EXT and INT insulation in Zone 5 – GreenBuildingAdvisor

The main considerations for the use of exterior and interior insulation combined are:

– the ratio of R-values for the exterior and interior insulation

– the vapor permeance of both and any other materials in the assembly

– the use of an interior vapor and air control layer and its vapor permeance

– indirectly, the use of air conditioning and its ability to chill surfaces below the temperature at which condensation forms

The ratio of R-values for exterior vs interior insulation is used to evaluate if the assembly will sufficiently reduce condensation risk at the interior surface of the exterior sheathing by keeping it warm enough, for a given set of interior and exterior temperature and humidity conditions.

The colder the exterior temps and the higher the interior moisture content, the higher the ratio of exterior R-value to interior R-value is required in order to keep the exterior sheathing warm enough to prevent condensation from forming on its interior surface. There are precalculated tables that suggest R-values for a given set of interior and exterior environmental conditions, and you can also input your own environmental conditions and calculate specific R-values for your use case.

If the interior R-value is high relative to the exterior R-value, then the sheathing will be colder because the interior insulation is preventing heat from warming up the sheathing. But if the exterior R-value is high enough, then it prevents enough heat loss to the environment and keeps the sheathing warmer.

Keeping the interior surface of the exterior sheathing warm also promotes drying of the sheathing, assuming there is a path for that moisture to dry towards either the interior or exterior.

The vapor permeance of both types of insulation is used to evaluate whether or not moisture adjacent to or in between the insulation layers will be able to dry out, either towards the interior, exterior, or both.

If both the interior and exterior insulation have low or no vapor permeance, e.g. interior spray foam with exterior foam board or spray foam, then there is very little ability for moisture that finds its way between these two layers to dry out. If there are any moisture-sensitive materials sandwiched between two layers with low or no vapor permeance, then moisture that finds its way between them will not dry out quickly or at all. An example of this would be using polyethylene on the interior, and interior spray foam, and/or exterior rigid foam or spray foam.

The use of an interior air and vapor control layer affects how much interior moisture can reach cold surfaces towards the exterior, and therefore potentially reduce the risk of condensation occurring on these surfaces. However, it requires that the installation of this layer be nearly perfect, with no air leaks, and that it remain airtight for the service life of the building. If this layer has low or no vapor permeance, it will also inhibit drying.

If air conditioning is used, then attention needs to be paid to whether or not moisture can migrate from the exterior to the interior and meet a cold surface where it may condense and cause issues. If this cold surface is the exterior of the WRB, applied over the outer surface of the exterior sheathing, then there is no issue, because the WRB is designed to shed water and prevent it from contacting moisture sensitive materials. If this cold surface is an interior air and vapor control layer, then it is a problem, because condensation inside the wall cavity will result as long as the air inside contains enough moisture. The use of insulation in these cases is only indirectly related, though, and this is mostly mentioned because the use of interior air and vapor control layers was discussed above.

I don’t recommend the use of polyethylene in any case for the reduced drying effect, meticulous installation required, and potential to create a cold condensing surface when combined with air conditioning.