Effects of Blocked Soffit Intake Venting - a Cause of Attic Condensation, Ice Dam Leaks & Attic Mold in buildings
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Blocked Soffit Intake Venting as a Factor in Attic Condensation Problems and Attic Mold
What causes attic condensation, mold, or dark stains on roof plywood?
How to inspect for working attic venting at soffits
How to check for true soffit intake vents versus faux soffit venting
How to detect roof venting deficiencies, attic insulation defects, and attic condensation problems
Questions & answers about attic moisture, condensation & ventilation: what are the effects of blocked soffit intake venting at roofs?
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.
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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.
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.
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.
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.
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.
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
Questions & Answers regarding this article
Questions & answers about attic moisture, condensation & ventilation: what are the effects of blocked soffit intake venting at roofs?
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Alan Carson Carson Dunlop Associates, Toronto, Ontario. Mr. Carson is a home inspection professional, educator, researcher, writer, and a principal of Carson Dunlop Associates, a Toronto home inspection and education firm. Mr. Carson is a past president of ASHI, the American Society of Home Inspectors
Some great illustrations of the proper under-roof ventilation pathways are offered by Carson Dunlop.
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 & 12/08
John Annunziata, P.E. - NY Metro ASHI during informal chapter discussions about roof and attic ventilation options (1986-1996).
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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 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).
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.
"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. "
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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