 Effects of Blocked Soffit Intake Venting - a Cause of Attic Condensation, Ice Dam Leaks & Attic Mold in buildings
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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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Blocked Soffit Vents Invites Attic or Roof Cavity Moisture, Condensation, & Mold
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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?
Even if continuous soffit venting was installed (seen from outside) it may be blocked in the attic (shown here).
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.
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Here are some examples of blocked intake venting in an attic:
- Insulation stuffed into building eaves: Some homeowners and even some insulation installers love to stuff insulation into the soffit to reduce drafts, in order to try to warm the attic and thus slow heat loss from the building. In some instances they even block soffit vents.
- Perforated soffit vents over solid wood: many older homes have had exterior siding and soffit coverings installed, often of aluminum or vinyl. But very often we find that perforated soffit vent panels were installed directly over
the older solid wood soffit covering. So even though an outside inspection shows that soffit or eaves ventilation appears to have been installed, in fact this is not the case. You can detect this condition easily:
from outside simply push up on the vinyl or aluminum perforated soffit panels. If they contact a solid surface they were installed over solid plywood or wood boards at the building eaves. From inside the attic, if there is
no insulation blocking the eaves, and if you do not see daylight, there is no working intake venting at the eaves or soffits.
- Deliberate Blocking at Eaves: in some older buildings, in an attempt to "warm up" a cold attic someone may have installed wood blocking between the rafters at the building eaves. In this case even
if soffit vents are installed there will not be adequate air inflow. Remove this blocking when designing good attic venting. Be sure you're removing eaves blocking by noting that you can, after its removal,
see into the soffit overhang at the building eaves. For safety, do not remove fire blocking found between wall studs which may show at an attic floor.)
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
 Photo of roof vent baffles installed in an 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.
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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. Newer open celled (Icynene®) closed cell foams are in use today.
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.
Questions and Answers on Un-Blocking Soffit Vents or Eaves Vents on buildings
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.
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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.
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.
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 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
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- 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?
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- 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
- "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
- Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
- "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
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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
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