Should building wall vent strips be used behind building sheathing ?
Do building wall vent strips create moisture problems in structures?
How does the dew point in the building wall or roof move when insulation is added?
How to avoid moisture & condensation problems in buildings
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Do we need to install vent strips behind foil-faced insulating wall sheathing? Is ventilation needed at the top and bottom of exterior walls sheathed with foil-faced insulating board?
Also see SIDING WOOD where we discuss wood siding failures when wood is installed over foam insulating sheathing board. Readers researching wood siding problems should also see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION as well as the other siding types listed at page left. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Is Venting a Wall Recommended for Exterior Building Walls Covered with Foil-Faced Exterior Sheathing?
Our photo at page top shows a super-insulated building addition constructed by the website author (DJF) in1978. Walls were framed with 2x6 studs, insulated with 6" of fiberglass batts, and sheathed on the wall exterior with 1-inch "High-R" foil-faced foam insulating board. An additional layer of 15-pound felt was installed over the wall, topped with furring strips to give a 1/4" air space ( for a total wall R-value of about R28) over which we installed plywood T-111 grooved building sheathing. The wall interior surface received a continuous 6-mil poly vapor barrier over which drywall was installed. This was an early experimental "superinsulated" design in response to soaring home heating costs during the U.S. oil price spikes in the 1970's.
By 1983 we heard that concern for possible wall cavity condensation had led manufacturers of foil faced sheathing to recommend corrugated vent strips nailed at the sills and wall top plates to ventilate the insulated wall - an idea that seemed odd at the time, since if we pass outside air behind insulating wall sheathing its insulating value would be wasted. Other reps suggested that vent strips be installed between the exterior sheathing and the insulating board - but that's not where the condensate would occur anyway.
With the benefit of 30+ years of experience with this structure, we have learned that: the felt was unnecessary, no wall vent strips were needed, no moisture condensation has occurred in the walls (where we made test cuts from the interior), and workmanship that included careful sealing around wall penetrations for the electrical boxes was important to keep indoor moisture out of the wall cavities.
The April 1983 Solar Age Magazine addressed the question of whether or not vents are needed behind exterior sheathing when foil faced insulating boards are installed, in Solar Age Magazine Q&A details "Dew Point Dilemmas" that helped resolve these questions. The text below paraphrases, quotes-from, updates, and comments an original article, (see links just above) from Solar Age Magazine and written by Steven Bliss.
Wall Vent Strip Question - are they needed?
I have been concerned with the moisture condensation issue for some time but have not found solid answers easy to come by. The local representative for a major foil-faced sheathing insulating board product tells me that vent strips should be used behind the exterior sheathing in climates with more than 8000 degree days. In milder climates, apparently their product does not create moisture problems in the walls. I still find this somewhat disconcerting since, although we have only 5000 degree days, the humidity is so high, especially in winter.
When I questioned the representative for an extruded polystyrene insulating board product about its use as an exterior sheathing or for use as roof insulation over exposed-beam ceiling, I was told that the dew point is effectively moved to the center of the insulating board when the wall cavity is uninsulated. This does not leave me convinced, however, and I am even less confident about what will happen if the wall or ceiling cavity is insulated. -- C.S., Seattle WA.
Wall Vent Strip Use Recommendations with Insulating Wall Sheathing Boards
We do not recommend venting a wall that uses foil-faced exterior insulating sheathing panels. First, if the level of ventilation is high enough to relieve moisture problems, it will probably chill the wall cavity as well. Second, recent research (1980's) at the USDA Forest Products Laboratory demonstrated that, in some cases, venting actually increases moisture accumulation in the building wall. Researchers suspect that the vented air is being replaced with moist inside air drawn through gaps and seams in the vapor barrier.
With a perfect vapor barrier [on the wall's warm interior side], the venting would be unnecessary in the first place. Leaving the insulating sheathing un-sealed at the joints, as most manufacturers recommend, should be adequate if a good inside vapor barrier is used.
As for the use of extruded polystyrene foam board insulation on the exterior of building walls, we have spoken to a number of manufacturers, builders, and researchers and have heard of no problems with or without cavity insulation. Since the insulating sheathing warms the wall cavity, less moisture is driven there by vapor diffusion. [DJF Note: subsequent research has shown that vapor diffusion is the minor player in wall or ceiling cavity moisture problems, and that air leaks at penetrations is what carries and accumulates moisture in building walls.] And with a perm rating of 1.2 per inch, the foam board sheathing provides some moisture relief. [DJF Note: compared with foil-facing that, if not perforated, has a very low perm rating, close to zero.]
Reaching the dew point (calculated as a function of inside building conditions) within the foam insulating board is generally not a problem, since the low rate of diffusion through the material effectively lowers the vapor pressure and dew point in that region to a safe level. See DEW POINT TABLE - CONDENSATION POINT GUIDE.
In unvented roofs, flat or pitched, the situation is complicated by the relatively impermeability of most roofing materials. A high-quality vapor barrier on the warm side of the insulation is recommended. In high-moisture situations, this may be combined with a vented air space between the insulation and the roofing underlayment or sheathing [DJF OPINION: this is the best construction practice and gives a longer cooler roof life as well.] This approach can be very effective in hot climates (using foil faced insulating board products) for cooling, or in cold climates to prevent the formation of ice dams.
Evidence of Moisture Penetration into Building Wall Cavities at Penetrations
DJF notes: there was no research actually demonstrating significant air or moisture movement through these butt joints or seams; probably most important are a very good interior vapor and air barrier, especially sealing at wall penetrations, combined with interior moisture control that keeps indoor humidity below excessive levels.
Our photo (left) shows thermal tracking (and possibly mold) around the electrical receptacle in a building wall that combined poor sealing around wall penetrations with high indoor moisture. Even before making a test cut to explore the condition of the wall cavity and its insulation, the marks on three sides of the outlet cover plate tell the story. The reason we see no moisture and air leak related marks along the left side of this wall outlet - can you guess? Yep - that's the side of the electrical box that was nailed snugly to the wall stud.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
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.
Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in BUILDINGS, and other topics.
Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
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