Using rigid foam insulation in basement.

[that_guy]

I've been reading up on using rigid foam insulation (both in this forum and in the Dow website) because I'll be finishing my basement in the next few months so I'm planning ahead.

I have two questions for anyone willing to spend a few minutes.

I understand we use 1x3 furring strips to fasten the rigid insulation to the walls and then these strips are used to fasten the dry wall, but:

1. Corners... the Dow site suggests using drywall corner beads on the inside and outside corners to fasten the drywall ... does this approach provide enough support?

2. What about baseboards? How are they fastened? Do we need to go back and find the furring strips after the wall's been painted? What if the wall isn't straight and we need nails closer than two feet apart?

Any help from people who've done this before is much appreciated. :lol:

...Thanks.

[that_guy]

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Any suggestions? Ideas?

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[Dragon]

The last time I did anything like this the insulation went between the furring strips, not over them.

But hey, this is Arkansas. You may want more R value if you live up north.

[bkrahmer]

Underground temperature is a lot cooler than room temperature in most areas of the country. If you have living space in a basement, it should be insulated to at least R20, IMHO.

[sleepy hollow]

I am framing and insulating my basement right now. I have a poured concrete foundation - about 1500 sq ft. total. I live in VA outside DC. Very humid climate with basically 4 seasons. It can get cold here, though not as frigid as points further north, of course. I did a lot of research to develop my insulation strategy. My house was built in 1985. Things have become more complex since then mainly because the insulation strategies used in the past were not so good for controlling moisture and condensation - leading to mold. Mold is the next big trial lawyer bonanza, IMHO. It will give John Edwards something to do if he does not win next week.

Anyway, I just wanted to do something sensible, and my local county inspectors were not very helpful in providing a good strategy or a rationale for a strategy besides. After reading a lot of stuff I finally deduced what I think is a good strategy for insulating and controlling moisture for my basement. Here is what I am doing:

For the concrete walls (about 94" high):

I am using 3/4 inch polystyrene with a barrier on both sides. HD sells these 4 x 8 sheets for about $5.50 each. I am gluing them to the concrete just before erecting a section of stud wall (I am not using furring strips). I am using liquid nails heavy duty adhesive. The stud walls provide the source of pressure to keep the foam up against the concrete until the glue sets up. I use scrap lumber between the studs and the foam to push the foam up against the foundation wall where there are gaps between the studs and the foam, which is quite common as the foundation walls are not perfectly plumb or straight, but the stud walls need to be.

For the rim joist areas above the foundation walls (about 13 1/2 inches):

I am using 1 inch polyisosinaurate (yellow foam - about $14 per sheet) with reflective side facing out toward the outside of the house. I am cutting these in roughly 24" by 12" sections and gluing them to the rim joists. Then I am caulking all gaps around the foam with basic 25 year latex caulk.

Inside of both foam applications, I will use unfaced fiberglass batts.

The rim batts are R-19 and the stud cavity batts will be R-13 probably, as soon as I can locate a supply of 24" unfaced batts. Neither HD nor lowes carries this size for some loony reason. I do not know many basements around here that are finished using 16" OC studs. The white foam (polystyrene) is about a 3 or 4 R value if I recall correctly, so the total for the walls will be about 16-17. Since my entire basement except for one end is below grade, I am satisfied with this. The yellow foam is about R6-7, so the rim joists will be about 25-6, which is where the basic need is in the first place.

Here is my reasoning for this scheme:

Air (with moisture) flows from warm to cool. Warm air vapor will condense on cool surfaces. With an unfinished basement, the temps are the same for the air and the walls (concrete), so moisture in the basement will stay in the relatively cool air. However, during the oppressively humid warm months here, the warm moist air outside will readily migrate through the wood rim joists and condense on any cool surfaces. In my house the first cool surface the air found was the kraft paper vapor barrier on the FG insulation in the rim joist areas. Part of my project includes removing and discarding the old kraft-faced, moldy FG batts, prior to installing the yellow foam with vapor barriers on both sides. I aim to prevent the entry of warm moist air (and radiant warmth) with the yellow foam.

As for the foundation walls, when the basement is finished, the finished space will be relatively warmer than the wall cavities behind the drywall, so the polystyrene acts as like a beer "cozy" to keep any warmer moist air that comes from the finished space (or that leaks in from the outside in summer) from condensing on the cool concrete. The FG batts are unfaced to allow air to circulate within the cavities and back and forth into the finished space (such as it will do through the drywall), and also into the rim joist area.

So, that's my story, and I'm committed to sticking to it at this point. Would have been better to insulate the exterior of the house, foundation and rim joists, when originally built by the original owners, but that is academic at this point, so this retrofit is the best second choice solution I have come up with.

Would welcome comments and opinions. Though in my experience with discussing vapor barriers and insulation, if there are 5 experts, there will be at least 6 opinions on strategy.

[bkrahmer]

BTW, Johns Manville makes 24-inch unfaced batts. I'm planning on using them in my 2x8 exterior stud walls. 1 layer of the unfaced, 1 layer of the faced on the inside. Total wall insulation = 30+.

[that_guy]

Thanks for the responses... and sleepyhollow, that's a lot of great information. This kind of approach is very close to the one I've always dreamt of doing some day... some day that is, when I am able to start a basement from scratch. My current challenge is one with an existing basement, so that changes things dramatically.

[that_guy]

.... I understand we use 1x3 furring strips to fasten the rigid insulation to the walls and then these strips are used to fasten the dry wall, but:

1. Corners... the Dow site suggests using drywall corner beads on the inside and outside corners to fasten the drywall ... does this approach provide enough support?

2. What about baseboards? How are they fastened? Do we need to go back and find the furring strips after the wall's been painted? What if the wall isn't straight and we need nails closer than two feet apart?

Any help from people who've done this before is much appreciated. :lol:

...Thanks.

Anyone else have answers and/or advice?
...Thanks.
:)

[sleepy hollow]

For any inside or outside corners, I would simply add furring strips wherever you needed a nailer. So, at an outside corner, add a furring strip on each wall plane butting at the corner. Or, if the strips are wide enough, you may be able to get away with just one furring strip that extends out from one wall surface to meet the same "plane" as the drywall along the other surface. Then you can nail both "planes" of drywall to the same furring strip.

This would also apply for baseboard. Just run a horizontal furring strip along the base of the wall using a width you like to assure the baseboard can be nailed where you want to put nails.

This way you have nailers underneath anything needing support, and drywalling will be much like it would be with stud walls.

[that_guy]

Sounds like a simple and yet effective idea. But just to make sure I understand, the furring strips between panels of rigid insulation are installed in "channels" in the insulation. At the corners, I will probably not have these, would I notch these out somehow?

[sleepy hollow]

Well, assuming the insulation is not yet installed, here is what I would do. First decide on the particulars of the insulation you plan to use. I'll use my case - polystyrene - since I am familiar with this stuff and it is going to work fine for me, and is pretty cheap.

If I were to use furring strips instead of stud walls, I would install my furring strips (FSs) first and then the insulation. You could do it a section at a time, which I would recommend because you will gain skill as you go. You will understand what to do with both the FSs and the insulation better with each section.

But, don't forget that the furring strips are there mainly to accept and support the drywall. Drywall needs to be supported vertically and horizontally. It also comes in 4 ft. widths and 8, 10, 12, etc. foot lengths. So, you really want to establish stud wall spacings for the FSs. That means either 18" or 24" on center spacing with corners covered as well. If your insulation is set up for 18" spacing, then use that. Less cutting and taping of the insulation pieces. I am not sure what R-value you need, but if extruded polystyrene is ok, you may wish to consider the 3/4" Polar Industries (or whatever) stuff I am using from HD. With 1X3 FSs you only have 3/4" of depth anyway.

So, you would begin by drawing layout lines for the FSs beginning in a corner and moving out from there. there would be 2 FSs in each inside corner so you have 2 nailing surfaces for the 2 drywall planes. Making sure the FSs are plumb and spaced 24" OC, move out from there for, say 12 feet. That would mean 4 FSs along one wall 24" OC making the distance a bit more than 12 feet from edge of one FS to edge of last FS at 12 feet. These 4 FSs would hold 3 foamboard panels.

Also, fasten FSs at the base of the foundation wall horizontally. you can fit them between the vertical FSs or you can put the base FSs down first and build the verticals up from there. Up to you. You may also want to put FSs at the top of the wall for nailing. I probably would.


Now you can fit the foam panel in between the horiz and vertical FSs. Just cut to fit. Hopefully you only need to cut to length.

I would fasten the panels with heavy duty Liquid Nails (the LN for Projects that is located near the foamboard displays is NOT the correct stuff, believe it or not.) You need to go to the caulk aisle and find the blue HD LN. Just apply about 3/4 of a tube to the back of each 8x4 panel and press it up against the concrete wall. The glue takes a minute to set up and you can also lean boards against it or whatever to hold it against the wall until it adheres on its own. Use foil mylar tape that is for flexible ducting (I found it at Lowes - like strapping tape, but silver and very thin), to tape the foamboard edges to the FSs. This will also hold the stuff in place.

Anyplace you need to cut the foam, use a utility knife on a scrap piece of plywood on the floor. The polystyrene cuts easily, though not so neatly, but neat enough. You can use a drywall square to measure and cut.

To fasten the FSs I would use a Remington .22 caliber fastener gun. Very fast. Would take forever to drill for anchors or drive cut nails.

Hope this helps. Maybe more than you need to know.

[that_guy]

Thanks S.H. for your detailed explanation, so if I understand this correctly, we're almost creating a frame with the furing strips and then installing the rigid insulation between the wood....right?

[sleepy hollow]

I think so. That is the only way I can see to control where you have nailers in the absence of a full blown stud wall installation.

[that_guy]

Got it. Thanks again.