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Insulating rim joists

9 years ago

Hello,
I have an old home that has never had insulation in the rim joists.. i just caulked every joist with regular caulk, and a couple bigger openings with spray foam.. also caulked the sill plates.
Everyone mentions rigid foam and then spray foam for the joist.. alot of work.. Why can't I just use batt insulation ? unfinished basement., if the joists have been caulked, can i not just put batt insulation in there and leave it?

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Comments (6)

rcs1
9 years ago
Hi davemax. I am going to do the same think except I am going to use Great Stuff and xps rigid foam. The batt insulation will cause mold to grow it is not a good barrier. In some states by code batt insulation is not recommended.
My other thought is to have the areas professionally spray foamed for a more secure insulation, but I am concerned about off gassing.
davemax
Original Author
9 years ago
Here is a response to the same question to Owens Corning: any thoughts?

We would recommend using unfaced cut-to-fit pieces of insulation and place them snugly into the space.

Fiberglass insulation does not contain water or a food source. Mold requires a food source, warmth, and moister to grow, none of which is provided by the insulation.

Best Regards,
Jill Laskey

Owens Corning Get Pink Team
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Winnipeg, cold? A bit. But every time I've visited, locals assured me "it's a dry cold you hardly notice--until your nose falls off." ** I'd use foam board as it serves as an air barrier and can likely be fitted tighter than rockwool, which would require an air barrier to be effective. However, I'd consider XPS rather than EPS, as it has a higher R Value per inch. The National Building Code, which serves as Manitoba's building code, requires a minimum whole wall R Value of R23.5 for abovegrade walls. Some of the diy type cannister foams could be used instead. I can't vouch for their ease of use. Here is a link that might be useful: Martin Holladay on Insulating Rim Joists This post was edited by worthy on Sat, Sep 20, 14 at 7:24
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I build in what may be a similar climate. (Please note that gardening zones are not climate or hygrothermal zones. See International Energy Conservation Code (IECC) energy zone map below.) I only use high-density spray foam on the rims. But the way you describe your retrofit, the combination of Roxul with sealing may be more appropriate if you see a need for accessing any of the pipes that would be buried in the foam. (I can't imagine why you would need to access the wiring.) Unlike fiberglass, on which mould grows when wet, Roxul will not wick water and does not have to be replaced after heavy wetting. However, unless great effort is put into cutting the insulation to fit, there will be gaps that reduce the R value. Whatever method you use, you might want to meet IECC minimums. Beside referring to the table above, you can use the map on this link to find IECC county-by-county insulation requirements. If this is an unfinished basement, termite shelter tubes will be visible on the foundation walls inside; and, for the same reason, be sure the foundation is exposed on the exterior. There's no particular moisture problem. Sources of ignition will have to be shut off during application. Don't forget the water heater. This post was edited by worthy on Wed, Oct 22, 14 at 0:02
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Hello, Many thanks, as always, for the speedy replies. I'm afraid, though, that I probably wasn't as precise as I should have been. My fault. I fully expect to use XPS between the joists perpendicular to the wall, cut to fit, and sealed with caulk or Great Stuff. No need to use the spray foam as they are doing in the pictures. What I was wondering about is what I should use to fill the space between the joist that runs _parallel_ to the cinderblock wall and the cinderblock wall itself. In my original picture, it's that dark gap that starts in the center at the top of the photo and then "runs" away from you toward the bottom of the photo. That gap is the entire height and length of the joist (but the gap is not even and there are various random mortar protrusions, some of which you can see in the picture). Should I use Great Stuff to fill that void, before I start adhering my XPS sheets below that? If not, what should I put up in that void (because I don't have a rim joist)? Thank you very much! Tiger
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mightyanvil is correct. While foam insulation is usually a great way to insulate crawlspaces, building codes prohibit foam insulation from being used underground in areas of known termite activity.
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rcs1
9 years ago
I understand what they are saying, but...I have done a lot of research on this and including on these forums rigid foam seems to be the way people are going with this. You can check out some great videos in you tube also. Can we know for sure our basements have no humidity or moisture issues? I know I have to run a dehumidifier in mine in the summer, but over all it is pretty dry. The other issue some folks have raised is even with the rigid foam you have to make sure it is pretty tight so that moisture does not build up behind it and cause rotting of the rim joists. So much to think about. That is why I may just go with the spray foam despite the cost. I have some pretty big holes where the vents are going through the floor. I can feel the cold on the floor in my living room and dinning room. If you google rim joist insulation you will find a lot of information and videos.
worthy
9 years ago
I remember when cigarette makers advertised their products on the recommendations of doctors.

The fiberglass does nothing to stop airflow and condensation so mould still grows on the rim joist wood.

See Building Science Corp. for the recommended methods of insulating rim joists.
davemax
Original Author
9 years ago
Moisture can still build up behind the rigid board in the joist as well and result in mold on the joist. and if the joist is caulked sufficiently prior to putting any insulation in there, that could eliminate the air flow.
rcs1
9 years ago
Yes davemax, that is why I am considering just doing the professional spray foam only and not the rigid foam + great stuff.

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