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Old 05-20-2013, 09:38 AM   #1
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Insulating basement: wall with ledge


I have a basement that has cinder block walls that have a "step" in them, that looks like this (pardon poort attempt at my ascii art):

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Info that I found on the web say that the first layer of insulation should be XPS (after prepping the wall). What are some of the common ways to install XPS on a wall like this?

My initial thought is to put 2 pieces on the vertical portions and cut out small pieces (with angled edges) for the sloped portion of the wall. Thoughts?

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Old 05-20-2013, 10:08 AM   #2
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Insulating basement: wall with ledge


Why not build a wall that's not touching the concrete and insulate that.
Then you would have a place to also run your wiring.

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Old 05-20-2013, 03:25 PM   #3
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Insulating basement: wall with ledge


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Originally Posted by joecaption View Post
Why not build a wall that's not touching the concrete and insulate that.
Then you would have a place to also run your wiring.
The articles I have read all seem to say that you should glue XPS to the concrete basement wall. Is it OK not to do that? (Not that I understand why XPS must touch the wall)
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Old 05-20-2013, 06:05 PM   #4
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Insulating basement: wall with ledge


I kind of had same issue as you except I had 8" concrete 4' up then wood the rest of the 4' (split foyer). I polyed the concrete and wood, built another wall against the poly (wood against concrete = mold) buttons it up real good and keeps in heat and cooled so well.
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Old 05-23-2013, 12:25 PM   #5
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Insulating basement: wall with ledge


hmm, ok so I need to poly it. Question is, how do I handle the angle in the middle?
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Old 05-24-2013, 09:43 PM   #6
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Insulating basement: wall with ledge


Where are you located, Mike? No poly in U.S. below grade, stops the moisture (indoor humidity) to rot the wood framing; http://www.buildingscience.com/docum...ong-from-start

http://www.buildingscience.com/docum...lation-systems

Build the wall full height as already said, need location for more info.

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Old 05-25-2013, 10:36 PM   #7
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Insulating basement: wall with ledge


http://www.buildingscience.com/docum...lation-systems

Excellent info.

This talks a lot about "walls must be able to dry to the interior" It also recomends installing rigid xps directly to the interior wall as a air barrier. Wouldn't such an air barrier prevent the interior wall from drying??
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Old 06-06-2013, 10:49 AM   #8
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Insulating basement: wall with ledge


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Originally Posted by Gary in WA View Post
Where are you located, Mike? No poly in U.S. below grade, stops the moisture (indoor humidity) to rot the wood framing; http://www.buildingscience.com/docum...ong-from-start

http://www.buildingscience.com/docum...lation-systems

Build the wall full height as already said, need location for more info.

Gary
I'm in northern NJ. My plan has always been to build the wall full height. My big question is, how do I poly the concrete wall where the angle is? Are you saying I should poly or not poly in the basement? Half the basement is below ground.
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Old 06-06-2013, 11:22 AM   #9
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Insulating basement: wall with ledge


How much depth difference is there? 1" 2". Take a picture. Have a 4' level plumb against the lower portion of wall and using a tape measure find out how what the difference is.
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Old 06-06-2013, 07:52 PM   #10
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Insulating basement: wall with ledge


No poly.

Rigid foam is fine and you can used the rigid foam to bridge the step gap if you want.

I would still have a framed wall to run mechanicals in regardless.
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Old 06-07-2013, 09:30 AM   #11
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How much depth difference is there? 1" 2". Take a picture. Have a 4' level plumb against the lower portion of wall and using a tape measure find out how what the difference is.
It's about 2-3 inches horizontally. I'll take a picture soon to show soon.

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No poly.

Rigid foam is fine and you can used the rigid foam to bridge the step gap if you want.

I would still have a framed wall to run mechanicals in regardless.
When you say rigid foam, do you mean the stuff here http://www.homedepot.com/h_d1/N-5yc1...&storeId=10051 (i.e. what HD calls rigid insulation)? I always thought that was poly, doh!

It's actually not a "step" but a ~45 degree angle so thats why I wasn't sure how best to do this. Does every square inch of the wall, angled or not, have to be touching the rigid foam or can I take short cuts provided that no part of the wall sees the light of day because the rigid foam is in the way but not every part is touching the rigid foam (i.e. there could be air gaps between the wall and the rigid foam)?

And yes, I will be building the straight framed wall regardless.

Last edited by saimike; 06-07-2013 at 09:39 AM.
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Old 06-07-2013, 09:58 AM   #12
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Insulating basement: wall with ledge


I think I'd use 2" XPS in the upper section, angled section, and lower sections. Use expanding foam to fill in the gaps. then frame against the lower portion of the wall. Use some blocking in the upper portion to keep the XPS in place.

By blocking I mean put a 14 1/2" 2x4 horizontally between the vertical studs. then push it back flush with the top portion of the wall. Toe nail it in place. If it is 3" then I'd make my blocking out of a 2x6.
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Old 06-07-2013, 10:01 AM   #13
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Insulating basement: wall with ledge


For interior basement insulation, I believe you need the ones without vapor/moisture barrier. The HDs in my area stock Dow with the barrier, and Owens Corning (pink Foamular) without the barrier.

Theoretically, rigid foam should be glued and/or screwed onto the concrete wall so that they are touching. But in reality, it’s really not possible. Walls are never perfectly flat. Once you start working with the foam boards, you will also find that they are not perfect rectangles. Small gaps could be caulked/taped. Larger gaps could be filled with Great-Stuff.

You should also look up fireblocking before you start this. Check with your town if it’s required. It could be difficult to fireblock after you put the foam up.
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Old 06-07-2013, 10:01 AM   #14
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Insulating basement: wall with ledge


Every bit of cold wall that is allowed to come in contact with warm inside air will produce condensation so yes you need to create (as best you can) a complete vapor barrier.
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Old 06-08-2013, 06:29 AM   #15
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Insulating basement: wall with ledge


Quote:
Originally Posted by saimike View Post
It's about 2-3 inches horizontally. I'll take a picture soon to show soon.



When you say rigid foam, do you mean the stuff here http://www.homedepot.com/h_d1/N-5yc1...&storeId=10051 (i.e. what HD calls rigid insulation)? I always thought that was poly, doh!

It's actually not a "step" but a ~45 degree angle so thats why I wasn't sure how best to do this. Does every square inch of the wall, angled or not, have to be touching the rigid foam or can I take short cuts provided that no part of the wall sees the light of day because the rigid foam is in the way but not every part is touching the rigid foam (i.e. there could be air gaps between the wall and the rigid foam)?

And yes, I will be building the straight framed wall regardless.
You are on the right track.

Rigid foam, sealed with spray foam, and built out with batt insulation after the wall goes up.

Seal up the rim joist/band area prior to putting up the rigid board.

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