Specify Finished Basement Assemblies Which Manage Moisture

The below-grade living space created by finishing a basement space creates a number of challenges: moisture concerns from below the slab and through the exterior walls, possible radon contamination, newer more stringent code-required insulation levels, and creating assemblies which can provide comfortable spaces which stay dry. This is another location where ID&C becomes paramount as basements have a higher probability for issues to arise.

Finished basements in new construction that are insulated on the exterior of the foundation wall offer a number of advantages. From a moisture control perspective, this type of assembly creates a warmer surface temperature of the foundation wall. As a result the foundation wall is less prone to condensation, and doesn’t need to be covered up with interior-facing insulation. This gives the assembly to the ability to dry towards the interior, allowing moisture from initial curing of the concrete or other sources to escape.

This approach is infrequently used however, due to challenges with integrating the exterior foam with the above-grade wall, protecting the above-grade portion of the foam over the long-term, and other challenges.

Instead, if we assume that the foundation walls have exterior waterproofing (but not insulation), then the inside of the foundation walls should be insulated and designed to dry to the interior. A set of recommendations to accomplish these goals is shown in Figure 3–4. Note that this assembly includes redundant moisture management strategies to increase the robustness of the assembly. In some cases a particular method or material could be substituted and work acceptably (e.g., use of an impermeable floor covering material), assuming that the related systems (e.g., capillary breaks and foundation drainage system) are installed and work well. In the interest of durability it’s better to use a belt and suspenders approach whenever possible.

Key elements of the basement wall assembly shown in Figure 3–4 include:

• As indicated by the blue drying arrows in the figure, this assembly is designed to allow any moisture accumulation to dry to the home’s interior. As such, avoid the use of a Class I vapor retarder like poly in the framed section of the wall.
• The layer of rigid foam insulation (typically EPS or XPS) against the foundation wall is a critical layer in the assembly. This layer is continuous, the foam is air impermeable and the joints are taped or sealed to prevent air leakage, and it keeps the cold foundation wall isolated to prevent condensation. In the case that the basement will not be finished immediately, keep in mind that exposed foam must meet code requirements for thermal protection (IRC R316.4).
• Prevent indoor air (and the water vapor it holds) from leaking into the wall assembly by using thenon-paper-faced gypsum board (increased moisture/mold resistance) layer as an air barrier. Sealthe drywall to the studs and top/bottom plates, and caulk any penetrations. Don’t forget to seal atthe ceiling level as well.
• Create a separation layer from the basement slab to the wall finishes. Moisture will wick intogypsum, wood, or other materials in direct contact with the slab so hold finishes up about ½” fromthe slab surface. Use sill seal or a similar product to create a thermal and capillary break betweenthe slab and the bottom plate.
• To minimize the food source for mold, consider using cold formed metal framing and fiberglassfaced gypsum board for basement finishes.
• To allow the basement slab to dry upward towards the homes interior (because it has a vaporbarrier below), allow the slab to dry before finishes are applied. Flooring product manufacturersoffer guidance on acceptable moisture levels. Further, ensure that floor finishes do NOT create asecond vapor barrier on the top side of the slab, because there is already one below and thebasement needs to be able to dry to the INSIDE. Higher permeance finishes (e.g., carpet andpadding above 1 perm) provide increased drying potential to the inside.