Rim Joist Insulation and fireblocking
My home is in Climate Zone 6 (Northern Iowa). I am insulating my rim joist areas similar to the first detail shown in “Three Ways to Insulate a Basement Wall” and GBA Detail 9-00101 “Existing Foundation Wall Insulation Retrofit 2 X 4 Wall With 2″ Rigid Insulation”. One difference is that my stud wall has about a 1″ gap between the stud wall and the rigid foam rather than tight against the rigid foam.
In my case, where the joists are perpendicular to the wall, there is about 2″ to 2 1/2″ of sill plate not covered by the rim board and the bottom edges of 2-2″ pieces of rigid foam insulation fastened against the rim board. Someone told me that the sill plate that remained uncovered would still be a thermal bridge and they recommended covering it with polyisocyanurate rigid foam. Neither of the previously mentioned details show rigid foam on top of the remaining uncovered surface of the sill plate. Detail 9-00101 does show it being covered by cavity insulation which I presume would be either fiberglass of mineral wool.
I would prefer to simply place 1/2″ drywall fire-block on top of the portion of the sill plate that is not covered, carry it across to the top of the top plate of the 2 X 4 stud wall, and use drywall screws to fasten it on both ends. I would then place a 3″ or 6″ batt of either fiberglass or mineral wool on top of the 1/2″ drywall fireblocking and against the vertical 2-2″ pieces of rigid foam in the rim joist area. Does that seem acceptable? I just want to make sure it doesn’t cause any condensation problems.
It has been mentioned in blogs and posts that rigid foam in the rim joist area is to be covered by drywall or plywood due to the flammability of rigid foam. Is that only if it is open to a room? Would the drywall or plywood not be needed if the basement stud walls and ceiling joists are covered with 1/2″ drywall, such that heat or flames from the room would not directly access the rigid foam in the rim joist area? The previously mentioned details do not show any drywall or plywood covering the face of the rigid foam in the rim joist area.
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Replies
Addressing the insulation of the rim joist areas in your home in Climate Zone 6 (Northern Iowa) involves careful consideration of thermal bridging, fire safety, and moisture management. Your approach and the concerns you've raised highlight important aspects of a comprehensive insulation strategy.
Addressing Thermal Bridging
Covering the sill plate with polyisocyanurate rigid foam is recommended to mitigate thermal bridging. This step is crucial because the sill plate, if left uncovered, can indeed act as a thermal bridge, allowing heat to bypass the insulation. Adding rigid foam insulation atop the sill plate will significantly improve the overall thermal performance of your foundation's insulation system.
Proposed Solution with Drywall Fire Block
Your idea to place 1/2" drywall as a fire block on top of the portion of the sill plate that is not covered and extending it to the top plate of the 2x4 stud wall is generally acceptable. This method would provide a fire-resistant barrier, which is a critical consideration when working with foam insulation. However, ensuring that this assembly does not create a surface where moisture can condense is important. Using fiberglass or mineral wool batt on top of the drywall against the vertical rigid foam can work, provided that:
The overall assembly allows for adequate drying to the interior, minimizing the risk of condensation.
The rigid foam insulation used is impermeable enough to prevent moisture from migrating inward from the exterior.
Fire Safety Concerns
Regarding the covering of rigid foam with drywall or plywood due to flammability concerns, the requirement typically applies to exposed insulation in living spaces or areas accessible to occupants. If the basement walls and ceiling joists are already covered with 1/2" drywall, creating a continuous barrier between the living space and the insulated rim joist area, additional covering over the rigid foam specifically in the rim joist area may not be strictly necessary from a fire safety perspective. However, local building codes and best practices should be consulted to ensure compliance and safety. The primary goal is to ensure that no exposed foam insulation is directly accessible within any habitable or storage areas.
Conclusion
Your approach is sensible, focusing on both thermal efficiency and fire safety. Incorporating rigid foam to cover all parts of the sill plate to prevent thermal bridging, using drywall as a fire block, and ensuring the entire assembly is safe and complies with local building codes are key steps. It's also advisable to consult with a local building inspector or a building science professional to confirm that your specific assembly meets all necessary requirements and to ensure that it won't lead to any unintended moisture issues. Your attention to both insulation effectiveness and safety considerations is commendable and will contribute to the overall performance and longevity of your home's insulation system.