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Old 05-07-2012, 09:41 PM   #1
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XPS Thickness


Hi guys

Been looking around for a while and absorbing all of the great advice. Getting heavy into my basement remodel and was wondering the thickness/R Value I need. I live in Nassau County, NY and have a basement that is 3/4 below grade. I was ready to buy the 2" owens corning XPS but the guys in HD said I could get away with 3/4" since I would be studding over it and adding at least 1/2" sheetrock over that (kind of funny they were out of the 2" but had hundreds of 3/4" in stock) I want it to work as a vapor barrier as well and from what I have read, XPS only acts as a vapor barrier if it's over 1 1/2" thick. Any thoughts? Thanks in advance
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Old 05-08-2012, 07:25 AM   #2
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If the XPS has a poly layer to it, it is a complete vapor retarder (i.e. class 1). The vapor retarder level has everything to do with the facing on the foam. The foam should have it somewhere listed on it.

You must get your seam detailing right and it has to be air tight.

3/4" is okay and if you are putting a 2x4 wall in front of it you will be way ahead of the game as compared to traditional insulation schedules.
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Old 05-08-2012, 12:02 PM   #3
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I believe 2" polystyrene is R10. Why would you go less?

Forget what you think you know about vapor barriers, you're on the right track. The guys at HD are morons.
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Old 05-08-2012, 12:11 PM   #4
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The only reason is that the thinner XPS is half the price. Agree about HD. Thanks
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Old 05-08-2012, 12:57 PM   #5
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the guys at buildingscience.com recommend 1" XPS minimum. this is for vapor retardation reasons. thinner than that and you may not get enough of a thermal gradient across the foam and may have condensation issues.
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Old 05-08-2012, 01:08 PM   #6
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My GC friend recommended for my basement remodel - 1/2" foil-backed XPS up against the foundation, then standard 2x4 framing, then R-13 between the studs.

I then also went ahead and put 3.5" Roxul along the rim.

I'm in SW Connecticut. I'm thinking that's plenty.
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Old 05-08-2012, 04:45 PM   #7
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OK just got back from HD, picked up some hose ends for my compressor. Needed some teflon tape for the threads but they have only generic with nothing printed on the package but "Made in China". I do believe the stuff should be UL listed.

Back on topic, the drywall does nothing to improve R value as it is not continuous and sealed. Dead air space does have an R value, but your wall assembly has openings wherever there is an electrical penetration, wall junction, and at the ceiling. Think about all that cold air filtering through your beautifully finished new basement.

Vapor barriers below grade are a problem because they don't allow the wall assembly to dry out. Moisture will migrate through the concrete wall, and you don't want to trap it. Polystyrene itself is not a barrier, and is not harmed by moisture. Stick with the XPS, and go 2" continuous. And if you can afford it, spray foam the box sill to seal the air leaks.

Good luck and have fun with it!
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Old 05-08-2012, 07:49 PM   #8
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Hey guys. 2 more questions. I was not planning to put batt insulation btw the XPS and the 1/2" sheetrock. Is this a bad thing because it will create an air gap? It's not clear in the building science articles I've been reading (0202). It says to add it if necessary. I thought the whole idea was to not have an air gap. Second, Is all R-10 XPS pretty much the same? I was going to go with a blue product called Styrofoam Cavitymate. It's 2" thick, R-10 Type X. I do not know what that meansThe water absorption rate is 0.1% by volume. The salesman told me its the same material that is used to float docks. Is this too impermeable? do I want more absorbtion? If I am getting moisture, wouldn't it get trapped btw the XPS and the concrete wall with nowhere to go to absorb? Thought I was home free. I just want to buy and install something. Thanks
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Old 05-08-2012, 08:15 PM   #9
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Absorption is not the issue...air movement is.

I would insulate the cavity at the same time but be sure to use unfaced fiberglass.

XPS installed and sealed tight to the wall is not going to pull any moisture into it.

If you have bulk moisture via a wall leak, that will show and likely still make it through the vertical pathway where the foam is attached to the wall.
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Old 05-09-2012, 09:15 PM   #10
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Blue and pink are both good, just don't get plastic or foil faced.

I don't add fiberglass between the studs, it's just an extra step. With R10 continuous you're good.
Attached Thumbnails
XPS Thickness-dscn2071.jpg   XPS Thickness-dscn1761.jpg   XPS Thickness-dscn1319.jpg  
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Old 05-10-2012, 12:29 AM   #11
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Savings: http://www.quadlock.com/technical_li...Insulation.pdf

No air gap: http://joneakes.com/jons-fixit-database/743

Fill the cavity: http://findarticles.com/p/articles/m...6/ai_n8582994/

Thickness of f.b.: http://www.buildingfoundation.umn.ed...timum-main.htm

Figure the dew point: http://www.greenbuildingadvisor.com/...ally-necessary

OR, just find your Zone; http://publicecodes.citation.com/ico...001_par001.htm

http://www.greenbuildingadvisor.com/...foam-sheathing

IRC, fire-blocking every 10' lineally between f.b., and continuous above the f.b to joist bays:http://publicecodes.citation.com/ico...002_par031.htm

Sill sealer under p.t. bottom plate for a thermal/air/capillary break to the cold, wet slab: http://www.greenbuildingadvisor.com/...-building-code

No air to the concrete: http://www.buildingscience.com/docum...wall-approach/

Foil-faced is alright, IF thick enough: http://www.buildingscience.com/docum...study-analysis

Gary
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Old 05-10-2012, 07:35 AM   #12
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Quote:
Originally Posted by GBR in WA View Post
Savings: http://www.quadlock.com/technical_li...Insulation.pdf

No air gap: http://joneakes.com/jons-fixit-database/743

Fill the cavity: http://findarticles.com/p/articles/m...6/ai_n8582994/

Thickness of f.b.: http://www.buildingfoundation.umn.ed...timum-main.htm

Figure the dew point: http://www.greenbuildingadvisor.com/...ally-necessary

OR, just find your Zone; http://publicecodes.citation.com/ico...001_par001.htm

http://www.greenbuildingadvisor.com/...foam-sheathing

IRC, fire-blocking every 10' lineally between f.b., and continuous above the f.b to joist bays:http://publicecodes.citation.com/ico...002_par031.htm

Sill sealer under p.t. bottom plate for a thermal/air/capillary break to the cold, wet slab: http://www.greenbuildingadvisor.com/...-building-code

No air to the concrete: http://www.buildingscience.com/docum...wall-approach/

Foil-faced is alright, IF thick enough: http://www.buildingscience.com/docum...study-analysis

Gary
Good points. The pics above are for entertainment purposes only.
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