# XPS Thickness



## Windows on Wash (Aug 30, 2011)

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.


----------



## davitk (Oct 26, 2008)

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.


----------



## cowbell9 (Jan 18, 2012)

The only reason is that the thinner XPS is half the price. Agree about HD. Thanks


----------



## itsnotrequired (Apr 30, 2010)

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.


----------



## Setzer1994 (Apr 12, 2012)

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.


----------



## davitk (Oct 26, 2008)

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". :laughing: 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!


----------



## cowbell9 (Jan 18, 2012)

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


----------



## Windows on Wash (Aug 30, 2011)

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.


----------



## davitk (Oct 26, 2008)

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.


----------



## Gary in WA (Mar 11, 2009)

Savings: http://www.quadlock.com/technical_library/bulletins/R-ETRO_Value_of_Basement_Insulation.pdf

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

Fill the cavity: http://findarticles.com/p/articles/mi_m0KWZ/is_3_6/ai_n8582994/

Thickness of f.b.: http://www.buildingfoundation.umn.edu/FinalReportWWW/Chapter-5/5-optimum-main.htm

Figure the dew point: http://www.greenbuildingadvisor.com/blogs/dept/musings/are-dew-point-calculations-really-necessary

OR, just find your Zone; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec001_par001.htm

http://www.greenbuildingadvisor.com...lating-minimum-thickness-rigid-foam-sheathing

IRC, fire-blocking every 10' lineally between f.b., and continuous above the f.b to joist bays:http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec002_par031.htm

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

No air to the concrete: http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Foil-faced is alright,* IF* thick enough: http://www.buildingscience.com/docu...merica-high-r-foundations-case-study-analysis

Gary


----------



## davitk (Oct 26, 2008)

GBR in WA said:


> Savings: http://www.quadlock.com/technical_library/bulletins/R-ETRO_Value_of_Basement_Insulation.pdf
> 
> No air gap: http://joneakes.com/jons-fixit-database/743
> 
> ...


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


----------

