Closed-cell spray foam and atypical rim joist area
wiscmarq
| Posted in Energy Efficiency and Durability on
Our basement’s floor joists rest on CMU filled with concrete. Each joist fits perpendicularly into hollow CMU (these unfilled CMU extend to the roof, forming the above-grade exterior wall). There are also short 2x8s, likely serving as blocking, between each floor joist and placed directly against the hollow CMU.
We want to create an air barrier in the rim joist area using closed-cell spray foam. Should we have any concerns about applying that foam directly over the interior 2x8s that cover the hollow CMU?
I’ve attached a photo for reference. The house is one-story, vintage 1950s, and in Climate Zone 5A (northern Illinois). Thank you.
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Applying closed-cell spray foam in the rim joist area is a common and effective method for creating an air barrier and insulating this critical junction in your home. Given the construction details of your 1950s house in Climate Zone 5A (northern Illinois), with floor joists resting on CMU filled with concrete and the presence of 2x8s against the hollow CMU, here are considerations for proceeding with spray foam insulation:
1. Moisture Considerations
Closed-cell spray foam is an excellent moisture barrier, which can prevent air and moisture intrusion. However, ensure that there's no existing moisture or water damage in the CMU or the 2x8s. Applying spray foam over damp materials can trap moisture, leading to potential deterioration over time.
2. Adhesion and Coverage
Closed-cell spray foam adheres well to most construction materials, including wood and CMU. There should be no concern about its application directly over the interior 2x8s that cover the hollow CMU, as it will create a continuous air and thermal barrier. The foam will also fill any gaps or cracks, improving the overall energy efficiency of your basement.
3. Structural Considerations
The 2x8s acting as blocking or bridging contribute to the structural integrity of your floor system by preventing joist rotation and spreading loads evenly. The application of closed-cell foam will not negatively affect these structural components; if anything, it may add a slight increase in rigidity.
4. Ventilation and Drainage
Before insulating, verify that the CMU wall does not require additional ventilation or drainage solutions, especially if the hollow blocks extend above grade. In some cases, ensuring that exterior water management (e.g., gutters, downspouts, and grading) is adequately addressed is crucial to avoid directing water towards the foundation.
5. Professional Application
Given the specific considerations of your home's construction and the importance of creating an effective air barrier without compromising structural elements or trapping moisture, professional application of the spray foam is recommended. A professional installer can ensure the correct thickness and coverage to achieve the desired R-value while considering the nuances of your basement's construction.
6. Local Codes and Recommendations
Check any local building codes or recommendations specific to your climate zone and construction type. There might be specific guidelines or requirements for insulating and creating air barriers in homes of your vintage and construction style.
In summary, applying closed-cell spray foam to the rim joist area, including over the 2x8s against the hollow CMU, is a viable and effective strategy for improving your basement's energy efficiency and comfort. Ensuring the area is dry, structurally sound, and that the application is performed by professionals will address any potential concerns and contribute to a successful insulation project.