In one of the latest moves toward energy conservation, a new building code requires the application of a technique known as “continuous insulation”. This technique involves placing a layer of insulation on the exterior of a building to deal with heat loss that results from thermal transfer at studs as a result of conduction. Ultimately, this technique attempts to improve the overall insulation value of walls.
There are a few issues with this requirement, however, such as determining where the dewpoint exists in the wall system. Continuous insulation frequently uses board insulation, or sheathing, on the outside of a building, which is then covered with siding, brick, stucco etc. The minimum insulation level, or R-value, of this method is about an R-5. However, the energy code requires a better overall wall value than an R-5, meaning that additional insulation is needed.
Architects will often add batt insulation between studs on the inside of a building to achieve the required insulation value. Depending on the assembly, this interior insulation can move the dewpoint closer to the inside, which can create condensation issues in the wall or on the sheathing, and make the selection and location of a vapor retarder critical.
The vapor retarder that you choose must prevent interior moisture from passing into the wall cavity. If the exterior sheathing is a continuous insulation layer with a foil face, then the wall won’t dry to the exterior, and a higher perm vapor retarder may be needed on the interior. This allows the moisture that collects in the walls to dry to the interior, as part of the normal heating and cooling seasons. The selection of insulation is also critical as closed cell foam can function as a vapor retarder, eliminating the need for one altogether. All of these building conditions need to be taken into account when designing a building.
Designers also face these new code requirements, and must meet the NFPA 285 standards for flammability rating. This test of the exterior wall system is challenging because the foam being used as continuous insulation can also fail this test, which mimics flame spread out of windows to the floor above. Your designers may have a limited number of materials that would meet these requirements, and as a result, they may call for more expensive materials and detailing to meet both of these requirements.
Contact Steve Flaten from our Building Sciences group for more information about continuous insulation.