Insulating Rim Joists
I am in the process of finishing my basement. My house was built in 2006 and is located in northern Iowa (climate zone 6). The basement walls are 8” concrete with Owens Corning Fold Form insulated concrete forms that have 2” of rigid foam insulation on the inside and outside of the walI. I have placed 2-2” pieces of XPS rigid foam, sealed around the rigid foam perimeter with Great Stuff spray foam, in the rim joist areas of the front and back of the house where the I-joists run perpendicular to the wall. At the ends of the house where the I-joists run parallel to the wall, I have placed 2-2” pieces of rigid foam against the I-joist that sits on the sill plate adjacent to the rim board. At all 4 walls, I have placed a bead of adhesive caulk along the junction of the interior edge of the sill plate and the top of the concrete wall. I have since become aware of fireblocking and a recommendation to fully insulate the top of the concrete wall. Based on that, the following paragraphs explain the remaining work I plan to do. I would like to know if the GBA Pros have any comments or concerns with what what I have already done and what I plan to do going forward.The attachments show drawings of the sill plate, I-joists, and rigid foam insulation, one for where the I-joists are perpendicular to the wall, and one where they are parallel to the wall.
For the walls where the I-joists run perpendicular to the walls, I plan to place 1-½” XPS rigid foam on top of the portion of the concrete wall that is not covered by the sill plate. For the ends of the house where the I-joists run parallel to the wall, the 2-2” pieces of XPS rigid foam cover all of the sill plate to the extent that there is nothing left to rest the ½” fireblocking sheet rock on. Therefore I plan to cover the exposed top of the concrete wall with ¼” sill sealer plus 1” thick treated lumber fastened to the existing sill plate. That will allow the ½” fireblocking sheetrock to be fastened on top of the new 1” thick treated lumber and then span across to be fastened on the underside of the nearest interior I-joist. There will be a 2×4 stud wall directly under that nearest I-joist. There will be a 1” to 2” space between the 2” insulated concrete form. rigid foam insulation. and the near edge of the studs.
An issue with the end walls where the I-joists run parallel to the wall is that the sill plate is tipped such that on the interior side of the sill plate there is a gap between the top of the concrete wall and the bottom edge of the sill plate. The gap varies from ¼” to ⅝”. I placed a bead of adhesive caulk along some of this gap and Great Stuff spray foam in some areas where the gap is larger. The reason the sill plate is tipped is that the contractor who built the house did not have the nuts for the anchor bolts snugged down to the sill plate. Also, the 1-⅛” thick rim board sits on the outside edge of the sill plate and the outside 2” of the sill plate sits above the 2” of the exterior rigid foam insulated concrete form. I have consulted with a structural engineer who recommended removing enough of the top of the 2” exterior rigid foam insulation to place a treated 2×6, anchored to the concrete wall, to support the outside edge of the sill plate. In my drawing I have shown the treated 2×6 on the outside of the concrete wall with ½” of XPS rigid foam on the outside of the 2×6. I am wondering if it would be better to use the ½” XPS rigid foam or ½” treated plywood on the outside of the 2×6. Either way, I am thinking I should seal the joint between the 2×6 / 1/2” XPS rigid foam or 2 X 6 / ½” treated plywood, and the 2” rigid foam insulated concrete form with caulk or Great Stuff spray foam.
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