Rim Joists Insulation in basement help


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Old 02-13-23, 03:28 PM
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Rim Joists Insulation in basement help

Hello, I don't have any experience with insulation and I am looking to remove my existing fiberglass installation that is on my basement rim joists (cleaning up mold) with that common pink insulation board that is sold at home depot by cutting/fitting it into each rim joist and then sealing it with great stuff. Many examples on youtube of this method so seems pretty common. Question is, home depot as the very large 2" pink insulation board and they also sell smaller pieces of 1" insulation board:

FOAMULAR NGX F-250 2 in. x 4 ft. x 8 ft. SSE R-10 XPS Rigid Foam Board Insulation
FOAMULAR NGX 1 in. x 2 ft. x 2 ft. R-5 Project Panel XPS Rigid Foam Board Insulation

Is it recommended to use the 2" insulation board for basement rim joists or could I use the 1" version? What are the differences between the two? Is one better other than the thickness? Just because I don't own a truck I may need to cut the larger insulation board in the parking lot to get it into my SUV.

Then I plan on putting new fiberglass insulation in from of the sealed insulation board.

Appreciate the help and any other tips.

Thank you!
 
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Old 02-13-23, 07:48 PM
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How much insulation you need depends on your climate zone and local building codes (if that matters to you). As for performance obviously more is better but that first inch makes the biggest improvement. The exposed rim is usually considered about r-2 without added insulation, pretty cold.

Also best practices involves covering rigid foam with something like drywall to limit fire exposure. Mineral wool might do both, add insulation and qualify as a thermal barrier. Again, check local codes.

In the past I have also cut 4x8 material in the parking lot, just plan your cuts to work with what you need to avoid waste and wasting time.

Also, even though the rigid will be sealing those cavities do some caulking in advance to ensure a good air seal.

Bud
 
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Old 02-13-23, 07:58 PM
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Kind of depends on what tools you have. You are going to be paying a premium price for the smaller precut panels.

I'd suggest you apply 2" of foam so that you have R-10... but whether that is 2 layers of 1" or 1 layer of 2" is up to you. It's definitely easier to cut the 1" if you are cutting it by hand.

You might find that the box store would cut 4x8 sheets down to a smaller size on their panel saw for you.
 
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Old 02-14-23, 05:32 AM
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Thank you very much. Couple additional questions as I am new to this type of work:

1. I should caulk before installing the rigid board? What kind if caulk do I get? Perhaps could you send a picture or link of what kind to get?

2. Regarding thermal barrier, is it fine to just put insulation in front of the rigid board? I am in NY so not familiar with requirements.

3. What's the best way to cut the rigid board in for example a parking lot? Do I use a T Square and a razor blade knife, do a couple swipes and try to snap it where I cut it with the razor? Is there a better way?

Thanks!
 
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Old 02-14-23, 06:41 AM
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A utility knife can work on one inch foam board but can't cut through the 2". Snapping it risks a poor edge. Actually a hand saw works well or a battery powered saber saw.
Although the foam board will serve as an air block when sealed some caulking prevents any circulation behind the foam. I often use PLC construction adhesive. I use the big tubes and wrap the tip in foil tape when done to seal it, lasts a long time.
Insulation in front of the foam board would be up to the local authorities. I construction, local authorities have the final say. I like having any fluffy stuff covered to prevent rodent nesting. I've pulled out some nasty fiberglass.
When you fit a piece into the rim space I often drag my knife along the edge at a 45 degree angle to create a valley for the caulk. Then use my finger or other to smooth it in and press it into the edge gap.
Be sure to caulk and seal the sill plate to the foundation as well.
Bud
 
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Old 02-14-23, 07:02 AM
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Thank you Bud for all the help. So my plan would be the following:

1. Pull out old insulation
2. Caulk the gaps around the wood in the rim joists with PLC construction adhesive.
3. Place 2" rigid foam board into the rim joist (measured with about half inch gap around to allow room for great stuff) and then seal it with "great stuff"
4. Place fiberglass insulation in front of the sealed rigid foam board as the final step.

Anything I am missing in these steps or see anything wrong?

Would you recommend using PLC construction adhesive to stick the rigid foam board to the rim joist? Or just place the foam board onto the joist and just use great stuff to seal around it?

Thanks!



 
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Old 02-14-23, 09:55 AM
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Construction adhesive cures slowly so might not hold while you apply the can foam. A piece of wood fit side to side with a screw angled left and right would secure the rigid while the foam sets up.

You could also use a 2.5" screw(s) right through the foam board to hold it in place. After foam has hardened you can remove the screws and seal the hole with a dab of caulk. I use torx wood screws everywhere, they are addicting.

Bud
 
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Old 02-14-23, 03:55 PM
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IMO you are wasting time and material by using spray foam. Plus you will just make a mess. Cut the foam tight enough that it stays in place "friction fit" then caulk the edges as you put each layer in.

I would use PL300 or Liquid Nails Latex construction adhesive.

And if you are using the 1" board, I would probably put 3 layers in (R15) and then skip the fiberglass or rockwool. If you need to cover it for firecode, glue 1/2" drywall over it.
 
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Old 02-14-23, 04:22 PM
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Thank you! Regarding covering the rigid board for firecode. Would placing fiberglass insulation in front of the rigid board be the same standard since it also has a "fire rating"?
 
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Old 02-14-23, 05:44 PM
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Inspectors usually prefer drywall. Isn't going to get ripped or fall out. As Bud said, your local authorities have the final say on that.
 
 

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