# Rigid foam



## Stein (Jan 7, 2012)

Hi,

I am attaching 1inch XPS foam to my poured concrete basement walls. I am then planning to frame a 2x4 wall in front of this and filling the voids between studs with regular fiberglass insulation for a higher R value. My question is do I need to seal the top and bottom of the rigid insulation or just tape the joints between then? I've asked numerous people and gotten different answers. Some say to leave it open so as not to trap moisture inside the wall and some say seal it completely.


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## cleveman (Dec 17, 2011)

install the xps on the exterior. what do you have on the exterior now?


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## Gary in WA (Mar 11, 2009)

Yes, foam the bottom and top. It has to be air-tight so absolutely no basement air gets to the colder concrete; http://joneakes.com/jons-fixit-database/743

Even air-seal the p.t. bottom plate with a sill sealer for a thermal/capillary/air break: http://www.greenbuildingadvisor.com...ressure-treated-sill-plates-and-building-code

Air seal the bottom edge (held 1/2" up) drywall at the slab/wall as that is your new air/thermal barrier; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002_par021.htm

Air seal the rim joists, also per minimum code; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002_par022.htm

For an "A"+ job, ADA; http://www.buildingscience.com/documents/information-sheets/air-barriers-airtight-drywall-approach/

Where are you located that 1" foam board will work?

And, welcome to the forums!

Gary


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## Windows on Wash (Aug 30, 2011)

cleveman said:


> install the xps on the exterior. what do you have on the exterior now?


I think the home is already built at this point if I am not mistake and +1 to GBR recommendations.

They need to be sealed.


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## cleveman (Dec 17, 2011)

I don't want to start another thread, but what would you guys suggest if he had 2" of polystyrene already in place on the exterior?


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## Windows on Wash (Aug 30, 2011)

cleveman said:


> I don't want to start another thread, but what would you guys suggest if he had 2" of polystyrene already in place on the exterior?


With 2" of foam to the outside, the thermal break is done where the foam is. Is the basement completely below grade at that point?

Additional foam to the interior does not hurt, just might not be necessary expense at that point.


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## cleveman (Dec 17, 2011)

I just try to get people to understand that the xps on the exterior is the best way to go. It takes care of condensation problems, protects the waterproofing membrane, and of course insulates.

I also want to encourage people to install it as a remodel because it is the "final solution" and I'm not sure even the most diligent worker will get a very good result trying to get their thermal/vapor barrier to work on the interior.

One day on a mini-backhoe and the whole exterior is exposed and wall problems, waterproofing, and drainage can all be addressed.

I also like to have the entire basement below grade and a brick veneer on top. However, I've had some luck with non-masonry siding when the grade has to stay down at least 4 inches by bringing the 2" polystyrene all the way up to the top of the wall and attaching some very heavy "fake masonry" vinyl siding to the sill plate, and flashing this area with some metal. Then I grade up against the "fake stone" vinyl to keep it in place. No problems thus far with this method.

This results in a sweat free, energy star basement. If it is later finished, then one can certainly put r-13 in the studded walls and you really have a nice situation then.

I'm sure there are thousands of others out there doing the same thing with new construction, but it always surprises me when I see this detail skipped. Plus, as noted before, I like to promote if for remodels as well.


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## Trucon01 (Jan 10, 2012)

Would 1 in of XPS be enough for concrete block walls or should you use 2 in? I say this because a 1 in board is almost $20 at HD and the 2 in board is $31. Thanks


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## cleveman (Dec 17, 2011)

you must be in Canada to pay those prices?


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## kuhlman (Dec 21, 2011)

Yep, $30-$35 dollars for 2", with foil on one side, here in Oregon.


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## Trucon01 (Jan 10, 2012)

I am in Northern VA and those are the prices at the Home Depots around here... Wish they weren't.


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## Beepster (Oct 19, 2009)

Trucon01 said:


> Would 1 in of XPS be enough for concrete block walls or should you use 2 in? I say this because a 1 in board is almost $20 at HD and the 2 in board is $31. Thanks


I chose to go 1", and stick frame with unfaced insulation. It was more cost efficient to get the same R. From what I researched at the Building Sciences sites is that you can do that but should monitor the humidity level in the summer and keep it below 50%.

B


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