Rigid foam on exterior and interior of rim joist?
walldocmp
| Posted in General Questions on
I have 2″ of Polyiso covering the exterior of my house including the rim joist. I would like to insulate the rim joist and like the idea of keeping the fluffy insulation out of the basement. The DOE shows using rigid foam on both sides of the rim joist. https://foundationhandbook.ornl.gov/handbook/images/Chapter%202/2-11S_no-cap.png
Seems like it could be a bad Idea and trap moisture. What do folks here think?
My second question is about insulating the foundation wall. I have 2″ of xps on the exterior of my poured cement basement wall. Can I used Rigid foam on the interior as well?
Thanks,
Mike
Zone 6 Maine
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Replies
Use EPS on the interior and you preserve some drying ability as EPS is much more vapor open than XPS or foil faced polyiso in comparable thicknesses. You can cut EPS pieces to fit between joists and seal them in place with canned foam (great stuff). Up to 2” thickness is commonly available in box stores, but be sure not to get FACED EPS since the poly facer is a vapor barrier.
You can use rigid foam on the interior of the basement wall too. Some like using some more vapor open like EPS, but in my opinion it doesn’t matter here, provided you have a capillary break between the concrete and the rim joist. If you have a capillary break in place, you can use any type of rigid foam here but be mindful of the fire resistance requirements on the interior which vary based on the type of space and the local jurisdiction. Personally I’d go with polyiso here, reclaimed if I could find it locally.
Bill
That sounds good to me. Thanks Bill.