The Importance of Considering Dew Point When Increasing Insulation in Existing Buildings
Many of Nova’s clients seek our help in retrofitting existing buildings, with the goal of reducing utility expenses. Often, we are asked to evaluate the impact of additional insulation on an exterior wall. This is a tricky subject to navigate and depends on a variety of factors.
From Building Science Corporation
In International Energy Conservation Code (IECC) Zones 5 and higher, retrofitting an exterior wall with additional insulation carries a risk of moving the dew point within the wall. This can create a condition in which moisture is retained within the wall and cannot evaporate. Often, the result is wet insulation, rotting framing members, mold problems, and associated indoor air quality risks. With careful planning, these risks can be mitigated.
While the theoretical dew point in a wall can be calculated, as a practical matter, moisture will condense on the interior of the wall sheathing, provided the surface is cold enough. To prevent this condition, the assembly must include adequate insulation on the exterior of the sheathing to maintain a warm surface temperature. Keeping the sheathing warm prevents it from serving as the condensing surface.
The general wisdom with respect to exterior insulation is “more is better.” Retrofit an exterior wall with too little rigid insulation and you run the risk of holding moisture in the assembly while not providing adequate insulation. However, an under-insulated exterior wall without a vapor barrier (rigid foam) will not have as strong a propensity to retain moisture.
The following are the two most current standards with regard to interior/exterior insulation ratios for preventing condensation on the interior of sheathing:
Building Science Corporation's Adaptation
Building Science Corp’s adaptation of the IBC and IRC as presented in “Flow Through Assemblies”, ASHRAE Journal, December 2015
According to Building Science Corporation’s adaptation of the International Building Code (IBC) and International Residential Code (IRC), the minimum continuous insulation to cavity insulation ratio in Zone 6 is 0.5, or 50%.
Stated another way, 50% of the assembly’s insulation should be located on the exterior of the sheathing.
Martin Holladay's Adaptation
Martin Holladay’s adaptation of the IBC and IRC as presented in “Calculating the Minimum Thickness of Rigid Foam Sheathing.”
Another reference is GreenBuildingAdvisor.com’s Senior Editor Martin Holladay’s adaptation of the same codes. According to Holladay, the subject property should be constructed with at least 36% of the insulation value on the exterior of the sheathing in Zone 6.
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Author: Andrew Calise, BPI Multifamily Energy Efficiency Group Manager Nova Consulting Group Inc. (207) 939-8974 firstname.lastname@example.org