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| Essay:insulation as exterior sheathinghttp://www.weccusa.org/wesh/winter2000.htmlApplying principles of good building science, we can dispel some myths about using well-sealed foam sheathing as an air barrier and rainwater drainage plain. First off, it takes a hole and a pressure difference for air to move. Buildings will always experience pressure differences between the interior and the exterior surfaces, but what about holes? Moisture vapor is mostly transported via air movement through holes. If air cannot move through a cavity, very little moisture vapor will move through that cavity, as well. So, by creating a continuous air barrier on the exterior of a building using well-sealed foam sheathing, air and moisture vapor movement will not be a problem. In theory, this is true, but practically speaking, given standard building practices, it is nearly impossible to completely seal the foam well enough to prevent all air movement. So, what do we do?
In our climate, a continuous vapor retarder/air barrier should always be installed on the interior surface of the building.
Then, the sealed foam can still be used as the primary air barrier and drainage plain. This eliminates the need for building paper or house wrap, keeps cavity insulation performing like it should and prevents rainwater penetration into wall cavities.
Another good argument for building with exterior foam sheathing is that by adding insulation to the exterior, wall cavity temperatures will be higher, and the condensing surfaces (sheathing) will be warmer, thereby reducing the potential for any moisture vapor that has entered the cavity from reaching it's dew point (point at which moisture vapor turns to liquid). In older homes, with 2x4 walls and limited insulation, cavity temperatures were naturally higher and condensation was rarely a problem. In new homes, condensation can occur if moisture vapor is allowed to penetrate into wall cavities, and if the dew point temperature is reached. Numerous studies have shown that exterior foam insulation can help to reduce or eliminate this problem. Of course, dew point temperatures within cavities will vary depending upon indoor relative humidity level and exterior temperature. So, what's the best way to control this?
In our climate, new homes should always have ventilation systems installed to control humidity and maintain indoor air quality. Indoor relative humidity should be maintained at no higher than a maximum of 40% in winter months.
As for the moisture that is naturally present in new lumber, as the lumber dries and releases moisture vapor into the cavity, if that moisture vapor is never allowed to reach it's dew point temperature, there will never be a condensation problem. Additionally, wood is not a good vapor retarder (it readily absorbs water and has a perm rating of approximately 17). Top plates will dry into the attic, absorb moisture released by other framing members and again, release that moisture into the attic. It may take longer, but the framing will dry. Of course, you wouldn't want to close up a wall that had been soaked by rain, or was abnormally wet for some other reason.
Finally, exterior foam not only insulates the cavity, but also insulates over the studs. With framing comprising approximately 25% of a typical wall assembly and with wood having an R-value of approximately 1.25 per inch, one-fourth of a building that only has structural sheathing, even with high levels of cavity insulation, will have very limited insulating value. Ina typical new home, windows account for approximately 20% of wall area. Taking this into account, without foam, almost 50% of the walls would have very low insulating values.
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