(C) Daniel FriedmanWall Vents Behind Sheathing?

  • SHEATHING, FOIL FACED - VENTS - CONTENTS: 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. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
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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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Exterior Wall Sheathing Vents?

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?

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.


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

Thermal tracking leaks at wall penetration (C) Daniel Friedman

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.

Original article

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 closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SIDING WOOD, FAILURES OVER FOAM BOARD where we describe wood siding failures when wood is installed over foam insulating sheathing board.

Or see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION as well as the other siding types listed at page top or at the MORE READING links at the bottom of this article

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