Insulation
I am working out the final drawings for a double wall house and I know that there can be significant thermal losses at the rim joist so I am trying to do something about that. My plan calls for moving the rim joist in on the mud sill that the floor joist butt up against by 3 ½ inches. This will give me room for 2” of Styrofoam plus the 2×12 on the outside. Basically the 2” of foam is sandwiched between 2 pieces of 2×12. My house is a 2 story so I plan on doing this to the second floor as well. So my question, is this worth doing, can I hope to gain anything by doing it, see any problems with it? Oh there is no foam going on the outside wall so I cannot cover the rim joist that way. Thanks.
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Replies
Paul,
Many builders use this technique -- recessing the rim joist to allow room for the installation of exterior rigid foam. Depending on your load paths, you may not need an exterior 2x12. In some cases, the exterior foam can be covered with a layer of OSB or plywood instead of another 2x12.
Paul,
You need to support both an inner and outer wall while minimizing heat loss. I've found that eliminating the "rim board" and using blocking between the floor 'joist' under the inner wall while extending the joist to hold both walls, followed by insulating the spaces between the joist is simple and relatively thermally prudent. Yes the joist are a wood heat path but the area they occupy is modest . Consider 2x joist on 16" centers means 1.5/16=0.09375 is wood while 14.5/16=.0.90625 is insulation With 10" cellulose (r 38) that's still r32 in the joist area. 24" joist spacing or I joists make it even better. Sometimes simpler is better even if it means accepting a wood " thermal bridge".