1999] no data are available on the effects of temperature duration on the durability of asphalt shingles.[7]
Nevertheless it seems obvious that being hotter longer is almost certainly going to mean wearing faster for asphalt shingles. We lack quantitative specifics, not concept.
Roof Temperatures: Roof Ventilation vs Roof Color & Roof Shingle Life (and Shingle Warranty)
A less exciting and less convincing argument but one that may matter to some is the observation that a ventilated roof is cooler in hot climates, potentially giving both lower building cooling costs and a longer life for asphalt shingles than the identical shingles installed on the same slope at the same exposure in the same climate, but on a hot un-vented roof design. Dr. Joe argues that shingle life depends less on roof temperature than shingle color. [5][6] He's correct in what I think he meant to say, but the statement needs clarification.
The chief effect of changing roof shingle color from, say black to white, is in fact a reduction in the surface temperature of the shingles when exposed to hot sun. It's not like wind and rain know what color the shingles are, nor that we use different asphalt binders for white mineral granules than we do for black ones. He may be right but I'm unclear on the argument. Most likely this is what was meant by the earlier statement.
I do agree that roof venting alone may not do much to reduce the temperature of shingles on the roof surface, as others have explained with some care.
The exterior surface of a [roof] shingle is almost exclusively governed by the balance of absorbed solar radiation [hence the importance of shingle color], convective surface heat loss (wind), and infrared radiation loss to the sky. Rose (1992) found that attic ventilation lowered the peak temperature of the sheathing surface directly below the shingles by about 10 degrees.
The peak temperature below the shingles on unvented cathedral ceiling roofs was also higher than on vented cathedral ceiling roofs, as long as a 1.25 inch ventilation slot was present between insulation and the sheathing. ... attic or roof ventilation lowers peak shingle temperature by less than 10 deg.F. because the effect of ventilation decreases close to the shingle's exterior surface. This increase translates into a relative change in absolute temperature of less than 2%.
As the rate of aging [of roof shingles] is likely related to absolute temperature, the effect of this modest rise in temperature is likely to be very small. The "color" of shingles has a greater effect on the temperature, and hence shingle durability, than does attic ventilation. Simpson and McPherson (1997) found that white roofs were as much as 36 degF. cooler than gray roofs and as much as 54 degF. cooler than brown roofs. [7]
Watch out: I and others also point out that many roof shingle manufacturers do not warrant their shingles if installed on an un-vented roof because higher shingle temperatures accelerate aging.[7]
Roof Leak Risks Need to Be Addressed When Comparing Vented vs Un-Vented Roof Designs
There are two principal moisture pathways into a "packed" sloped roof
- From the building interior via penetrations, following air leaks in the ceiling around light fixtures, electrical boxes, etc. Much more moisture moves on air leaks through penetrations than moves through solid continuous drywall itself.
- From the building exterior: via leaks either due to an error in construction ( a flashing mistake or improper shingling or nailing, for example), or due to luck and the forces of nature - a tree limb falls on the roof, or there is an unusual storm, wind, ice damage.
When enthusiasts recount differences between the performance of a "packed" or "hot roof" design (presumably very high-R and "tight") and a ventilated roof design (more trouble, more cost, sometimes poorer insulation and higher energy cost) they sometimes fail to report that when a leak into the packed roof occurs, water stays there a long time - inviting rot, carpenter ants, mold.
At the Journal of Light Construction building conference in Boston way back in 1985 we heard from the hot roof camp, we heard research reporting that most moisture leaking into (and thus potentially wetting) building cavities was through penetrations, and following those talks, we saw slides of horror stories by a roofer, Henri De Marne's, whose business was going to New England's hot-roofed cathedral-ceiling homes where a leak had gone unnoticed long enough for the repair to require more than just fixing a leak. The repairs for the cases he documented required a complete roof tear-off, roof sheathing removal, wet insulation removal, moldy drywall removal, replacing rotted rafters, and then finally, reconstruction.
Our photo (left) illustrates rotted roof sheathing and rafter damage in a older home that went undiscovered until the ceiling covering was removed.
It would appear that
- An vented attic roof design is at less risk of trapped moisture than any cathedral ceiling design
- An un-vented cathedral ceiling can remain dry provided everything is perfect in construction and over the life of the roof
- A vented cathedral ceiling design, provided sufficient air space is provided to allow adequate air movement, can reduce the risks of the effects of moisture leaks into the roof from below.
- The common advice that attic ventilation is credited with minimizing ice dams needs to be reconsidered for the case of high-R-value cathedral ceiling roofs as well as for well insulated attics that are also protected against wind wash insulation problems.
Ending Remarks About Hot vs Vented Roofs
Joe has much better education and credentials than I do (Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow), compared to About Us and Daniel Friedman's Resume, and he's an exciting speaker, and truth be told, he's better looking too. We agree on much of the theory but I maintain some practical objections elaborated below. First and more significant: TenWolde and Rose have written a seminal work summarizing the issues and their conclusions on the topic of attic and cathedral ceiling ventilation.[7]
We conclude that while attic ventilation can be beneficial under some circumstances and climates, it should not be viewed as the principal strategy to eliminate moisture and other problems in the attic and the roof. Rather, attic ventilation should be part of a broader range of control strategies. Taking all factors into account, we make the following specific recommendations:
- Indoor humidity control should be the pri8mary means to limit moisture accumulation in attics in cold and mixed climates; we recommend attic ventilation as an additional safeguard
- To minimize the danger of ice dam formation, heat sources in the attic and warm air leakage into the attic from below should be minimized. Additional measures, including attic vents or temperature-controlled mechanical attic ventilation, should be considered. However, mechanical ventilation should not repressurize the attic.
- We recommend venting of attics in cold and mixed climates. However if there are strong reasons why effective attic vents are undesirable, unvented attics can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below, or vice versa.
- The necessity and effectiveness of vents in cathedral ceilings in cold and mixed climates is still a contested issue. Unvented cathedral ceilings can perform satisfactorily in cold and mixed climates if the cavity is properly insulated, measures are taken to control indoor humidity and minimize air leakage into the roof cavity, and a vapor retarder is installed in the ceiling.
- Ventilation should be treated as a design option in cold wet coastal climates and hot humid climates. Currently technical information does not support a universal requirement for ventilation of attics or cathedral ceilings in these climates.
- Research should be directed toward better understanding of the factors that affect shingle durability and toward minimizing air leakage into the attic from below.
In summary, for each of the most commonly cited claims of benefits offered by attic ventilation - reducing moisture problems, minimizing ice dams, ensuring shingle service life, and reducing cooling load - other strategies have been shown to have a stronger and more direct influence. Consequently, attic ventilation should be shifted away from its position as the centerpiece and focus of regulation. The performance consequences of other design and construction decisions should be given increased consideration.
Consistent with those authors I propose that increased consideration should be given to the effects on the building, roof, heating and cooling costs, and maintenance costs associated with
- the gap between design and implementation that leaves air leaks, water leaks, other defects
- variations in use and occupancy and indoor moisture levels in buildings
- the effects of wear and tear on building roofs resulting in leaks into the structure
Watch out for real world snafus, damage, leaks in roofs. In sum, I'm left unsure about the gap between new construction designs and a perfect world where roofs never leak and the roofs I and more importantly, repair and renovation roof contractors have found when we inspected, tore apart, and repaired leaky roofs of both insulated cathedral ceiling homes, and homes with vented roof cavities.
- In hot humid climates an un-vented roof is a defensible approach that may avoid both condensation and heat transfer problems.
- In both warm climates and in cool northern climates an un-vented roof is a defensible approach that avoids risk of chimney effects affecting building heating and cooling costs
- Moisture problems originating outside the building such as due to damage or poor workmanship involve an element of luck and appear to have been excluded by experts and studies in the field, notwithstanding the experience of repair-roofers and home inspectors that those leaks appear on a great many homes, can cause severe rot damage, costly mold damage, and that leaks eventually happen on a great many if not most homes that remain standing for 20 years or more.
- Attic or roof cavity condensation due to indoor moisture sources vary widely as individual building moisture conditions vary (we start our roof inspection by looking for evidence of basement or crawl space flooding) as well as by quality of construction and the number of penetrations in the building ceilings and walls, not to mention the effects of air pressure differences within the building. Delivering a durable hot dry roof is sound in theory but hard to deliver in practice.
Samuelson (1995) pointed out that "... to guarantee no indoor air movement into the attic, the ceiling has to be airtight and the pressure of the attic needs to be higher than that of the indoor air (i.e. pressurized attic or depressurized living space)"[7] - not normal conditions in most climates.
- An under-roof cathedral ceiling nor attic ventilation system, to help the building in any way, needs to be properly designed and installed with adequate intake venting at the eaves, outlet venting at the ridge, and with careful sealing of air leaks from the occupied space into the attic or roof cavity (to avoid heat losses or increases in both moisture movement into the roof cavity and increased heating or cooling costs).
- The packed un-vented hot roof system can work very well in the climates where recommended if everything is perfect in construction and if no leaks ever occur in the roof system - that is, if nothing ever goes wrong. If leaks do occur, from either direction, damage is likely to be more severe than in the older vented roof cavity approach.
Our expert Steven Bliss commented to offer a final word on this topic:
I agree with you that, in the real world, this is not such a good idea. If there's a flashing leak or other roof leak, you could have a pretty soggy mess that stays wet for a long time and could cause structural decay. Plenty of people are building hot roofs, but I wouldn't -- except maybe one with spray urethane which won't absorb much water like cellulose would. [37]
For more details about cathedral ceilings, moisture, ventilation, and insulation, see CATHEDRAL CEILING INSULATION and ICE DAM PREVENTION. See HEAT TAPES & CABLES on Roofs for Ice Dams. And see ROOF VENTING ENERGY SAVING DETAILS.
Also see COOLING LOAD REDUCTION by ROOF VENTS.
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- [2] DCI Products' description of the SmartVent™ tapered under-shingle attic ventilation intake strip is provided at www.dciproducts.com/html/smartvent.htm We have not see data citing actual air flow rates compared with the airflow through a typical continuous vent strip at an un-blocked overhanging building soffit, but this type of product is should be considered at least where a roof structure does not provide a soffit where intake venting can be easily installed. Out of a concern that some roof eave and ridge vent products do not pass nearly as much air as others, we'd like to see airflow data comparisons. [Thanks to G.K. for this update, August 2008]
- [3] Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. (727) 595-4211 mark@BestTampaInspector.com 11/06
- [4] Daniel Friedman - principal author Daniel Friedman, editing, expanding, adding to comments from John Annunziata, P.E. - NY Metro ASHI informal chapter discussions
- Eric Galow, Galow Homes, Lagrangeville, NY. Mr. Galow can be reached by email: ericgalow@gmail.com or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions.
- [5] Building Science Corporation, Joseph Lstiburek, PhD., P.Eng., ASHRAE Fellow, Website: http://www.joelstiburek.com/
- [6] "Joe's Top Ten List of Dumb Things to Do in the South", Joseph Lstiburek, Ph.D., P.Eng., ASHRAE Fellow, web search 5/16/12, original source: http://www.joelstiburek.com/topten/south.htm
- [7] "Issues Related to Venting of Attics and Cathedral Ceilings", Anton Tenwolde, William B. Rose, (both ASHRAE members), Ch-99-11, web search 5/16/12, original source: http://www.penta.ca/resources/articles/issues_related_to_venting_of_attics_and_cathedral_ceilings.pdf [copy on file as Roof_Vent_Issues.pdf ]
- [8] 1997 ASHRAE Handbook - Fundamentals. Atalanta.
- [9] Beal, D. and S. Chandra., 1995 "The measured summer performance of tile roof systems and attic ventilation strategies in hot humid climates", Thermal Performance of the Exterior Envelopes of Buildigns VI., pp. 753-760, ASHRAE
- [10] Baker, MC. 1967, Ice on Roofs, Canadian Building DIgest 89, Ottowa, Ontario: National Research Council Canada
- [11] britton, R.R. 1948, "Condensation in Walls and roofs." Technical Papers 1, 2, 3, and 8, Washingto D.C. Housing and Home Finance Agency
- [12] BLP (Buchan,Lawton,Parent LTd.) 1991, Survey of Moisture Levels in Attics. Ottawa, Ontario, Mortgage and Housing Corporation, Research Div.
- [13] Burch, D.M. & S.J. Treado. 1978. Ventilating residences and their attics for energy conservation - an experimental study. In: Summer attic and whole-house ventilation, NBS Special Publication 548. Gaithersburg MD, National Bureau of Standards
- [14] Burch, D.M., G.N. Walton, G.A. Tsongas. 1997. A mathematical analysis of moisture and heat transfer in the roof cavities of manufactured houses. In: Proceedings of the Fourth International Symposiumon Roofing Technology, September 1997, Gaithersburg MD., pp. 390400
- [15] Dutt, G.S. 1979, Condensatsion in attics: are vapor barriers really the answer? Energy in Buildings, Vol. 2 pp. 251-258.
- [16] Dutt, G.S. and D.T. Harrje. 1978. Forced ventilation for cooling attics in summer. In: Summer attic and whole house ventilation, NBS Special Publication 548. Gaithersburg MD National Bureau of Standards
- [17] FHA. 1942. Property standards and minimum construction requirements for dwellings. Washingto D.C., Federal Housing Administration
- [18] Forest, T.W. and I.S. Walker. 1993. Attic ventilation and moisture, final report. Ottawa: Canada Mortgage and Housing Corporation
- [19] Grot, R.A. and CI Siu. 1978. Effect of powered attic ventilation on ceiling heat gransfer and cooling load in two townhouses. In: Summer Attic and Whole-House Ventilation, NBS Special Publication 548. Gaithersburg MD.: National Bureau of Standards
- [20] Hinrichs, H.S. 1962. Comparative study of the effectiveness of fixed ventilation louvers. ASHRAE Transactions 68. Atlanta. American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.,
- [21] HHFA. 1949. Condensation control in dwelling construction. Washingto D.C., Housing and Home Finance Agency
- [22] Jordan, C.A. and E.C. Peck, F.A. Strange, and L.V. Teesdale. 1948. Attic condensation in tightly buildt houses. Technical Bulletin No. 6. pp. 67-84. Washington D.C.: Housing and Home Finance Agency
- [23] Latta, J.K. 1973. Walls, widows and roofs for the Canadian Climate.National Research Council, Canada, Ottawa, Ontario.
- [24] Rose, W. 1992. Measured values of temperature and sheathing moisture content in residential attic assemblies. In: Thermal Performance of the exterior envelopes ofr buildings V., pp. 379-390. Altlanta, ASHRAE.
- [25] Rose, W. 1995. Attic construction with sheathing-applied insulation. ASHRAE Transations, Vol 101 (1). Atlanta: ASHARE
- [26] Rowley, F.B., A.B. Algren and C.E. Lund. 1939. Condensation of moisture and its relation to building construction and operation. ASHVE Transations, Vol 44, No. 1115, Pitt5sburgh: American Society of Heating and Ventilation Engineers
- [27] Samuelson, I. 1995. Temperatures and relative humidities in ventilated and unventilated attics.Measurements and calculations in attics with loose fill insulatiojn of mineral wool and cellulose fibre. SP Reports 1995:68, Swedish National Testing and Research Institute, Borls, Sweden (In Swedish)
- ]28] SC(Scanada Consultants Ltd.). 1996. Ice dam research data analysis. Final report. Ottawa: Canada Mortgage and Housing Corporastion
- [29] Simpson, J.R. and E.G. McPherson. 1997. The effects of roof albedo on coolign loads of scale model residence in Tucson, Arizona. Energy in Buildings, Vol 25, pp. 127-137.
- [30] Tobiasson, W., J. Buska and A. Greatorex. 1994. Ventilating attics to minimize ice at eaves. Energy and Buildings, Vol. 21, pp. 229-234
- [31] Wolfert, C.K., and H.S. Hinrichs, 1974. Fundamentals of residential attic ventilation. Princeville,Ill.: H.C. Products Co.
- [32] "Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
- [33] "Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
- [34] "Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
- [35] "Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
- [36] "Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
- [37] Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
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- Building Research Council, BRC, nee Small Homes Council, SHC, School of Architecture, University of Illinois at Urbana-Champaign, brc.arch.uiuc.edu. "The Small Homes Council (our original name) was organized in 1944 during the war at the request of the President of the University of Illinois to consider the role of the university in meeting the demand for housing in the United States. Soldiers would be coming home after the war and would be needing good low-cost housing. ... In 1993, the Council became part of the School of Architecture, and since then has been known as the School of Architecture-Building Research Council. ... The Council's researchers answered many critical questions that would affect the quality of the nation's housing stock.
- How could homes be designed and built more efficiently?
- What kinds of construction and production techniques worked well and which did not?
- How did people use different kinds of spaces in their homes?
- What roles did community planning, zoning, and interior design play in how neighborhoods worked
- "Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
- "Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
- "Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
- ...
