To avoid these ads, REGISTER NOW!

Basement rigid foam insulation question

B

BWWgarage

Well-known member
Joined
Feb 9, 2023
Messages
342
Hi. I’m looking to finish out my basement in NW suburbs of Chicago. I bought 2” rigid foam board insulation for exterior walls. The foam board is higher than the cement walls. My gut says I should cut to fit the cement wall height and then cut in addition sections on the framing.

What would you guys do?

Thanks!

​

 

Attachments

  • IMG_7026.jpeg
    IMG_7026.jpeg
    709 KB · Views: 153
To avoid these ads, REGISTER NOW!
G

gizardlizard

Well-known member
Joined
Aug 29, 2019
Messages
731
Location
Madison, WI
The rim joist is the bigger issue and air leaker. Cut 2” of styrofoam to closely fit the rim joist. You need to remove all the fiberglass batting first. Using spray foam, spray all the gaps in the rim joist area and while still wet, press the close fitting styrofoam into it. Then, spray around the styrofoam again. When dry, take your fiberglass batting you removed and put over the styrofoam. The styrofoam will be your vapor barrier. Be sure it’s the pink stuff. Owen’s Corning is the most commonly used here in Wisconsin. As far as your foundation goes, I’d be using fiberglass batting after you frame the foundation. Better R value than the foam board and cheaper.
 
OP
B

BWWgarage

Well-known member
Joined
Feb 9, 2023
Messages
342
loganb said:
How are you planning on finishing/covering it and attaching to the wall?
Click to expand...
Hi. Thanks for the question. Foam adhesive to the wall. Framing over it with drywall. I don’t think I need to insulate the framing further.
 
OP
B

BWWgarage

Well-known member
Joined
Feb 9, 2023
Messages
342
gizardlizard said:
The rim joist is the bigger issue and air leaker. Cut 2” of styrofoam to closely fit the rim joist. You need to remove all the fiberglass batting first. Using spray foam, spray all the gaps in the rim joist area and while still wet, press the close fitting styrofoam into it. Then, spray around the styrofoam again. When dry, take your fiberglass batting you removed and put over the styrofoam. The styrofoam will be your vapor barrier. Be sure it’s the pink stuff. Owen’s Corning is the most commonly used here in Wisconsin. As far as your foundation goes, I’d be using fiberglass batting after you frame the foundation. Better R value than the foam board and cheaper.
Click to expand...
Thanks for the response. I was just watching some YouTube videos that showed similar and makes a lot of sense. Thanks!
 
A

allinon72

Well-known member
Joined
Jul 5, 2010
Messages
3,307
Location
Indianapolis
gizardlizard said:
The rim joist is the bigger issue and air leaker. Cut 2” of styrofoam to closely fit the rim joist. You need to remove all the fiberglass batting first. Using spray foam, spray all the gaps in the rim joist area and while still wet, press the close fitting styrofoam into it. Then, spray around the styrofoam again. When dry, take your fiberglass batting you removed and put over the styrofoam. The styrofoam will be your vapor barrier. Be sure it’s the pink stuff. Owen’s Corning is the most commonly used here in Wisconsin. As far as your foundation goes, I’d be using fiberglass batting after you frame the foundation. Better R value than the foam board and cheaper.
Click to expand...

Do this. This very discussion, among others, is how I landed on just spray foaming my basement. The up front cost is higher but let me tell you, I have no regrets on comfort now that it's a finished space.
 
OP
B

BWWgarage

Well-known member
Joined
Feb 9, 2023
Messages
342
I think I’m going to cut the insulation panels to bottom of plate for a tighter fit to foundation.
 

Attachments

  • IMG_7027.jpeg
    IMG_7027.jpeg
    824 KB · Views: 137
B

billconner

Well-known member
Joined
Jul 20, 2021
Messages
6,971
Location
Thousand Islands NYS
Any suggestions for the rim joist parallel to joists? It's a 12" icf basement wall with drywall and the gap is very small - can't get my hand in. Suggestions other than spray foam contractor.
 
H

Hohn

Well-known member
Joined
Aug 25, 2016
Messages
2,719
Location
Diesel Central, Indiana
gizardlizard said:
The rim joist is the bigger issue and air leaker. Cut 2” of styrofoam to closely fit the rim joist. You need to remove all the fiberglass batting first. Using spray foam, spray all the gaps in the rim joist area and while still wet, press the close fitting styrofoam into it. Then, spray around the styrofoam again. When dry, take your fiberglass batting you removed and put over the styrofoam. The styrofoam will be your vapor barrier. Be sure it’s the pink stuff. Owen’s Corning is the most commonly used here in Wisconsin. As far as your foundation goes, I’d be using fiberglass batting after you frame the foundation. Better R value than the foam board and cheaper.
Click to expand...
This guy knows^^.

You need to seal out air and vapor from your rim joist your you will have mold because that rim joist is so cold. You want to keep warmer (wetter) interior air from contacting it. Hence the strategy recommended above.

You can just fill the whole thing with commercial CLOSED CELL spray foam (which is also a vapor barrier if over 2” thick).

What I did to save time was I cut pink board (2”) up the height of the concrete and lined the basement with it all sealed together with tyvek tape. Then I hired a spray foam guy to come in a fill the rim joists and glue it into the top of the pink boards.

I then framed the basement out with full 2x4 stud framing inside the pink boards. I lost a small amount of space, but since my foundation walls were far from flat, this was the easy way to get straight walls that my drywall guy wouldn’t kill me over.
 
F

finn

Well-known member
Joined
Mar 27, 2005
Messages
16,370
Location
The UP, God's country
gizardlizard said:
The rim joist is the bigger issue and air leaker. Cut 2” of styrofoam to closely fit the rim joist. You need to remove all the fiberglass batting first. Using spray foam, spray all the gaps in the rim joist area and while still wet, press the close fitting styrofoam into it. Then, spray around the styrofoam again. When dry, take your fiberglass batting you removed and put over the styrofoam. The styrofoam will be your vapor barrier. Be sure it’s the pink stuff. Owen’s Corning is the most commonly used here in Wisconsin. As far as your foundation goes, I’d be using fiberglass batting after you frame the foundation. Better R value than the foam board and cheaper.
Click to expand...
The fiberglass against the concrete tends below grade to smell musty over the years. I wouldn’t use it below grade, since it can’t breathe.
 
To avoid these ads, REGISTER NOW!
F

finn

Well-known member
Joined
Mar 27, 2005
Messages
16,370
Location
The UP, God's country
Hohn said:
This guy knows^^.

You need to seal out air and vapor from your rim joist your you will have mold because that rim joist is so cold. You want to keep warmer (wetter) interior air from contacting it. Hence the strategy recommended above.

You can just fill the whole thing with commercial CLOSED CELL spray foam (which is also a vapor barrier if over 2” thick).

What I did to save time was I cut pink board (2”) up the height of the concrete and lined the basement with it all sealed together with tyvek tape. Then I hired a spray foam guy to come in a fill the rim joists and glue it into the top of the pink boards.

I then framed the basement out with full 2x4 stud framing inside the pink boards. I lost a small amount of space, but since my foundation walls were far from flat, this was the easy way to get straight walls that my drywall guy wouldn’t kill me over.
Click to expand...
We did something similar. Used the small pieces of leftover foam board in the rim joist crevice, then had the spray foam guys foam right over that.

Got rid of those cutoffs, and a little better R value when we were done.
 
C

coldh2o

Well-known member
Joined
May 21, 2013
Messages
1,442
Location
Ontario, Canada
Hohn said:
This guy knows^^.

You need to seal out air and vapor from your rim joist your you will have mold because that rim joist is so cold. You want to keep warmer (wetter) interior air from contacting it. Hence the strategy recommended above.

You can just fill the whole thing with commercial CLOSED CELL spray foam (which is also a vapor barrier if over 2” thick).

What I did to save time was I cut pink board (2”) up the height of the concrete and lined the basement with it all sealed together with tyvek tape. Then I hired a spray foam guy to come in a fill the rim joists and glue it into the top of the pink boards.

I then framed the basement out with full 2x4 stud framing inside the pink boards. I lost a small amount of space, but since my foundation walls were far from flat, this was the easy way to get straight walls that my drywall guy wouldn’t kill me over.
Click to expand...

This is exactly what I did. Cutting, fitting, foaming board in the rim joist cavities will be more frustrating than wallpapering with your wife.

Once I framed out the walls I also filled them with rockwool, for additional insulating. I did not add a vapor barrier on the inside of the rockwool, since the foam board acts as a vapor barrier and I did not want to trap moisture in the rockwool.
 
H

Hohn

Well-known member
Joined
Aug 25, 2016
Messages
2,719
Location
Diesel Central, Indiana
coldh2o said:
This is exactly what I did. Cutting, fitting, foaming board in the rim joist cavities will be more frustrating than wallpapering with your wife.

Once I framed out the walls I also filled them with rockwool, for additional insulating. I did not add a vapor barrier on the inside of the rockwool, since the foam board acts as a vapor barrier and I did not want to trap moisture in the rockwool.
Click to expand...
I did rockwool in a couple places (bathroom) where the sound insulation is desirable. The rest is kraft faced fiberglass.
 
H

Hohn

Well-known member
Joined
Aug 25, 2016
Messages
2,719
Location
Diesel Central, Indiana
billconner said:
Any suggestions for the rim joist parallel to joists? It's a 12" icf basement wall with drywall and the gap is very small - can't get my hand in. Suggestions other than spray foam contractor.
Click to expand...
Personally if the gap is that small, I’d seal that gap and treat that joist like it’s the rim joist. The important thing is to put a barrier against warm moist air reaching the band joist. If that barrler is on the band joist, it works. But if it’s one joist over, it will still work.

ICF foam should be closed cell. So sealing at the tight gap (tyvek tape?) and then sprayfoaming over the top should work well. You might need to trim back some drywall to get a good seal to the ICF itself.

Always remember the heirarchy of home sealing, which is in this order:
1) water
2) Air
3) Vapor.

There’s no point sealing out vapor if you haven’t first sealed out air that carries vapor. And there’s no point in sealing out the air if you haven’t already addressed the bigger issue of water sealing.
 
G

gizardlizard

Well-known member
Joined
Aug 29, 2019
Messages
731
Location
Madison, WI
A musty smelling basement has zero to do with fiberglass insulation and all
To do with lack of moisture control (Dehumidification) which will happen if the basement is finished or not.
 
F

finn

Well-known member
Joined
Mar 27, 2005
Messages
16,370
Location
The UP, God's country
gizardlizard said:
A musty smelling basement has zero to do with fiberglass insulation and all
To do with lack of moisture control (Dehumidification) which will happen if the basement is finished or not.
Click to expand...
******** on his one.

Fiberglass insulation in a confined space, against a cold wall that’s thirty degrees below room temperature will be musty.
 
G

gizardlizard

Well-known member
Joined
Aug 29, 2019
Messages
731
Location
Madison, WI
coldh2o said:
Yes - because of lack of moisture control.
Click to expand...
EXACTLY. Mold can’t grow when it’s dry. Do you think walls above grade aren’t in a confined space? The insulation is trapped between studs and top and bottom plate. Why does mold predominantly on bathroom walls more than other rooms? Lack of moisture control.
 
OP
B

BWWgarage

Well-known member
Joined
Feb 9, 2023
Messages
342
IMG_0137.jpegReviving this one as I put the Proj on hold after doing the pink board + foam. I’m taking my time off this week to get back at it.

(1) Raise basement ceiling height framing
2” r10 pink board. Great foam to seal. R15 3.5” “rock wool”. I was going to drywall (mold resistant) untaped over to keep insulation from moving around. This will be behind 2x4 finished walls and not visible once basement is finished.

(2)Rim joists:
I added 2” (r10) pink board and great foamed it in. Hindsight, I should have done 4”(r20) as there 2” left in cavity. Strange it’s only 3.5” deep? I’d like to add another 2” of insulation and I think I need to do with another layer of r10 pink board as rock wool/fiberglass batts don’t come in 2”.

My other question: I don’t plan to finish the ceiling and will just paint spray it black. Code says you should have the pink board covered (flammable) and that section will be visible above the finished walls since I won’t drywall the ceiling. I was going to cut mold resistant / fire”proof” drywall to cover the insulation for code and also to make a look better. Is there a better way to cover the pink board vs drywall that also meets fire code for being exposed and is also paintable?

Thanks!
 
You must log in or register to reply here.
To avoid these ads, REGISTER NOW!
Top Bottom