Controlling moisture balance with watertight vapor-open assemblies

Exterior-insulated walls

Instead of insulating walls within the stud cavity, exterior continuous insulation can be used in any climate, and this approach to insulating wall assemblies can also be effective for concrete or CMU walls. In these walls, insulation goes on the exterior of the wall back-up.

Exterior continuous insulation is an effective way of achieving highly insulated wall assemblies and can substantially reduce thermal bridging. In addition to these thermal benefits, exterior insulation can also provide a robust assembly with respect to vapor diffusion and condensation.

The use of exterior continuous insulation changes the temperature profile through the wall assembly and, consequently, the back-up wall—be it sheathed steel stud, concrete, or CMU—is maintained at a temperature relatively close to interior conditions. Additionally, since a water-resistive barrier is applied to the back-up wall behind the insulation in these assemblies, moisture-sensitive materials are generally all located on the interior side of the insulation and water-resistive barrier where they remain both warm and dry, resulting in a very durable wall assembly in all climates.

Split-insulated walls

In some cases, it can be advantageous to use split-insulated wall assemblies; that is, assemblies where insulation is provided both in the stud cavity and on the exterior of the sheathing. Usually these are used as they can provide the necessary R-value in a relatively compact (i.e. thinner) assembly. As one might guess, these walls essentially provide a mixture of the performance of stud cavity insulated and exterior insulated wall assembly.

Given that split-insulated walls fall somewhere between stud cavity-insulated walls and exterior-insulated walls with respect to performance, there are important considerations to make with respect to vapor diffusion. Generally, the exterior insulation will be enough to keep the sheathing closer to interior conditions, but if it is a relatively small fraction of the overall insulation in the wall, the sheathing will remain at temperatures close to the exterior. In this situation, the vapor permeability of the insulation should be considered.

When vapor permeable stone wool insulation goes outboard of the sheathing, it has the effect of warming the stud space and exterior sheathing—the more exterior insulation, the warmer the cavity and sheathing. An interior or exterior Class I vapor retarder may not be required; a Class II or III vapor retarder may be necessary to prevent condensation or high relative humidity (RH) levels from occurring. This will depend on the thickness of exterior insulation and the vapor pressure gradient (expected interior and exterior conditions). It ultimately depends on ensuring a proper ratio of exterior air permeable insulation R-value to total insulation R-value. For moderately cold climates and most indoor conditions within commercial buildings, the installation of a thin layer of stone wool outboard of an insulated (152 mm [6 in.]) stud wall is sufficient to ensure good performance when applying Class III vapor retarder (e.g. latex paint) on the drywall’s interior. Because wood is hygroscopic, keeping RH in the wall low is critical to preventing mold and decay.

Detailing and roof assemblies

In flat commercial roofs, vapor-open insulation does not provide any value in relation to vapor control, as they will always require a roofing membrane which is vapor permeable, but do not disqualify it as a good choice in this application, as stone wool insulation provides several other benefits. In fact, in comparison to other roofing insulation materials, stone wool’s complete performance (i.e. thermal stability [the R-value of some foam insulation will decrease over time due to blowing agent off-gassing], dimensional stability [e.g. foam shrinks, creating gap], acoustic performance, and fire resistance) makes it an ideal solution.

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