Controlling moisture balance with watertight vapor-open assemblies

Vapor-open insulation

The primary reason to choose vapor-open insulation is to allow for superior drying. Wall assemblies will almost always leak, so plan ahead and use a vapor-open insulation product to facilitate the drying of that moisture when it inevitably happens. But the wall assembly itself and the strategy for vapor permeability will depend on the type of application. The plan will vary if it is a retrofit versus a new construction.

Regardless of the assembly, material selection is critical. Vapor permeable insulation such as stone wool will allow for greater drying than can be achieved with vapor impermeable insulation such as foam plastics: XPS, polyisocyanurate, and spray polyurethane foam insulation. This greater drying ability generally results in improved durability of the wall assembly.

The exterior of Clippership Wharf in Boston incorporated a prefabricated wood frame construction with fire-rated OSB sheathing. This approach, coupled with the city’s climate zone and unique location challenges, called for the use of a vapor permeable air- and water-resistive barrier. No matter how moisture entered the system, through construction or intense wind-driven rain and snow, the team needed to be confident that any incidental moisture could escape and not cause serious damage to the building enclosure. Photo courtesy The Architectural Team and Lendlease

Wall assembly design

Whether the construction is new or a retrofit, there are three typical approaches to insulating walls: all interior cavity insulation, all exterior continuous insulation, or split insulation between the interior and the exterior side of the wall. The selection depends on several factors, such as construction type and climate. Climate will dictate the direction of the vapor drive as well as the dew point and, consequently, where to install the vapor barrier in relation to the insulation. In the south, for example, the priority is preventing moisture from coming in from the exterior; in the north, the opposite is true.

Stud cavity-insulated walls

Exterior rigid insulations may be used as the exterior vapor retarder; however, their permeability is relatively low, which restricts drying to the exterior. A vapor-open exterior insulation, such as stone wool, used in conjunction with a vapor permeable membrane of at least 10 perms, provides a forgiving assembly with increased exterior drying. Additionally, this assembly can tolerate leaks and construction moisture to a certain degree. The 5 to 10 perm (semi-permeable) vapor control can be achieved by selecting an appropriate fluid-applied, self-adhered, or mechanically fastened water control membrane applied over the sheathing or concrete masonry unit (CMU) back-up wall. Alternately, when using plywood or OSB sheathing, a more vapor-open water control layer is permissible, since the plywood or OSB itself provides the vapor control.

In colder climates, where only stud cavity insulation is useable in a building, thermal bridging in walls can increase the risk of condensation by causing increased heat loss, resulting in colder surfaces where condensation may form if the temperature of the surface falls below the dew point. Builders may install continuous insulation on the exterior of a building to reduce this risk.

For warm climates, installing an interior vapor retarder can create a substantial risk of condensation within the wall assembly. A common, unintentional example of this is the use of vinyl wallpaper. The vinyl wallpaper acts as an interior vapor retarder and, in combination with inward vapor drive, can lead to moisture accumulation within the wall assembly. Alternatively, in warm and humid climates where air conditioning is provided, diffusion occurs primarily from the exterior to the interior and an exterior vapor retarder should be provided.

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