# Vapor barrier against concrete foundation?



## capecodder (Jun 5, 2011)

Im going to be framing my basement. I was wondering if I will need to use a plastic vapor barrier where the verticle 2x4 studs run up against the concrete walls?

Thanks!


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## hyunelan2 (Aug 14, 2007)

In cold regions, the vapor barrier goes on the warm side of the insulation - if you're not using XPS boards. Frame the walls 1" off the concrete, not up against the concrete. Then install insulation and cover with the vapor barrier before drywall.

Note: newer methods of insulation and framing dictate using XPS panels against the concrete before framing.


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## capecodder (Jun 5, 2011)

Great! Thanks for the info. These XPS panels...are they something that can be purchased through home depot? or a lowes?

Thanks again!


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## Gary in WA (Mar 11, 2009)

That would be Case #3, possible mold and convective looping of the fiberglass without the air space, definite c.v. with the air space; http://www.buildingscience.com/docu...merica-high-r-foundations-case-study-analysis

http://joneakes.com/jons-fixit-database/743

Gary


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## hyunelan2 (Aug 14, 2007)

Good info about convective looping, thanks. So according to that 2nd link, without XPS you should push the fiberglass all the way against the concrete. You should still frame the walls off of the concrete though, correct? Or is it also a myth that it's bad to have untreated lumber in direct contact with the concrete?


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## Gary in WA (Mar 11, 2009)

Concrete walls will wet the f.g. insulation degrading it's R by 66%. The air space is bad on concrete, the rigid seals the air out along with the air-tight drywall approach (ADA) and a sill sealer for an air break, it also acts as a capillary/thermal break to the slab/earth below. Canada is about the only place to still use a v.b. in basements even with their own dated literature - pp.14, 19, 29. 32, 42, 49-55, 59, read "a" and "e" and "g" there. pp 59 of article- and last paragraph.... ftp://ftp.cmhc-schl.gc.ca/chic-ccdh...ngual/Vapour_Permeance_Volume_1_Web_sept5.pdf

Others of interest: http://www.carb-swa.com/articles/in the news/HomeEnergy_The Challenges of Basement Insulation.pdf

http://libdspace.uwaterloo.ca/bitstream/10012/3242/1/Ueno MASc Thesis.pdf

http://www.buildingscience.com/docu...ing-hygrothermal-modeling-basement-insulation

Gary


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## jasin (Jan 22, 2012)

capecodder said:


> Im going to be framing my basement. I was wondering if I will need to use a plastic vapor barrier where the verticle 2x4 studs run up against the concrete walls?
> 
> Thanks!


If you use low density closed cell spray foam in between the studs you do not need a vapor barrier. If you use just regular batting insulation, something i'd never use in a basement, then you need a vapor barrier on the warm side.


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## jasin (Jan 22, 2012)

GBR in WA said:


> Concrete walls will wet the f.g. insulation degrading it's R by 66%.


That's why you seal the walls before studding and putting in insulation. :wink:


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## jasin (Jan 22, 2012)

I hope the OP is is using exterior grade studs. You are not suppose to use regular interior studs in a basement.


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## Beepster (Oct 19, 2009)

jasin said:


> I hope the OP is is using exterior grade studs. You are not suppose to use regular interior studs in a basement.


...??? Your last three posts have me scratching my bald head.

The bottom plate in contact with the floor should be treated wood, but the rest of the lumber can be regular 2x4.

Don't Drylock the basement walls, that just directs the moisture further down.

B


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## hyunelan2 (Aug 14, 2007)

GBR in WA said:


> Concrete walls will wet the f.g. insulation degrading it's R by 66%.


_Let me preface this by saying I'm not trying to argue that FG batts are better, just trying to understand._

So, if you are reducing an R13 fiberglass batt by 66%, you now have an ~R4.3 value. Isn't this about just as good as a 1" XPS (R5) with lower cost?


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## jasin (Jan 22, 2012)

Beepster said:


> ...??? Your last three posts have me scratching my bald head.
> 
> The bottom plate in contact with the floor should be treated wood, but the rest of the lumber can be regular 2x4.
> 
> ...


If the studs are anywhere near an unsealed concrete wall they should be exterior grade. Concrete walls do get wet. see post #6 & 8


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## Beepster (Oct 19, 2009)

If your concrete walls are going to get wet, you deal with that issue before you finish off your basement.

I have 1" XPS glued to the wall and am 2x4 framing over that with R13 unfaced insulation in between. I believe after reading over all of the "Building Sciences" links that GBR has provided that that is the current thinking on how you finish off basements. Use 2" XPS if you want to.

B


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## Gary in WA (Mar 11, 2009)

jasin said:


> If you use low density closed cell spray foam in between the studs you do not need a vapor barrier. If you use just regular batting insulation, something i'd never use in a basement, then you need a vapor barrier on the warm side.


-----* In between does not stop moisture where the studs are. Vapor barriers on the warm side below grade are not recommended.* They have problems, even with SPF: http://docs.google.com/viewer?a=v&q...Gcfigu&sig=AHIEtbRnzexpOuy2DmtpoAYQmvK_bHtRKg

"That's why you seal the walls before studding and putting in insulation. :wink:" --------- sealing the walls with anything is counter-productive. This just forces the water in the concrete wall elsewhere, usually to the footing/wall joint to enter the basement with more volume now at the slab/wall joint.

"I hope the OP is is using exterior grade studs. You are not suppose to use regular interior studs in a basement."------- 
You need to explain that one to me. I hope you are confusing studs with plywood = exterior/interior...

2" is better in colder locations; http://www.buildingfoundation.umn.edu/FinalReportWWW/Chapter-5/5-optimum-main.htm

Gary


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## Gary in WA (Mar 11, 2009)

Cc, I would use some Grace Ice&water shield on the fire-stopping studs sistered to the layout studs every 10', per Code, with the foamboard between. Use a sill sealer under the p.t. bottom plate (capillary/thermal/air break) and for a thermal break between the fire-stop stud and the layout stud. P.t. on the concrete WALL is not required with the G.I.&.W. Be sure to fire-stop over top of foamboard/joist cavities above. http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec002_par031.htm

http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_3_sec017.htm

ADA the drywall: http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

And air-seal the rims w. f.b.: http://www.buildingscience.com/docu...joist/files/bscinfo_408_critical_seal_rev.pdf

Gary


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