insulating rim joists pockets

Old 12-24-18, 09:05 AM
Thread Starter
Join Date: May 2009
Location: central
Posts: 57
Upvotes: 0
Received 0 Upvotes on 0 Posts
Cool insulating rim joists pockets

A merry Christmas to everyone. Have a question about insulating my rim joists in basement. This may be a bit long so bear with me. We live in Michigan and its cold now even without the snow. We have a ranch with a full basement and craw space. Have work shop and family room downstairs, it seems to get chilly after awhile being downstairs. I have put fiberglass in much of the joist pockets and basement walls are not insulated, I have a few vents cut into side of heat duct but that is all. Even so it seems to stay in the 65 degree area. Have been ready and looking online and find that fiberglass is not that good, lots of people say to use spray foam....would love to but I have not won the lottery yet. Many also say to use foam board to fill pocket and then spray expanding foam to fill gaps, still a bit pricey but do able. I know I will have to spray foam on the section where the sill plate sits on the concrete wall (for basement). I do know I will have to put a piece of R14 ROXUL over foam board for fire break. If anyone has any ideas, feel free to give them....always listening.... Thanks all
Old 12-24-18, 01:18 PM
Marq1's Avatar
Join Date: Sep 2016
Location: USA MI
Posts: 9,745
Received 1,211 Upvotes on 1,099 Posts
The spray foam and ridged foam are primarily there is stop air infiltration. If you had air tight rim joists the type of insulation would not make any difference!
Old 12-25-18, 11:51 PM
Join Date: Oct 2008
Location: New England
Posts: 9,460
Received 47 Upvotes on 43 Posts
Hi homes,
As Marq1 mentions air sealing is a high priority. A joint between two rim boards hidden behind a floor joist can leak a lot of air. Best to picture frame all cavities and seal all joints plus rim to foundation.

Fiberglass insulation fails as an air barrier resulting in warm moist air reaching a cold rim. That is where a layer of rigid comes in to be sure the first inside surface any air can reach is above the dew point. There is a balance between fiber r-value and rigid r-value that determines that condensation potential. I can dig out some guidance on that for your climate region if you go that direction. You mention Roxul, it would work better than fiberglass.

As for the walls, check local code requirements but from a practical point of view ($$$) I emphasize insulating the above grade foundation area plus one or two feet below grade. Below that the soil is not as cold. But the same condensation concerns exist as mentioned above.


Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: