OT: Insulating rim joist

SeaOfRed75

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Dec 5, 2010
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In the process of finishing my basement. Before I get too far need to decide whether to insulate the rim joist or not.

Believe it can make the basement more comfortable/energy efficient from what I've read. However I've read conflicting things about what to use and maybe to do it at all. Fiberglass is a no no from what I've read as its vapor permeable and can cause mold and mildew to grow in it.

Froth pack for closed cell foam sounds the best for air sealing and insulation, but Ive read some thougths that this can lead to condensation on the other side of the rim joist leading to rot as it cant ever dry out if any moisture vapor gets in there.

Thoughts from any contractors or all around handy folks?
 

TruHusker

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Sep 21, 2001
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I have done most of my basement as I remodeled it. It had regular insulation and it was fine but not very tight.

I used 2" foam board and cut it so it fits right. You can put two layers if you want. I put sheet rock over that and screwed through the foam into the rim. I also caulked around everything from the inside out. I didn't worry about moisture but if you are, just hold it out with a spacer board and that will give you a dead air space. It made a big difference in my basement and was easy to do. I would figure what you want for R value and go from there.
 

SeaOfRed75

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I have done most of my basement as I remodeled it. It had regular insulation and it was fine but not very tight.

I used 2" foam board and cut it so it fits right. You can put two layers if you want. I put sheet rock over that and screwed through the foam into the rim. I also caulked around everything from the inside out. I didn't worry about moisture but if you are, just hold it out with a spacer board and that will give you a dead air space. It made a big difference in my basement and was easy to do. I would figure what you want for R value and go from there.
Not overly concerned about moisture. Brick home and have at least 12"+ from grade to sill plate all around house. So I would think that would help?

Considering dow froth pack that you can get at HD, Lowes, or Menards. About $250 at most. Would cover what I need and with the spray foam I think you get R value and an air seal. Dont have to cover with drywall either iirc unlike Xps foam.
 

TruHusker

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Not overly concerned about moisture. Brick home and have at least 12"+ from grade to sill plate all around house. So I would think that would help?

Considering dow froth pack that you can get at HD, Lowes, or Menards. About $250 at most. Would cover what I need and with the spray foam I think you get R value and an air seal. Dont have to cover with drywall either iirc unlike Xps foam.

Think about the rim joist. It is a solid piece around the outside so where does the cold come from? You have a dead air space between your brick and sidewalls. The cold is from transfer, not leakage, assuming you have sill deal and even then, not much.

No, you don't have to finish them with sheet rock. I should have said I did an open ceiling look a d so my rim joist show between the floor joists.

I am not sure how you will manage the spray foam. What do you level to or I guess maybe your don't care. Just spray it and it will cover when you out a ceiling in.
 
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Crazyhole

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This is the kind of thing that drives contractors crazy. Not to diminish your concern, but it's just the frigging rim joist. Stuff some Kraft faced r-19 in there and call it good. If there is a chance of mold, you're going to have mold everywhere, not just in the rims.
 

SeaOfRed75

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This is the kind of thing that drives contractors crazy. Not to diminish your concern, but it's just the frigging rim joist. Stuff some Kraft faced r-19 in there and call it good. If there is a chance of mold, you're going to have mold everywhere, not just in the rims.
Thats
Think about the rim joist. It is a solid piece around the outside so where does the cold come from? You have a dead air space between your brick and sidewalls. The cold is from transfer, not leakage, assuming you have sill deal and even then, not much.

No, you don't have to finish them with sheet rock. I should have said I did an open ceiling look a d so my rim joist show between the floor joists.

I am not sure how you will manage the spray foam. What do you level to or I guess maybe your don't care. Just spray it and it will cover when you out a ceiling in.
I meant that from what ive read with spayed in closed cell you dont need a firestop with say drywall. But with foam board like xps i think you do.

And yeah would just cover the spray foam eventually with ceiling.
 

SeaOfRed75

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This is the kind of thing that drives contractors crazy. Not to diminish your concern, but it's just the frigging rim joist. Stuff some Kraft faced r-19 in there and call it good. If there is a chance of mold, you're going to have mold everywhere, not just in the rims.
Im starting to feel the same. Read contradicting things everywhere I turn. Beginning to think screw it and just put in regular insulation there and call it good. Think you might be right on the mold.
Would rock wool be a better option than fiberglass? More expensive I know but would likely reduce chance of mold correct?
 

Crazyhole

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Im starting to feel the same. Read contradicting things everywhere I turn. Beginning to think screw it and just put in regular insulation there and call it good. Think you might be right on the mold.
Would rock wool be a better option than fiberglass? More expensive I know but would likely reduce chance of mold correct?

You aren't going to get fiberglass insulation tight enough to create a mold problem. The issues that people have had with mold is when they've used plastic vapor barriers and made the house too tight. People are acting like icynene is the panacea to mold problems but in reality if we just went back to standard practices from the 1980s we would be fine. It all goes back to try and make a house as energy efficient as possible but the initial expense outweighs the long term benefits and comes with unintended consequences.
 

SeaOfRed75

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You aren't going to get fiberglass insulation tight enough to create a mold problem. The issues that people have had with mold is when they've used plastic vapor barriers and made the house too tight. People are acting like icynene is the panacea to mold problems but in reality if we just went back to standard practices from the 1980s we would be fine. It all goes back to try and make a house as energy efficient as possible but the initial expense outweighs the long term benefits and comes with unintended consequences.
So unfaced batts, kraft faced batts, rock wool?
 

Crazyhole

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So unfaced batts, kraft faced batts, rock wool?
Fiberglass, Kraft faced batts. If you want more r-value then go with r-25 but r-19 is plenty for rim joists.

I'll give this advice with one caveat: if you have floor trusses instead of TJI or dimensional joists then you are better off with spray foam.
 
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Crazyhole

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One other thing: you've mentioned having to put sheetrock over DOW styrofoam. Just to be clear, that isn't because DOW is any more flammable than any other insulation, it's because if there is a fire it puts out toxic gas as it smolders. If you are doing large areas with it then yes, you should definitely cover it with drywall but in an area as small as a joist space I wouldn't sweat it. If the house is on fire to the extent of it creating a problem, the house is gone anyway and you'll be overcome by the smoke anyway if you don't get out.
 
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Jun 5, 2020
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Underestimating the importance of house insulation is the biggest mistake a person can make.
Besides the fact that insulation is essential for maintaining the temperature inside the house, it is also crucial for keeping the walls dry and safe.
I also insulated my park house recently.
There are many reasons why park home insulation is essential, but I am sure that most of you are aware of this.
These aspects are fundamental if you need a solid and warm house in the future.
Finally, I wish you success, stay safe, and have a wonderful day!
 
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HuskerLove1

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I insulated the ceiling when finishing my basement - definitely worth the effort.

One suggestion, if you're using regular rolls of insulation make sure to cover every inch of your body with clothing - that fiberglass is no joke when it gets on your skin :/
 
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dinglefritz

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In the process of finishing my basement. Before I get too far need to decide whether to insulate the rim joist or not.

Believe it can make the basement more comfortable/energy efficient from what I've read. However I've read conflicting things about what to use and maybe to do it at all. Fiberglass is a no no from what I've read as its vapor permeable and can cause mold and mildew to grow in it.

Froth pack for closed cell foam sounds the best for air sealing and insulation, but Ive read some thougths that this can lead to condensation on the other side of the rim joist leading to rot as it cant ever dry out if any moisture vapor gets in there.

Thoughts from any contractors or all around handy folks?
Good heavens who drags up these old threads? BTW, spray foam is the ONLY way to go.
 

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All time great under rated game...just look at those amazing graphics!!!!
 
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