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CarlsonRower 12-04-2011 07:00 PM

Is this a feasible way to insulate a basement floor?
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I am planning on doing a full basement renovation (currently it is 100% unfinished).

Prior to doing any work I want to insulate it correctly.

I want to use 2" foam board on the floor and walls... tape all the seams... then sub out concrete work... which would be laying down a floating concrete pad for stained concrete (2" thickness?).

I will post the concrete questions in the correct section.

My question here is... is this normally done? Is this a disaster waiting to happen? Will this give me mold etc? Is R-10 on the floor overkill?

I live in Metro Detroit Michigan and we've had some of THE WORST flooding I've even seen in all my time here... my basement has been BONE dry. So I would just be concerned of some general build up etc.

I am VERY new here so let me know what you all think!

Thanks again!


crankcase 12-04-2011 07:18 PM

I saw on TV an episode of this old house where they insulated a basement concrete floor. 6mil poly was laid on the concrete, then 2x4's spaced 16" OC and secured with Ramsets, 1.5" foamboard in the spaces,then 3/4" subfloor of your choice. I believe they glued and screwed the subfloor down.

I would plan out your interior walls and have extra 2x4"s in place for securing the walls to the new floor.

You would loose less headroom this way and I'm not too sure about the integrity of a thin 2" concrete slab over foamboard.

titanoman 12-04-2011 07:21 PM

I'm just a carpenter, so I don't know. Has pouring concrete over rigid insulation been done before? It seems to me you're going to end up with cracks all over the place as rigid insulation has some give to it. I'm sure a concrete guy will come along soon...

HandyAndyRR 12-04-2011 10:13 PM

I am not a seasoned concrete vet, but I have some experience with it. I would not be comfortable pouring 2in on concrete directly on foam board. I would think too much give in it. Also, I am not sure, but there is a chance that the concrete would not react well with the foam, as in it may melt it. The concrete curing process produces heat, but that is not my main reason to think it may cause and issue. Cement is caustic and may chemically "burn" the foam. I am not sure about this, just a thought, I'm sure a concrete guy will know the true answer. You could also do an in floor radiant heat since you are doing the pour anyway. Just my .02 on the situation.

CarlsonRower 12-04-2011 10:17 PM

I know that in floor heat is a possible solution but the basement is about 1500sqft.... so the idea of heating up that much space seems like it would bump up my utility bill considerably. I'll crunch some numbers to find out thou!

Is there anyway that I could insulate the floor? And is there anyone that did a basement reno in a Midwestern area WITHOUT insulating the floor? Is this something I should even worry about doing?

joecaption 12-04-2011 10:27 PM

I live in a slab house so the floors are always cold so I came up with the idea of trying something I saw in a magizine I had that's just for contractors.
I laid a layer of 3/4 blue foam on the floor and taped the seams, then went over that with 3/4 Advantec subflooring, I used 4" Tap Con screws in predrilled holes to force it all down tight to the slab. I happened to have some Allour vinyl strip flooring so I used that for the finished floor.
Once I installed my base board you can step hard right next to the baseboard on the floor and the floor does not move at all.
The floor is now much warmer.
Concretes pored over foam board all the time in my area. But it would be 4" of concrete not 2" and as everone said you will loose head room.

CarlsonRower 12-04-2011 10:33 PM

GAH!!! 4" slab? THAT hurts.... :(

What part of the country are you in? Curious if 3/4" foam board would be good enough for my part of the country. I can't find any websites that talk about required R-value for a basement floor :wink:

I would be ok with losing SOME headroom because I am not putting in a drop ceiling but if we're talking over 4"... that is getting hard to handle heh.

joecaption 12-04-2011 10:37 PM

I'm in VA but any place I've lived it's been 4" in driveways, garages, even walkways.

DrHicks 12-04-2011 10:42 PM

I've laid hardwood laminate in a couple basements, using the appropriate underlayment. It doesn't warm the floor or anything, but it's sure a heckuva lot better than bare concrete. And the price was right. I wouldn't hesitate to do it again.

jklingel 12-05-2011 12:48 AM

I covered an unheated basement concrete floor w/ two layers of 1/2" OSB (I'd use plywood now) that were screwed and glued together, staggered joints. The OSB just sits on the floor, on MANY, MANY plywood shims (the cats who poured the floor did a horrible job many years earlier). I installed tap-and-glue laminate flooring and have had no issues. Did this in 2000. The floor is marginally warmer, but the area with rug and pad on it feels much better. Were I to do this again, I'd put down 2" of 60 or 100 psi blue foam, 3/4" T&G plywood, 1/4" of underlayment particle board glued to that, and drill holes to bolt the flooring to the concrete. (Less than 2" of foam is of marginal heat benefit, IMO.) I am not experienced w/ concrete enough to know if 2" over foam would crack, but I would at least use 100 psi blue foam if I went that route, and something to ensure that the old concrete floor is FLAT (tough to do, likely). If you want heat, they make plywood pre-grooved for PEX pipe; 3/8" PEX is virtually the same as 1/2" for heat output. You have to heat somehow, and radiant floor is pretty nice, but it limits what flooring you can use over it.

CarlsonRower 12-05-2011 06:23 AM

Hmmm good info on the less than 2" being a waste of time. I thought as much looking at the R-value but wanted to make sure.

For the bathroom I am going to put in electric radiant heat but I was hoping to avoid it everywhere else.

Looks like i'll have to call up a few decorative concrete guys to see how feasible this plan is.

I am dead set on using the stained concrete because of the look, durability and easy cleanup of it. :)

stuart45 12-05-2011 01:42 PM

Minimum 65mm reinforced sand/cement screeds are sometimes used on top of insulation here, although 2 in sounds a bit thin.

jomama45 12-05-2011 05:58 PM

Personally, I believe that putting down 2" of insulation is a waste of money, and you'll never recoup any of the savings, considering the ground is typically 55 degrees year round under your basement. Your're really only fighting 10 - 15 degrees, unlike a roof for example, where the temp difference can be 70+ degrees in winter. Unless, of course, you were installing hydronic heating under the floor, which would require the insulation.

I also wouldn't suggest pouring the floor under 3" thick either especially over foam. If you decided to do so, I'd suggest drilling between 5-10 holes in each sheet of foam so the concrete had direct contact with the concrete floor below. And 25 psi foam is the min. rated foam for this.

Gary in WA 12-06-2011 10:19 PM

Find your zone on the map or closest City below;

Footnote “d”;

Slab on foam;


CarlsonRower 12-07-2011 12:50 PM

Great info guys!!!

After talking it over with people I think I am just going to go with a radiant floor system. Really the price is a few hundred more (not including the high efficiency water heater) and I will actually get what I want!

I have posted some questions regarding that on this thread:

Again thanks for all the input!


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