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Old 11-13-2015, 01:20 PM   #1
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Question about varying levels of insulation


I have a walk out basement that I'm renovating. The wall that underground has a stair landing that limits the amount of insulation I can put in to about R-7-10. I have a wall I the opposite side that is walkout is and I'm not limited on so I was thinking of doing 2 inch xps foam glued to the exterior brick/concrete blocks as a thermal break and than 2x4 wall filled with more insulation (thinking mineral wool) which should give me an r value of... 20 or so at least I'm hoping.

So I have 2 questions.

1) for the wall that I can glue xps foam to, it's an above ground wall and therefore I'm guessing that the 2 inches of xps foam just glued to the wall is ok since I don't have to be super worried about water. Do I need to worry about the vapor barrier? I plan on doing foil faced to make it a vapor barrier. I can put more spray foam in the cavity to move the vapor barrier out further. I'm in Maryland if that helps.

2) does it make sense to have more insulation on the second wall or is it a lost cause since one wall is only r 7 to 10? Wasn't sure if the lesser wall would have the limited performance removing any value in insulating more elsewhere in the room.

Thanks for the help!

Mike
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Old 11-13-2015, 03:02 PM   #2
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1. No. XPS will provide all the vapor control you need. A vapor barrier isn't even required in your climate. Make sure the board is well adhered and sealed up. Be sure to air seal the rim/bandjoist areas as well.

2. Cut and cobble rigid board or get some spray foam. Put what you can in there without reinventing the wheel.
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Old 11-13-2015, 03:33 PM   #3
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Quote:
Originally Posted by Windows on Wash
2. Cut and cobble rigid board or get some spray foam. Put what you can in there without reinventing the wheel.
So is it worth it taking the second wall above and beyond r value of 10 or will it be severely limited by the first wall that I can't take above r-10?
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Old 11-13-2015, 04:15 PM   #4
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Would they be staggered and look weird or do the stairs provide the potential to hide that transition?

More is more, but not if you have to move a whole bunch of stuff.
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Old 11-13-2015, 04:24 PM   #5
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Quote:
Originally Posted by Windows on Wash
Would they be staggered and look weird or do the stairs provide the potential to hide that transition? More is more, but not if you have to move a whole bunch of stuff.
They are opposite walls so there wouldn't be a stagger or anything to give it away. The only thing I'd be losing is 2 inches of floor space. If the insulation is going to save me money it's worth it to be, if it won't be effective due to the only walls limits than I might just do 6 inch studs or something to be a bit more economical.
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Old 11-13-2015, 04:36 PM   #6
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Basements are traditionally under and poorly insulated.

Do it right the first time around and the space will be lights out more comfortable and efficient.
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Old 11-13-2015, 11:22 PM   #7
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A concrete wall is a poor insulator so the r-7 to 10 you propose on the limited wall will make a big difference. For the opposite wall, what you propose is just fine. I would not limit the second wall due to concerns about the other.

If any of the rigid foam is going to be left exposed (not covered with drywall), you need to consider an approved thermal barrier. Dow Thermax, if available in your area, is a foil faced rigid foam board that meets a higher standard. Local codes vary so you should bounce your choice off of them. There are also special thermal paints and of course a layer of drywall that can meet most requirements.

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