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VENTILATION in BUILDINGS
AIR BYPASS LEAKS
AIR LEAK DETECTION TOOLS
AIR LEAK SEALING PROCEDURE
AIR POLLUTANTS, COMMON INDOOR
AIR SEALING STRATEGIES
BASEMENT CEILING VAPOR BARRIER
BASEMENT HEAT LOSS
BASEMENT LEAKS, INSPECT FOR
BLOWER DOORS & AIR INFILTRATION
BRICK WALL DRAINAGE WEEP HOLES
CATHEDRAL CEILING VENTILATION
CEILINGS, DROP or SUSPENDED PANEL
COMBUSTION AIR for TIGHT BUILDINGS
COOLING LOAD REDUCTION by ROOF VENTS
CONDENSATION on WINDOWS & SKYLIGHTS
DEW POINT CALCULATION for WALLS
FIREPLACES & HEARTHS
FLAT ROOF MOISTURE & CONDENSATION
GREEN BUILDING CONSTRUCTION
HEAT LOSS in BUILDINGS
HEAT LOSS DETECTION TOOLS
HEAT RECOVERY VENTILATORS
HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS
HOUSEWRAP AIR & VAPOR BARRIERS
HOUSE DOCTOR, how-to be
HUMIDITY LEVEL TARGET
ICE DAM LEAKS
INDOOR AIR HAZARDS TABLE
INDOOR AIR QUALITY & HOUSE TIGHTNESS
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSULATION AIR & HEAT LEAKS
INSULATION INSPECTION & IMPROVEMENT
INSULATION R-VALUES & PROPERTIES
LOG HOME GUIDE
MOISTURE CONTROL in BUILDINGS
ODORS GASES SMELLS, DIAGNOSIS & CURE
ROOF VENTILATION SPECIFICATIONS
ROOF ICE DAM LEAKS
SHEATHING, FOIL FACED - VENTS
STAIN DIAGNOSIS on BUILDING INTERIORS
STUCCO WALL METHODS & INSTALLATION
SWEATING (CONDENSATION) on PIPES, TANKS
THERMAL MASS in buildings
THERMAL TRACKING Indicates Heat Loss
VAPOR BARRIERS & AIR SEALING at BAND JOISTS
VAPOR BARRIERS & HOUSEWRAP
VAPOR CONDENSATION & BUILDING SHEATHING
VENTILATION in BUILDINGS
WIND WASHING INSULATION at EAVES
WINDOWS & DOORS
This article explains the effects on buildings caused by locked soffit intake vents and we explain how blocked soffit venting causes or contributes to attic condensation, moisture, and potential mold contamination problems in buildings. We also explain that attic or roof exit venting without adequate soffit intake venting increases building heating cost.
This article series describes inspection methods and clues to detect roof venting deficiencies, insulation defects, and attic condensation problems in buildings. It describes proper roof ventilation placement, amounts, and other details. Also see CATHEDRAL CEILING INSULATION.
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The photograph at page top shows an attic whose intake venting is blocked by fiberglass insulation.
The photograph at left shows severe mold on the underside of roof sheathing in a 1920's cape cod attic where there was no under roof ventilation.
The page top photo suggests (by the absence of visible mold) that luckily we may not find a mold problem in every poorly-vented attic or under-roof space. But after we confirmed in our lab that the mold was Penicillium sp., in our opinion the attic at left needed to be cleaned. The risk of problem mold bothering building occupants was increased when the attic was in a knee-wall area adjoining a bedroom.
When removing problem mold from an attic we must also correct the moisture problem by both removing the moisture source and by correcting any attic venting defects. See Correcting Roof Ventilation for more details about correcting under-roof ventilation. See the Mold Information Center for guides to inspecting, testing, and removing mold in buildings.
Why is Blocked Soffit Intake Venting a Factor in Attic Condensation Problems?
The photograph shows building eaves blocked by mineral wool insulation. In this case the builder and insulator were fortunate because even though roof venting was blocked, there was not an attic moisture problem.
And we were pleased to note the absence of significant ice dam leak stains on the roof sheathing or rafters. Still, opening the soffits for venting can result in a cooler roof surface and longer shingle life, even if there are no moisture problems in the attic.
Here are some examples of blocked intake venting in an attic:
Why Does Blocked Soffit Intake Venting Increase Building Heating Cost?
It's simple. If we make the mistake of providing exit venting from a roof cavity or attic, such as a nice open ridge vent or gable end vents, we also need about twice as much (by square inches) of intake venting at the building eaves. Otherwise here is what happens:
Heat and warm air flows into and is lost from the building roof cavity or attic - warm air rising creates upwards convection currents in the building.
The rate of movement or "strength" of the up-flowing warm air current from the building occupied space increases as it enters the attic and finds a ready exit vent at the ridge or gable ends. (We prefer continuous ridge vent to assure even ventilation across the roof deck underside).
As air flows readily out of the exit venting high on the roof (ridge vent or gable end vents) it creates a negative pressure with respect to the air pressure in the building occupied space.
But if there is not adequate intake venting of outside air, that same negative pressure tends to draw still more conditioned air (or heated air) out of the building space. Essentially we are increasing the heat loss from the building.
Conversely, if there are open soffit vents to allow free flow of air into the attic (or cathedral ceiling roof space), the negative pressure or "vacuum" created by the exiting attic air is more easily satisfied by inflowing (cooler, more dry) outdoor air than it is by leaking air from the occupied space. That slows building heat loss during the heating season.
Why aren't gable end vents the best idea for attic venting?
Gable end vents alone do not uniformly cool and dry the whole roof underside.
Gable end vents combined with a ridge vent tend to become intake vents feeding air flow currents created by air exiting at the ridge, thus failing to draw air up along the roof underside, failing to cool and dry that area, even if soffit intake venting is present.
How do I Unblock Obstructed Roof Eaves by Installing Roof Vent or Soffit Baffles in the Attic
If your building has adequate intake venting at the soffits or eaves, and good outlet venting at the ridge, you may still find problems with attic condensation, attic mold, or roof ice dams (in freezing climates) if the attic insulation blocks the venting system.
An inexpensive solution is the addition of styrofoam roof vent baffles which are placed between every rafter pair.
The attic roof vent baffles baffles hold the insulation away from the roof deck to permit air to enter the attic or under-roof space.
We use this same design under roofs that cover cathedral ceilings, but on occasion find that more air space and air flow up under the roof sheathing are needed in those structures.
Eaves blocked by foam insulation: some older homes were super insulated during the energy crisis in the U.S. in the 1970's by pumping urea formaldehyde foam insulation (UFFI) into building cavities.
We may spot this foam oozing into an attic even if it's not visible elsewhere in the building (though you can find it by strategic probing and inspecting at building cavities). If excess UFFI pumped into building walls has blocked attic insulation, it may need to be removed.
See UREA FORMALDEHYDE FOAM INSULATION, UFFI for a detailed discussion about formaldehyde and indoor air quality issues concerning this UFFI urea formaldehyde foam insulation.
The best place for locating or placing attic insulation, from the view of avoiding attic condensation and ice dams, is in the attic floor or up the sides of attic knee walls.
This leaves a cold, drafty attic, but it means longer shingle life and no attic condensation problems.
Avoid placing insulation between the rafters unless special venting measures are also taken.
Question: Faux Soffit Vents - Can I Vent My Soffits by Working From Inside the Attic?
Hello- I just finished reading your piece on attic ventilation and unfortunately believe that work done on my house was incorrect. I am a 71 year old female who hired a gutter man to put new gutters on my house. The soffits needed painting (there had never been any vents) and I gave him the job of replacing the soffits.
I now believe that he just covered the old soffit with vinyl soffits that had perforations every few feet without cutting any holes. This past winter, I had a problem with icicles forming along the edge of the new gutters. I called him about the problem, but he claims that it was a bad winter.
He has agreed to return next week, as the gutters are leaking in a few places. I don't think that the attic is venting at all except for the gable vents. My question is this : Can holes be cut from inside the attic and what would be the best way to attempt this? Would I be better off calling an insulation company to check out the attic?
Technically, there are 2 soffits - one the original plywood and the vinyl one on top. The first order of business is to get into the attic and see if he cut any openings. If not, it sounds as though the entire job needs to be removed. If a continuous channel is cut, would I not be able to reuse the vinyl soffit? I'll attempt to send a photo that I took during the winter that shows icicles along the gutter and you can also note the type of vinyl soffit that was used. Again, thanks for your input. - P.
Reply: Usually access from in the attic is too limited. Check for venting, and add from outside if it's absent.
You can't cut holes into the soffit from inside most attics - it's just too difficult to crawl down into the roof edge and almost impossible to reach into the soffit with a saw.
Besides, drilling a few holes just won't provide adequate air inflow.
The work to provide air intake venting at a building's eaves or soffits is almost always performed from outside.
First, let's find out if the gutter guy did what we fear: installing perforated "soffit vent panels" over solid plywood soffit coverings - what we call "faux soffit vents" because you see the vent panels but they aren't doing a thing.
There are two easy ways to check this. If you can get into the attic when it's dark (or bring a flashlight but then turn off the attic lights), see if you see light when looking into the eaves - if so there were openings cut. If not there were either no openings or they are blocked.
Our photo of our friend Jess Aronstein's attic (Poughkeepsie NY) shows light at the house eaves - so we expect that there was pretty good intake venting at that location.
From outside, if the soffit undersides are safely and readily accessible (don't work from a ladder if you're not fit and experienced, don't work alone, don't put a ladder on an icy surface, etc) then often we can push upwards on those vinyl soffit panels. If they are over solid wood they'll feel solid - nothing moves. If the soffit vent panels were installed only after the original solid soffit wood or plywood was removed, the vinyl panels will flex upwards easily for 1/2 inch or even more.
If the wood soffit covering was "vented" just by cutting a few intermittent holes before the vinyl was installed, chances are it's going to be inadequate. We need roughly twice as much air intake at the house eaves as we have air outlet along the ridge.
The best procedure would be to remove the vinyl panels that your gutter guy installed, cut in a continuous soffit vent strip along the entire soffit - continuous, not intermittent.
See our photo (left) of continuous soffit vent strips.
[Click to enlarge any image]
We like the openings and vents to be just behind the fascia board - it's easier to install (one saw cut instead of two), it lets us leave the rest of the soffit plywood covering in place, and by its location we reduce the chances of wind-wash (movement of loose fill attic insulation away from the wall tops in the attic) and we reduce the chances of wind-blown rain entering the attic space.
But if the soffit plywood was in poor shape, stained, rotted, needs painting, a better alternative is to simply remove the soffit wood entirely and replace all of it with perforated vinyl panels.
See our photo at left.
Be sure you've also got a working exit vent along the ridge, and check that the vent openings into the attic space are not blocked by insulation
Question: do I need to replace soffit intake venting that has been blocked by a building addition?
We extended out roof line out twenty four feet and built a garage covering 26 feet of soffit. A contractor told us because this 26 feet of soffit was now in the garage, we would have trouble. What do we do?
Contractors contradict each other and one says do nothing. Here are some pictures. On the first you can see how we extended the roof line. Follow the roof line right to left and see the white soffit end and wood addition begin. The Amish built us a log garage attachment (making a straight roof line because of the ground slope) to former garage we turned into a living room. Given property considerations, this was the only way we could add space.
[Click to enlarge any image]
Next photos show the inside of the garage which shows the rafters meeting the logs on one end, and that 26 feet of soffit is now "in the garage".
The house is only 2 years old and main portion is 1100 sq feet without old garage converted. Egg cartons between all rafters.
Any advise you could give is greatly appreciated. We live in Wisconsin. - Eric Zingler 12/24/2013
At above left we see the new roof extending off of the edge of the3 original roof eaves, where the perforated white soffit covering remains in place on the original roof overhang. At above right is the new roof eaves end seen from the garage interior - showing that any intake ventilation has been blocked at the wall top.
In general, if a garage addition onto an older home caused a portion of the original roof to become "indoors" by building a new, higher or over-roof over the garage that itself would be vented as well. And if a contractor built a second roof atop an original roof, leaving the original shingles in place he may have violated local building and fire codes.
But the case you describe is not a roof-over but a roof-extension - extending an existing roof line out from an existing eave line. So the fire code issue does not pertain. What does remain is the under-roof ventilation question you raised and that it took me some time to understand.
On your photo above I marked in blue the original roof venting system in theory - we don't know if it is actually working. On that photo I marked in red the rear roof slope vent path that is now blocked by the new roof extension, and I marked in dashed green a possible new air path that may be needed to reactivate the red section.
My comments begin with remarks on the three options you were given
From the photos and your notes I see the following options
If I had built this addition I'd have removed the original roof fascia and sistered the upper ends of the new addition garage rafters to the last 24" of the original roof rafters. That would have left an adequate airflow opening along the original roof eaves. I would have built a similar roof overhang (looks like you have 24" or more) at the new garage roof lower end or eaves and vented it.
Watch out: I'm not clear on how the new roof was attached to the original one. If it was just tacked onto the fascia board of the original eaves I'm not sure this is a structurally adequate connection.
Watch out: if the original under-roof venting design did not leave adequate airflow space between the top of the ceiling or under-roof insulation in the original house, or if the ridge vent does not provide adequate outflow opening size along the entire ridge, then this entire question is moot because the original house under-roof ventilation system is not working anyway.
If your original roof is not adequately vented you are faced with a decision to convert the building to a "hot roof" design. (HOT ROOF DESIGNS: UN-VENTED ROOF SOLUTIONS
In sum, I would not do a thing before first investigating the original roof venting design to be sure that there is one, and I might, if the garage is to be un-heated, wait to insulate its roof and wait to see the effectiveness of air intake into the old roof eave soffits entering from the garage space. BUT if you plan to heat the garage I'd review again the thoughts I outlined above.
Reader follow-up: I welcome all opinions and will check [here from time to time to see if anyone else has a different suggestion]. It looks like I have two options: open window or have the two inch channel from old roof to end of new. Bottomline, it looks like cracking a window can handle the matter until I can afford the latter.
Reply: Cracking a window makes sense for a temporary measure - assuring that there is plenty of air available to feed the natural in-draft at the soffits that is caused by warmer air exiting at the ridge. The reason I prefer continuous soffit intake venting is that rafter pairs form a bay that conducts airflow along the roof underside. If you are not insulating the garage ceiling then it's not an issue. But if you are you want an air path between every rafter pair and an air intake for each of those bays.
Question: replacing aluminum soffit vents with vinyl reduces the actual vent opening area - seeing attic mold
7/8/14 Richard said:
Three years ago, I had my 34x24 hip roofed house have its roof and soffit vents replaced. The original ridge was continuous aluminum vent and so was the original soffit vents. The new soffit vents are vinyl and continuous but are only 3.5 to 4 inches wide.
When the contractor originally replaced the soffit vents I expressed concern that just looking at the vinyl soffit vents would provide much less air into the attic. The contractor assured me it would be enough [ventilation in the attic] but now I've noticed mold in the attic. And some limited amount of water had puddled on some boxes under the ridge.
So then I started looking at calculations.
I live in NH- Do I need adjustments of both ridge and soffit?
I have a concern that the water on the boxes was an indication that the soffit venting was insufficient.
Are the any rules of thumb that contractors should be using when replacing aluminum vents with vinyl ones?
Thanks for the interesting and important analysis. Indeed some contractors are not into reading instructions, building journals etc. We joke that the instructions are just used to kneel upon while doing the actual work.
Rule of thumb for venting a standard gable roof, soffit to ridge ratio: Basically we want the soffit intake to be 2x the outlet vent opening. Indeed the smaller perforations in some soffit vent coverings can have the effect you describe.
Some hip roofs add a spot vent on the smaller triangle hip roof ends at the triangular apex near the end of the ridge to add exit opening. I'd consider that improvement if we also need more hip roof exit venting for a hip roof design.
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Frequently Asked Questions (FAQs)
Question: soffits are covered with insulation, how do I get venting intake?
(May 30, 2014) Re-posting comment from Anonymous on 5/29/14 without his caps-lock on
My roofer wants to cut into decking a small area here and there along gutter line because my soffits are covered in attic with insulation. Is this a good method? Doesn't it leave some shingling hanging without support? Concerned. Other than that, he said smart vents are dumb vents except in the south bc they can cause ice damming in winter in gutters and back up that ice into shingled areas. Other than that there are baffles which seem ok. I like his idea, but cant find it on internet.
Anon I don't quite have a clear idea what's going on, but if by "decking" you mean the roof deck, I think you are talking about looking for a way to get intake ventilation into the attic.
The best solution is to bring insulation out over the wall top plate and at the same time keep it off of the underside of the roof deck and give an air path by inserting baffles (carboard or styrofoam) sold for that purpose, between every rafter pair - work done from in the attic.
Then either go to a perforated soffit cover or cut a vent opening continuous along the soffit to accept a perforated vent strip.
We do agree with roof deck removal as an intake port along the entire lower roof edge just for the case of roofs built with no roof overhang at all. IN this case a Hicks Starter vent or equivalent is added to support the shingles while providing a (modest) intake vent opening.
Watch out: even adding a starter vent and cutting off roof decking for a couple of inches won't work if the space inside is filled with insulation. So you're back to inserting baffles, which means there was no reason to cut roof deck in the first place.
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