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Insulating an old house. POLY ISO Advice


fixerDIYupper's Avatar
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Join Date: Oct 2013
Posts: 14
NY

01-10-14, 05:59 PM   #1  
Insulating an old house. POLY ISO Advice

We're redoing an older home. The walls are plaster and lathe with no insulation. The construction is (true) 2x4. We had to take down some of the plaster walls as the plaster was beyond repair. We took down all the exterior walls to the studs. I'm thinking of putting in 4" (or 3") poly iso between the studs. I don't think I need to put up a vapor barrier, unless I'm wrong. Also, if I put the poly iso between the 16"OC studs, are the studs going to create a thermal bridge situation? What can I do to avoid this, and does this sound like a good plan in general?

Thanks

 
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Bud9051's Avatar
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01-10-14, 06:18 PM   #2  
If you fill the cavities with a batt material and then cover the inside with a 1" layer of polyiso you will have a very good air barrier with a layer to break the thermal bridging. Take a look at Roxul for the batt option, it is extremely dense. The real 4" cavity will require some handy work to completely fill with the standard 3.5" material, but done neatly the results will be great.

Whatever approach you select you don't want to leave an air gap in the cavity.

Bud

 
fixerDIYupper's Avatar
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01-11-14, 05:41 AM   #3  
Thanks Bud. I did look at the Roxul material. I thought I could accomplish a higher R value with 4" boards. Are you saying I would be better off if I used something that is not so rigid and filled all the small creatives, and used a rigid board on top of that? What about using just 4" boards cut to fit inside the wall cavities, and then adding a layer of 6mil before the sheetrock? Thanks again...

 
planigan's Avatar
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01-13-14, 12:43 PM   #4  
Just thought I'd chime in with a +1 for Roxul. I'm using in a similar project (older house, plaster lath, true 2x4 framing). The stuff is more expensive than fiberglass but so much easier to work with. It's a really, really nice product.

If your house is anything like mine, you'll find that none of the stud cavities are uniformly spaced and that many of the studs aren't exactly parallel. The "friction fit" of Roxul makes it easier to cut and place in irregular cavities.

 
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