Continuous High Capacity Eaves or Soffit Intake Venting Provides Adequate Intake Air Under Roofs
This article 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.
These recommendations are based on roofing industry standards, roof covering manufacturer recommendations, and on review of the literature on building insulation and ventilation, as well as on 30 years of
building inspections, on the observation of the locations of moisture, mold, ice dams, condensation stains, and other clues in buildings,
and on the correlation of these clues with the roof venting conditions at those properties.
We have also measured changes in airflow, temperature,
and moisture before and after installing roof venting.
As we show in this pair of photos (above and below), continuous soffit intake venting will provide optimum intake air flow between every rafter pair.
On buildings with very large gable end vents, lots of insulation in the attic floor, and perhaps lucky house siting, I have seen attics that were perfectly dry and free of condensation, ice dams, and mold.
But these have been the exception, not the rule, at least for inspections in northern climates subject to cold winters and hot humid summers.
Installing Continuous High Capacity Eaves or Soffit Intake Venting Works Best to Avoid Attic Moisture, Mold, & Ice Dams
Here are examples of inadequate intake ventilation: vents at the soffits are intermittent or "spot vents" or are simply too small.
Continuous soffit/eaves intake venting is the proper location for the intake air, in order to assure that the entire under-side of the roof sheathing is vented and kept dry.
Where I inspect attics with "spot vents" in the soffits (those little round
louvered vents ranging from about 3/4" diameter to 2" in diameter, are completely ineffective, never moving enough air.
Venting needs to be provided between every rafter pair at the eaves and ridge. You won't achieve this if venting is intermittent along the soffits or eaves of a home.
Don't install intermittent or occasional or faux soffit intake venting or vents with too little opening area such as we show in the photo at left.
Not only are the openings too small to pass enough air (obstructed further by the louvers and insect screens), intermittent soffit intake vents or little round or rectangular soffit spot vents are singularly ineffective in providing good under-roof or attic ventilation.
Where we inspect attics where even larger vent openings are provided in the soffits or eaves, if the openings are intermittent, we see wet and often moldy roof sheathing on those roof sections where no venting is provided, even though at other roof sections where vents are present
the sheathing often looks clean and dry.
This is very strong evidence that air is not moving up the under-side of the sections of roofing where no vents are
Continuous ridge venting is the optimum exit path for warm rising air in an attic, thus pulling new cooler, drier outside air into the under-roof area from between every rafter pair. (C)Daniel Friedman - copyright violation trap.
But also remember the danger of adding a ridge vent without soffit vents (the worst) or soffit vents without a ridge vent (bad) or only gable-end vents (usually bad).
The up-draft of air from the building (convection
current of rising warm air which moves up through most buildings) will be increased and will mean unnecessary heat loss if you have a ridge vent to vent air out without also providing good intake venting at the soffits or eaves.
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ok to strip off old plywood soffit covering
could you take the plywood off and jus run the soffit? - Paul Gilbert 8/13/11
Certainly, Paul, as long as there is adequate support for the vinyl or aluminum perforated soffit covering the plywood soffit that was in place can usually be omitted.
But do not just leave the soffits entirely open with no covering whatsoever - doing so invites an invasion by birds, squirrels, raccoons, etc.
I have a bit of a problem with your last statement; too great and intake area is going to be a problem as well, IMHO. I am curious about the engineering with regard to airflow in the case of continuous soffit venting. Unless you have the same area (or greater) for the air to exit, does the air flow not just go slack (no proper draft) and possibly create the problem you are trying to prevent?
I have continuous soffit venting and ridge venting (which frankly is a horrible idea without the right amount of pitch and again IMHO a bad idea in general...automated extraction is a MUCH better answer!) and the air flow is pathetic - it has no direction and don't even bring in the range of factors created by a breeze, more engineering!
A continuous soffit vent of let's say 3" around the perimeter of a home is going to have a much larger opening area then the ridge vent or gable vent not creating a proper draft and allowing the air to go slack. (with exception of automated extraction fans) This issue has been over-simplified, it is much more complex than what I have read here. - Robert 9/30/2012
Really? Not quite, Robert. We need about 2x as much intake venting at the house eaves as exit venting at the ridge in order to be sure that we don't start drawing conditioned air out of the building itself. Decades of field experience as well as numerous building studies and, among building professionals, hundreds of thousands of onsite inspections of buildings in various configurations of venting and venting devices confirm the data.
I agree that some modern ridge vents may not be passing as much air OUT as we'd like, but having extra intake venting is not going to hurt - since the attic or roof cavity is not a flexible balloon, more air won't enter the attic than can exit at the ridge.
But Robert, your idea, of automated "extraction" of roof moisture is unfortunately either a fantasy ( the fans that "extract" attic air don't draw in cool outside air and instead extract air from the building, increasing heating and cooling costs), or works only at significant net cost (for example the cost of adding humidity controlled equipment), and in all cases, risk uneven drying of the under-roof surfaces unless continuous intake and outlet venting are there to direct the air flow where it's needed.
Some of the other problems we see with "extracting" vent systems that rely on an attic or roof vent fan include these:
The attic or roof or gable end fan does not ventilate evenly under the roof, leaving wet areas
The attic or roof or gable end fan operates by temperature, venting only in summer, not in winter when there are condensation worries
The attic or roof or gable end fan creates a serious fire-increase hazard should it operate during a house fire, or reqires extra costly controls
It's often risky to be over-confident in our own opinions, no matter how carefully reasoned, as too often our own experience, study, and knowledge are just much less than that of the larger body of experts interested in the question. Take a look at the ASHRAE, DOE and other expert citations at the end of this article and you can figure that we're not just making this stuff up.
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Question: how can I install a vinyl soffit covering if there is dentil trim at the wall top?
I am considering replacing the soffit on the front upper overhang of my house with vinyl invisivent soffit, but have an existing dentil moulding. Is there a way to install the new vinyl soffit without covering the dentil moulding so that I can preserve this distinctive architectural feature? - C.M. U.S.A.
http://www.certainteed.com/resources/CTS150.pdf from Certainteed describes the soffit covering product you mention.
Normally a vinyl soffit covering is supported on the outer or fascia edge by L-channel and against the building wall by L or F channel strips.
If you have space to nail the supporting F or L channel to the house wall above the dentil trim you describe, you're in business. If there is no space even for the supporting channel then you don't have room to install the soffit covering, as its thickness is not much less than that taken up by the supporting vinyl channels. You'd have to move the dentil trim down.
You need about 3/4" of space to support the edges of the vinyl soffit covering.
But all of this is speculating and arm waving - as I haven't seen the details of your existing soffit structure and house trim details. If you'd like to send us some photos I can comment further.
(Feb 6, 2014) gene said:
hi we have moisture coming out of our soffit and cant figure out why . we have gable end vents and no ridge venting, with small 3 inch holes in the soffit. its only doing it on the wall that the shower is on its about a ten foot section we tried cutting the 3 inch holes bigger but it didn't help it just made more ice cycles , so that tells me we have heat loss now?
Gene, it would be no surprise that moisture may accumulate in a soffit, particularly if the vent openings are too small (which IMHO is the case with 3-inch openings unless you've cut a 3-inch strip for the full soffit length).
Moisture accumulates in cold weather (warm moist air entering soffit, hits cool conditions, moisture condenses out) and can form ice
I agree that your description may mean that there is more building warm air movement into the attic and out through soffits (probably through other vents & openings) than you'd want - but to make sense of the heat loss picture one would want to take a look in the attic - for the amount and care with which insulation was installed - look for openings, missing insulation &c. And in the finished area look for leaks especially around wall & ceiling penetrations.
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John Annunziata, P.E. - NY Metro ASHI during informal chapter discussions about roof and attic ventilation options (1986-1996).
The Smart Vent™ by DCI roof intake venting provides an intake at the lower edge of roof decking for difficult cases. See www.dciproducts.com/html/smartvent.htm
The AccuVent™ attic ventilation roof baffle produced by Berger permits insulation to extend over the top plate as far forward as possible. See www.bergerbuildingproducts.com/pdfs/AccuVentAtticVent.pdf
GAF Cobra® and other GAF roof ventilation products: see www.gaf.com/Content/GAF/RES1/roof/RS_whyuse_ventchart.asp?viewer=&module=
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
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.
"Unvented Roofs, Hot-
Humid Climates, and
Asphalt Roofing Shingles
Research Report - 0306
Building Science Corporation", Building Science Corporation, 30 Forest Street,
Somerville, MA 02143
Quoting from the article abstract:
"When constructing unvented roofs with asphalt shingles in hot-humid climates, a vapor barrier must be
installed between the asphalt shingles and the roof deck." - Web Search 5/21/2010
Thanks to reader - Mike Martino
for discussing attic ventilation retrofit ideas - August 2010
 Air Vent, Inc., Tel: 1-800-247-8368, Email: firstname.lastname@example.org, web search 08/11/11,
 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.
ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries. The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
2005 ASHRAE Handbook : Fundamentals: Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
"2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
"The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
Energy Savers: Whole House Systems Approach to Energy Efficient Home Design [copy on file as /interiors/Whole_House_Energy_Efficiency_DOE.pdf ] - U.S. Department of Energy
"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
Ice Dam Leaks in building attics and roof cavities, how to inspect for evidence of leaks, identify causes, and correct bad attic ventilation, improper roof venting, and these causes of attic mold or roof structure damage
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