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Basement insulation

Posted by rexg (My Page) on
Tue, Dec 2, 08 at 16:32

I am in the middle of finishng my basement out and now seem to possibly have a problem. When I framed the interior walls I use 2x4's with a plastic sheeting as a vapor barrier between the interior walls and the concrete wall as instructed by a builder friend. I was planning on now insulation with r-13 fiberglass insulation. Now from reading this forum and doing more research I seem to have made a mistake. My house is 8 years old and has never had a water problem on the inside. I waterproofed the walls while it was being constructed both inside and out and it has an excellent foundation drainage system. I sit on a very sloped lot (drops 10 ft. in 30 ft.) and there is a 10 ft. covered porch around three sides of my house, so the ground stays very dry next to the house except for the backside which seems to get very little rain based on the position of the house and the direction in which most of our rain comes. I have read that I should now put xps board on the concrete walls but I can only do that now by removing the plastic (would have to cut it out) and then glue the foamboard between the studs, the walls are already wired. Could I just cut the plastic and stape it back to the studs then glue the foamboard or should I completely remove it? I don't need a whole lot of r vaule here in Georgia so I'm more concerned about any possible dampness or mold in the future.


Follow-Up Postings:

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RE: Basement insulation

Could I just cut the plastic and stape it back to the studs then glue the foamboard

Yes. Though the board should be mechanically fastened.

See Building Science for designs that work in hot-humid and mixed-humid climates. (Georgia is split half one, and half the other and I don't know which half you're in.)

Here is a link that might be useful: Building Science Corp.


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RE: Basement insulation

Thank you, I'm in the northern half. How many fasterners for a 4x8 sheet? I checked at the local Lowes and 2" foamboard is running about $26 per sheet, sure makes for expensive insulation but cheaper than the spray form company I contacted they wanted $2500 to do 1260 sq. ft.! Part of my basement had normal wood framing, rather than a concrete wall so I guess I can use regular insulation there. Glad I found this forumn before I ended up with a bad mold problem.


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RE: Basement insulation

One last follow-up question. I live in a log house which is sitting on a 10 inch wide concrete foundation. The log are 6 inches thick and the floor system sits inside the logs but independent of the log walls. Do I really need in insulate the rim joist in my basement? I have never seen any moisture or felt in air movement, just seems like a lot of work and cost for what I would get back. Could I just use something like Great stuff and seal the top and bottom? I actually injected great stuff between the logs and the rim joist when the house was under construction.


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RE: Basement insulation

See EERE recommendations for maximum R value on the map below.

You can save money by using 1" XPS on the foundation walls followed by fiberglass batts between the wood framing. (As shown by Building Science Corp.)

Sealing at the rim is recommended, as with the foam you're using. (I'm not clear on the construction you're describing.) The conditions in the walls will change when you finish the basement and, in effect, enclose the foundation; moisture will accumulate if it's not finished appropriately.

Here is a link that might be useful: EERE Recommended Insulation Levels


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RE: Basement insulation

Spoke with a construction friend about the 1" xps. He agreed I needed to remove plastic sheeting and use the xps on the walls. He suggested that I cut the the 4x8 sheets into 12" x 8' lengths then attach to the concrete walls then use great stuff max. expanding foam the fill the void between the foam board & studwalls. He said it would also fill the gaps and help lock in the foam. He thought the additional cost of the great stuff foam would probably equal out to the cost and time involved in trying to cut the xps to a tight fit. Does this make sense?


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RE: Basement insulation

What are the 4x8 sheets? XPS? EPS? I don't understand why you would cut them into narrower sheets.

In any case, once you adhere the insulating board to the concrete walls as tightly as you can, you can use pretty much any other insulation or sealant you want on the balance. Small cans of expanding foam are most economically used to fill irregular nooks and crannies.


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RE: Basement insulation

xps and the walls are already framed.


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RE: Basement insulation

Fast worker! Post a pic.


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RE: Basement insulation

Actually I was answerwing your question in that the foam is xps and the reason I was cutting it because I had framed the walls against the concrete foundation before I found this forumn. The general contractor who build my house advised me to put plastic sheeting against the concrete wall as a vaper barrier and then frame against it followed by R-11 FG and sheetrock. Seems to be a popular ways of insulating a beasement around here from what others have told me. I am concerned about mold and now I will need to go back and remove the plastic and fit the xps foamboard in between the studs. Cutting the 4x8 sheets into 12" strips is more an economy thing than any thin else. That stuff sure is not cheap, about $14 (1" thick) a sheet at HD. It will take a while but cheaper than have the walls spray foamed by a professional and easier than takening the walls and starting over. There is a slight opening between the rear of the 2x4 walls and the concrete foundation and I hope the great stuff will fill in the voids. My walls are very dry and after 8 years I have never seen any water or even dampness. I run a dehumidifier most of the year to keep the humidity under control and my builder did a great job of putting a good foundation drain system and grading away from the house. It was suggested that I use the new mold proof sheetrock but I don't know anything about it. Thank you.


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RE: Basement insulation

Now I get it: the framing is at 12" on centre and you're using the hand held foam for the space between the studs and the concrete wall. That will work fine. I've done many basements and have never bothered with the mould proof drywall. (It's actually just mould resistant and not all that resistant if you look closely at the test literature.)

Here is a link that might be useful: Mold Resistant Drywall. NOT!!


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RE: Basement insulation

Would the green water resistant sheetrock that you use in showers and bathroom be better to use on the walls framed next to the concrete walls?


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RE: Basement insulation

"Greenboard" was removed from the International Residential Code as a tile backer board in 2006.

Knowledgeable builders have avoided it for more than 20 years. It's no better in a basement. Use mechanical dehumidification to keep relative humidity below 35-50%, watch for water leaks and insulate properly.


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RE: Basement insulation

I know you have said I needed to attach the foam board with mechanical fasteners but my carpenter friends said I might just want to use 1x2's and toenail it with a trim gun. He said it would be a lot faster and easier. Suggested about one very 16" or so. Mt concrete is very hard and even though I have a hammer drill it would be a lot of work to drill all those hole and screw in the concrete fasteners. I checked at Lowes and those fasteners like "tapcon" are rather pricy. Any problems I would have to worry about using the wood strips. I understand the fasteners are there to keep the foamboard tight against the concrete and it seems that the wood strips would accomplish the same thing. Also I have access to a lot of wood pallets I could cut up for strips for free.


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RE: Basement insulation

1x2's and toenail it with a trim gun.

I've mentioned using boards on other threads. But that works best if there's no wood framing in the way, so you can adhere a bunch of the foam boards at once.

But I don't think a trim gun is powerful enough to penetrate concrete. You can use a powder charge nailer e.g., Ramset.

If you're thinking of nailing the corners of the foam into the studs, the problem is that that will not keep the foam tight to the concrete wall.


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RE: Basement insulation

Actually I was thinking toenailing the strips to the insides of the studs.


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RE: Basement insulation

One last question and I hope I'm ready to start. If I foam the rim joist as has been mentioned in this forumn with just a 1" thick layer of closed cell foam ( my wife is getting impatient and wants me now to hire a foam company and get it done!) and if I was planning to put in a drop ceiling/ sheetrock in only a part of the basement do I also have to cover the foam where there is no ceiling in the rim joist separate. I will be covering all the foam in the walls with shetrock but I am putting in a small woodshop in one part of the basement and I was not planning on putting in a ceiling.


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