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Analyzing Wall Assemblies

Selecting an exterior wall assembly might not be one of the most exciting decisions made when designing a new house, but it certainly is one of the most critical.   In this article we present some of our investigations into the thermal performance of some typical, and some less- typical, new single-family home exterior wall assemblies in the Philadelphia area, using WUFI.   These spreadsheets and graphs represent an investigation into four wall types:

  1. Double wythe brick masonry with plaster interior finish (reproducing a typical existing Philadelphia rowhouse)
  2. Wood frame construction with 5 -1/2” of batt insulation in the stud cavity only
  3. Wood frame construction with both 5 -1/2” of cavity batt insulation and exterior rigid insulation
  4. Wood frame construction built to Passivhaus standards

While the original double wythe brick walls of many of Philadelphia’s rowhomes (wall type A) provide a solid structure with both fire resistance and acoustical benefits, they offer very little in terms of thermal insulation.  Figures 1 and 2 show that Wall Type B, typical of much recent new construction (5 ½” of fiberglass batt in the stud cavity) shows a nearly 75% reduction in Thermal Transmission and nearly 80% improvement in R-Value when compared to Type A walls.

While the typical cavity-insulated wall assembly is a vast improvement from the original masonry construction, it’s not ideal.   Insulation in the stud cavity increases the R-Value of the wall, but it doesn’t address the problem of thermal bridging through the studs.   Furthermore, the location of the insulation can create a high temperature differential through the wall components that leaves the exterior OSB/plywood wall sheathing quite cold in the winter months (see Figure 3), making the insulation susceptible to condensation.   Condensation trapped in a wall can reduce an insulative material’s effectiveness and can result in mold growth and rot in the wall if there is no way for that moisture to dry out. The way we typically avoid that is to provide a continuous vapor retarder at the warm side of the insulation, that way, any water vapor that does get into the wall assembly has a means to escape to the outdoors on the cold side of the wall.

Wall type C address this temperature differential issue by adding a layer of rigid insulation attached to the exterior face of the exterior sheathing.   The additional exterior insulation results in a 16% improvement in thermal transmission vs. the insulated-cavity-only wall assembly (type B).   The additional exterior insulation also bumps the average temperature of the interior face of OSB/Plywood sheathing over the 40ºF mark, which is a typical dew point for the Philadelphia area in February.  The WUFI analysis demonstrates that adding insulation outside the wall sheathing improves both thermal performance  and  moisture control in the wall, resulting in a building that’s more cost-effective to heat and cool, and more durable.

The Passivhaus wall assembly takes that approach to a much higher level.  In this assembly, not only is there exterior insulation and cavity insulation, but there is also an insulated interior “service cavity” through which all electrical/plumbing/miscellaneous penetrations are made.   In addition, all seams and joints are thoroughly sealed and taped, and a continuous thermal barrier and air barrier is established at the structural stud cavity.  This keeps all moisture-laden air away from the insulated cavity by eliminating penetrations of the barrier. As a result, as the WUFI analysis shows, a Passivhaus assembly results in a nearly 63% decrease in Thermal Transmission   through the wall and a 50% increase in the overall assembly R-Value compared to wall type C.   The combined increase in R-Value and decrease in Thermal Transmission translate to a more stable temperature through the wall assembly, drastically reducing energy loading.

While this analysis focusses on new construction, the same analysis can be applied to energy retrofits of existing buildings.