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Old 05-15-2013, 08:05 AM   #1
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Insulating basement below grade


Hi I'm finishing a room in my basement and am evaluating my options for insulating my cinder block walls that are below grade except for maybe the top 18 inches. Local code says no vapor barrier. Does that make my only options spray foam in the studs or xps glued to the block before framing? I am confused if xps will be considered a vapor barrier and cause me to fail inspection. If xps works is 1" or 2" necessary?

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Old 05-15-2013, 10:32 PM   #2
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Insulating basement below grade


Vapor barriers, Class 1 (poly sheeting) are no longer required below grade in most of U.S., where are you located?

http://publicecodes.cyberregs.com/ic...001_par003.htm

http://publicecodes.cyberregs.com/ic...001_par005.htm

Depends on location for foam thickness to raise the dew-point temperature in the wall cavity, preventing condensation on the wood framing.
At 2" XPS is a vapor retarder; pp.3-5; http://www.buildingscience.com/docum...apor+retarders

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Old 05-15-2013, 10:37 PM   #3
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Insulating basement below grade


I'm in milwaukee so zone 6. My basement is dry. Just looking to take appropriate precautions.
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Old 05-16-2013, 09:46 AM   #4
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Insulating basement below grade


Go with Roxul in between studs if you don't want to glue XPS to the walls,it gives a nice R-15 versus R-13 for fiberglass.No itch to it and very easy to cut and size.
Remember,if you put XPS on the walls,it must get covered,if it catches fire,it emits toxic gas that will kill you mighty dead.
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Old 05-16-2013, 11:33 AM   #5
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Insulating basement below grade


So if I use roxul I just frame next to cinder block and then add the roxul? Any idea on cost? I see Home Depot can special order but no cost is listed. Do I need to use treated studs then against the cinderblock if I don't use foam?
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Old 05-16-2013, 01:37 PM   #6
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Insulating basement below grade


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So if I use roxul I just frame next to cinder block and then add the roxul? Any idea on cost? I see Home Depot can special order but no cost is listed. Do I need to use treated studs then against the cinderblock if I don't use foam?
Stand the studs off the wall so they arent in contact with it,say 1/2 to 3/4 of an inch.
If you are going to use it,Lowes has it in stock for 39.99 a bag.
I recently did about 100 lineal feet of basement wall 8 ft high and I think I used 9 bags.
Doesn't rot,decay,mold up and is Class A fire rated,good stuff.
Your bottom plates need to be pressure treated,additionally,they should have sill sealer (foam roll) stapled to the bottom,that way no wood is in contact with the floor.

Last edited by spring3100; 05-16-2013 at 01:45 PM.
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Old 05-17-2013, 11:44 PM   #7
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Insulating basement below grade


Using 3 coldest months for you:http://www.weather.com/weather/wxcli...graph/USWI0455
an average of 25*F and room temp of 68*F, with; R-5 XPS/R-15 Roxul = safe to relative humidity of 30%RH


R-10/15 = 38%RH

R-15/15 = 45%RH

Work with the local AHJ on the stairs. Use PIC board insulation if needed. Roxul alone (no foamboard) will condense moisture on the CMU and back edges of framing; above grade/3' below grade areas at/above 40%RH basement air. You need to realize these levels are figured on 43*F (AVERAGE) temp for the concrete block inside face. So, any temperature below 25*F outside (for most of your coldest 3 months), you risk condensation from the indoor air in the winter, plus outside air in the summer; http://www.buildingscience.com/docum...lation-systems

http://www.buildingscience.com/docum...g-your-basment

Leaving any air gap between cavity (air-permeable; fiberglass, rock wool, etc.) insulation/CMU, insulation/drywall, or foamboard/CMU will give convective loops;http://joneakes.com/jons-fixit-database/743

https://docs.google.com/viewer?a=v&q...fHFsk1RrK9G2HQ

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Old 05-18-2013, 12:23 PM   #8
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Insulating basement below grade


I leave the studs off the wall,I push the Roxul against the wall,you can stuff Roxul between studs and wall also,rips into small pieces easily.
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Old 05-19-2013, 08:41 PM   #9
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Insulating basement below grade


Roxul makes boards that you put right up against the basement wall, and then you can do the framing. Link to video I have no experience with Roxul, just seems like this would be easier than doing studs first and then stuffing insulation behind them.
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Old 05-26-2013, 09:55 AM   #10
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Insulating basement below grade


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Originally Posted by bryanp22 View Post
Hi I'm finishing a room in my basement and am evaluating my options for insulating my cinder block walls that are below grade except for maybe the top 18 inches. Local code says no vapor barrier. Does that make my only options spray foam in the studs or xps glued to the block before framing? I am confused if xps will be considered a vapor barrier and cause me to fail inspection. If xps works is 1" or 2" necessary?
I do not believe xps would be considered a vapor barrier but call the local inspector to be sure.

1" should give you an R5 value and 2" should give you an R10 value, thats up to you.

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