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-   -   Insulating basement: wall with ledge (http://www.diychatroom.com/f103/insulating-basement-wall-ledge-180017/)

saimike 05-20-2013 09:38 AM

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?

joecaption 05-20-2013 10:08 AM

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.

saimike 05-20-2013 03:25 PM

Quote:

Originally Posted by joecaption (Post 1183183)
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)

jonesb183 05-20-2013 06:05 PM

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.

saimike 05-23-2013 12:25 PM

hmm, ok so I need to poly it. Question is, how do I handle the angle in the middle?

Gary in WA 05-24-2013 09:43 PM

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

Stcrosby 05-25-2013 10:36 PM

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??

saimike 06-06-2013 10:49 AM

Quote:

Originally Posted by Gary in WA (Post 1186949)
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.

brockmiera 06-06-2013 11:22 AM

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.

Windows on Wash 06-06-2013 07:52 PM

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.

saimike 06-07-2013 09:30 AM

Quote:

Originally Posted by brockmiera (Post 1196275)
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.

Quote:

Originally Posted by Windows on Wash (Post 1196579)
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! :bangin:

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.

brockmiera 06-07-2013 09:58 AM

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.

leungw 06-07-2013 10:01 AM

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.

brockmiera 06-07-2013 10:01 AM

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.

Windows on Wash 06-08-2013 06:29 AM

Quote:

Originally Posted by saimike (Post 1196860)
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! :bangin:

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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