Un-Vented Roof Solutions: how to avoid condensation, leaks, attic mold, insulation mold, & structural damage to roof framing when roof venting is not possible
Hot roof designs: suggestions for un-vented or hard-to-vent building roof cavities
How to detect roof venting deficiencies, attic insulation defects, and attic condensation problems
Questions & answers about un-vented cathedral ceiling under-roof spaces
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..
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This article roof and ceiling ventilation series describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings.
Un-Vented Roof Solutions: how to avoid condensation, leaks, damage when roof venting is not possible
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, that approach risks 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 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
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.
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
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.
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.
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.
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.
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.
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.
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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]
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
Daniel Friedman - principal author Daniel Friedman, editing, expanding, adding to comments from John Annunziata, P.E. - NY Metro ASHI informal chapter discussions
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
"The Elimination of Unsafe Guardrails, a Progress Report," Elliott O. Stephenson, Building Standards, March-April 1993
"Are Functional Handrails Within Our Grasp" Jake Pauls, Building Standards, January-February 1991
Access Ramp building codes:
UBC 1003.3.4.3
BOCA 1016.3
ADA 4.8.2
IBC 1010.2
Access Ramp Standards:
ADA (Americans with Disabilities Act), Public Law 101-336. 7/26/90 is very often cited by other sources for good design of stairs and ramps etc. even where disabled individuals are not the design target.
ANSI A117.4 Accessible and Usable buildings and Facilities (earlier version was incorporated into the ADA)
ASTM F 1637, Standard Practice for Safe Walking Surfaces, (Similar to the above standards)
American Plywood Association, APA, "Portland Manufacturing Company, No. 1, a series of monographs on the history of plywood manufacturing",Plywood Pioneers Association, 31 March, 1967, www.apawood.org
Asbestos: How to find and recognize asbestos in buildings - visual inspection methods, list of common asbestos-containing materials
Asbestos products and their history and use in various building materials such as asphalt and vinyl flooring includes discussion which draws on Asbestos, Its Industrial Applications, D.V. Rosato, engineering consultant, Newton, MA, Reinhold Publishing, 1959 Library of Congress Catalog Card No.: 59-12535 (out of print).
Asbestos Identification and Testing References
Asbestos Identification, Walter C.McCrone, McCrone Research Institute, Chicago, IL.1987 ISBN 0-904962-11-3. Dr. McCrone literally "wrote the book" on asbestos identification procedures which formed
the basis for current work by asbestos identification laboratories.
Stanton, .F., et al., National Bureau of Standards Special Publication 506: 143-151
Pott, F., Staub-Reinhalf Luft 38, 486-490 (1978) cited by McCrone
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
"An Example of Colonial Paneling", Norman Morrison Isham, The Metropolitan Museum of Art Bulletin, Vol. 6, No. 5 (May, 1911), pp. 112-116, available by JSTOR.
Dust from the World Trade Center collapse following the 9/11/01 attack: the lower floors of this building contained spray-on fire-proofing asbestos materials.
"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
"Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
"Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
"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
"Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
"Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
Falls and Related Injuries: Slips, Trips, Missteps, and Their Consequences, Lawyers & Judges Publishing, (June 2002), ISBN-10: 0913875430 ISBN-13: 978-0913875438 "Falls in the home and public places are the second leading cause of unintentional injury deaths in the United States, but are overlooked in most literature. This book is unique in that it is entirely devoted to falls. Of use to primary care physicians, nurses, insurance adjusters, architects, writers of building codes, attorneys, or anyone who cares for the elderly, this book will tell you how, why, and when people will likely fall, what most likely will be injured, and how such injuries come about. "
Fiberglass: Indoor Air Quality Investigations: Health Concerns About Airborne Fiberglass: Fiberglass in Indoor Air from HVAC ducts, and Building Insulation
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
Lighting, proper use of: proper aiming of a good flashlight can disclose hard to see but toxic light or white mold colonies on walls.
Pergo AB, division of Perstorp AB, is a Swedish manufacturer or modern laminate flooring products. Information about the U.S. company can be found at http://www.pergo.com where we obtained historical data used in our discussion of the age of flooring materials in buildings.
Plank House Construction: weblog from plankhouse.wordpress.com/2009/01/25/plank-house-construction/ and where plank houses were built by native Americans, see
Large 1:6 Scale Plank House Construction / P8094228,
Photographer: Mike Meuser
06/12/2007 documented at yurokplankhouse.com where scale model Museum quality Yurok Plank Houses are being sold to raise money for the Blue Creek - Ah Pah Traditional Yurok Village project.
Re-Bath, tub lining products is a bath tub relining manufacturer and distributor located in Tempe, Arizona - see rebath.com
Slips, Trips, Missteps and Their Consequences, Second Edition, Gary M. Bakken, H. Harvey Cohen,A. S. Hyde, Jon R. Abele, ISBN-13: 978-1-933264-01-1 or
ISBN 10: 1-933264-01-2,
available from the publisher, Lawyers ^ Judges Publishing Company,Inc., www.lawyersandjudges.com sales@lawyersandjudges.com and also from the InspectAPedia Bookstore (Amazon.com)
The Stairway Manufacturers' Association, (877) 500-5759, provides a pictorial guide to the stair and railing portion of the International Residential Code. [copy on file as http://www.stairways.org/pdf/2006%20Stair%20IRC%20SCREEN.pdf ] -
How to Clean Moldy Wood Framing & Sheathing How to clean/seal mold from/on exposed lumber or plywood subfloor or roof sheathing indoors - some suggestions based on our field and laboratory research
Lighting, proper use of: proper aiming of a good flashlight can disclose hard to see but toxic light or white mold colonies on walls.
Manufactured & Modular Homes: Modular Building Systems Association, MBSA, modularhousing.com, is a trade association promoting and providing links to contact modular builders in North America. Also see the Manufactured Home Owners Association, MHOAA, at www.mhoaa.us. The Manufactured Home Owners Association of America is a National Organization dedicated to the protection of the rights of all people living in Manufactured Housing in the United States.
Stuff that is not mold but is often mistaken for it - things you may not want to test. Also, not all "black mold" is toxic - here are examples of harmless black mold.
Mold-Resistant Building Practices, advice from an expert on how to prevent mold after a building flood and how to prevent mold growth in buildings by selection of building materials and by anti-mold construction details.
Slips, Trips, Missteps and Their Consequences, Gary M. Bakken, H. Harvey Cohen, Jon R. Abele, Alvin S. Hyde, Cindy A. LaRue, Lawyers and Judges Publishing; 2 edition (April 2006), ISBN-10: 1933264012 ISBN-13: 978-1933264011
Steps and Stairways, Cleo Baldon & Ib Melchior, Rizzoli, 1989.
"The Dimensions of Stairs", J. M. Fitch et al., Scientific American, October 1974.
"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
Weaver: Beaver Board and Upson Board:
Beaver Board and Upson Board: History and Conservation of Early Wallboard, Shelby Weaver,
APT Bulletin, Vol. 28, No. 2/3 (1997), pp. 71-78, Association for Preservation Technology International (APT), available online at JSTOR.
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