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Foil-faced building wall sheathing installation:
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?
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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 from Solar Age Magazine and written by Steven Bliss.
Wall Vent Strip Question - are they needed?
Wall Vent Strip Use Recommendations with Insulating Wall Sheathing Boards
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.
as well as VAPOR BARRIERS & CONDENSATION in buildings for details.
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.
Continue reading at HOUSEWRAP / SHEATHING WRAP or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs) about wall sheathing vents
Questions & answers or comments about ventilation requirements behind foil faced building wall sheathing.
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