Welcome to the DoItYourself Forums!

To post questions, help other DIYers and reduce advertising (like the one on your left), join our DIY community. It's free!

Add Foam Board to Inside?


Yukon Youngun's Avatar
Member

Join Date: Sep 2005
Posts: 154
MO

12-20-15, 08:59 PM   #1  
Add Foam Board to Inside?

I have about forty feet of existing wall that was left without drywall when I bought the house and I finally am getting around to covering it. The existing fiberglass batts will need some cleaning up first and I was thinking to add a layer of 1" foam sheet too. Aside from removing the facing, are there any other concerns about this approach?

I am thinking to run the foam right up into the rim joist spaces as well. Since these are 9 ft. walls, I expect to need three pieces to cover the entire height anyway, so it seems reasonable to notch the smaller, top piece to fit around joists.

 
Sponsored Links
joecaption's Avatar
Member

Join Date: Jan 2011
Posts: 3,967
VA

12-21-15, 06:58 AM   #2  
Is this in a basement?
A picture would be nice so we can see what your seeing.

 
Yukon Youngun's Avatar
Member

Join Date: Sep 2005
Posts: 154
MO

12-21-15, 03:47 PM   #3  
Yes, in a walkout basement. Basic 2x6 stick construction westward facing with 4 windows below an upper deck. Summer sun is rather brutal on this side of the house and the lay of the land and house concentrate stronger winds across this side, making cold weather also brutal.

I added house wrap and new siding a couple years ago, sealing up around windows, doors and foundation, and thought the additional layer of foam would help a bit with the r-value.

Between cats, kids and, and poor installation by the original owner the batts need a little attention. On the other end of the basement, I had to replace a few batts, add some filler, re-position and re-staple others. Will be the same at this end, but at least it will go faster with the longer dw panels.

Still working on the picture...

 
Yukon Youngun's Avatar
Member

Join Date: Sep 2005
Posts: 154
MO

12-22-15, 12:46 PM   #4  
This is a small view of a typical window section. Figure four or five feet between...

 
Yukon Youngun's Avatar
Member

Join Date: Sep 2005
Posts: 154
MO

12-28-15, 10:06 AM   #5  
Essentially, is there anything technically that would hinder me from adding a layer of foam on the inside of this exterior wall over the existing fiberglass? I intend to remove the existing kraft facing, tape the seams of the foam and caulk or spray foam around the perimeter of the foam for purposes of the interior vapor barrier and then drywall over it all.

 
Bruce H's Avatar
Member

Join Date: Oct 2001
Posts: 866
MN

12-28-15, 05:40 PM   #6  
A vapor retarder is defined as a material having a perm rating of 1 or less. For example, 6 mil poly has a perm rating of .03 or so. If you're talking about sheet foam, I don't believe an extruded polystyrene (for example) would qualify; it has a perm rating around 1.5, depending on thickness. I would suggest putting a layer of poly over the foam, then rock it. Poly is cheap.

If there is any electrical, etc. installed, you may need to modify those to accommodate the thickness of the foam. I personally think the addition of a layer of foam is a good idea.

 
calvert's Avatar
Member

Join Date: Jan 2014
Posts: 466
PA

12-29-15, 04:33 AM   #7  
Have you ever felt the sheathing in the winter to determine if there is any condensation on it?

Adding foam can be a good upgrade but you must be conscious of any potential air paths into the wall such as at electrical boxes and perimeters of the wall. Keep in mind that the higher the level of insulation, the colder the sheathing temperature will be. Any air flowing into the wall, with a level of humidity, will be more likely to condense.

 
Yukon Youngun's Avatar
Member

Join Date: Sep 2005
Posts: 154
MO

01-08-16, 04:26 PM   #8  
No, I have not checked for condensation, but the idea is to seal as best I can for just that reason. As it is, I need to do something to seal up segments and seams in the kraft facing anyway (they used 8' batts in 9' walls, patching pieces in and not stapling the edges all that well) so caulking around the poly would be part of the plan.

I expected to move the boxes too, but have always wondered how to seal them. I have just smeared some caulk around the wire entry holes in the past...

 
Bud9051's Avatar
Member

Join Date: Oct 2008
Posts: 9,772
ME

01-08-16, 04:38 PM   #9  
They make air tight electrical boxes, but caulking can work, as long as no one has to go back and change anything.
Vapor barriers become less important when you do a good job of air sealing. modern codes now require vapor barriers only in the far north and deep south, but not all local code department have jumped on. Check locally.

Bud

 
Search this Thread