This section focuses on interior components and finishes from the interior face of framing and inward. These could include vapor retarder membranes, drywall, paneling, wallpaper, plaster, and coatings such as primer, paint and stain.

Interior Finishes

When specifying interior coatings or finishes it's important to consider both volatile organic compounds (VOCs) as well as the product's potential impact on wall assembly drying potential. VOCs are best controlled by minimizing emissions at the source and specifying low- or no-VOC products and coatings. Ventilation will then help reduce VOC levels through dilution.

The recommended wall assembly is designed to dry to the interior and it is essential the interior finish does not inhibit drying. Typical finishes such as wood and drywall, both with most types of paint are permeable enough to allow for drying. Concerns increase when the finishes can be thought of as a plastic coating of sorts, with vinyl wallpaper being a good example. 

Image of painted walls.
Painted walls allow drying to the interior. Photo credit Behr.

The vinyl wallpaper is not permeable and will not allow assemblies to dry out. When this occurs, the wallpaper can start bubbling, drywall can soften, mold and mildew can form in the wall cavity, and the framing and sheathing can rot. 

Vapor Retarders

In walls we are typically looking at using either a semi-permeable class II or permeable class III vapor retarder and rarely an impermeable class I. If a code-specified minimum level of exterior rigid insulation is installed, a class III vapor retarder can be used. A good example of this is latex paint. 

2018 IRC code minimum for class III vapor retarder:

Climate Zone 2 x 4 wall 2 x 6 wall
Marine 4 CI R ≥ 2.5 CI R ≥ 3.75
5 CI R ≥ 5 CI R ≥ 7.5
6 CI R ≥ 7.5 CI R ≥ 11.25
7 CI R ≥ 10 CI R ≥ 15

CI= Continuous Insulation

If the code-specified minimum level of exterior rigid insulation is not installed, then a class II vapor retarder is required. Examples of these are Membrain, Intello, and kraft paper. These are also considered smart vapor retarders as their permeance changes depending on moisture levels. 

Image of intello membrane
Proclima Intello smart vapor retarder. Photo credit 475 High Performance Building Supply.

We generally recommend against class I vapor retarders in walls, such as sheet polyethylene, due to the extreme care and attention to detail required for a successful installation. They can work but are considered riskier than class II and II vapor retarders. They eliminate drying to the interior and the wall must be designed to dry to the exterior. There are also few, if any, compelling advantages to this type of assembly, and given the increased risk, they should be avoided.

Sometimes with ADUs in a mild climate, the designer wants to avoid using exterior rigid foam. In such a case, we recommend using a class II vapor retarder such as Intello which can also serve as netting for dense pack cellulose insulation. 

Image showing moisture transport due to vapor diffusion vs air leak.
Air leaks transfer substantially more moisture than diffusion. Image Credit Building Science Corp.

Lastly, under typical conditions for a residential home, risk of damage due to water vapor is low. Water driven damage is largely due to bulk water and air leaks (air can hold a lot of moisture). We must meet code requirements, but we shouldn’t lose too much sleep over vapor retarders. 


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