# Is this a feasible way to insulate a basement floor?



## crankcase (Sep 21, 2010)

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.


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## titanoman (Nov 27, 2011)

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...


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## HandyAndyRR (Dec 2, 2011)

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.


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## CarlsonRower (Dec 5, 2011)

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?


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## joecaption (Nov 30, 2011)

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.


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## CarlsonRower (Dec 5, 2011)

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.


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## joecaption (Nov 30, 2011)

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


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## 95025 (Nov 14, 2010)

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.


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## jklingel (Dec 21, 2008)

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.


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## CarlsonRower (Dec 5, 2011)

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.


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## stuart45 (Jun 20, 2009)

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


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## jomama45 (Nov 13, 2008)

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.


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## Gary in WA (Mar 11, 2009)

Find your zone on the map or closest City below; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec001_par001.htm

http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002_par011.htm

Footnote “d”; http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_11_sec002.htm

Slab on foam; http://www.buildingscience.com/documents/reports/rr-0309-renovating-your-basment

Gary


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## CarlsonRower (Dec 5, 2011)

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:
http://www.diychatroom.com/f2/installing-radiant-heating-basement-general-basement-questions-125891/


Again thanks for all the input!

Chris


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## astersia (Jan 13, 2008)

We had this exact process done in the basement of our walkout ranch (1850 sf)

We had to dig for new footings and the basement bathroom pipes, so it "made sense" to jackhammer the entire basement and install hydronic radiant. It is the nicest area in the house during the winter, and so much more comfortable than any finished basement I've been in. 

But, we had great contractors who knew what they were doing, installed a 5 zone system and included dampers in the the upper level ducts so the basement hydronic is balanced seasonally with the upstairs. Then there is perimeter insulation for the absolutely necessary thermal break, and gravel/ foam underneath it all. Insulating from the ground is absolutely necessary or the heat goes into the ground rather than into the floor. We left steel trowel (smooth) finished concrete for the floor, no staining although we considered it. Just deciding where to place the control joints so they looked, and functioned, correctly required a meeting of the minds.

We even added a zone to the upstairs master bath because warming the floor up there while we had access to the subfloor made sense.

It was a HUGE project. Just getting rid of the concrete required a small bobcat in and out of our basement door for a while. We had a nearby area we could dump the concrete, otherwise it would have been $ for another process to haul it away.

This was in 1997-8, the system still works fine.

My impression is that the systems have come a long way since, but knowledgeable local contractors seem indispensable. If you go DIY, be prepared for a lot of heavy work. We are v experienced DIY'ers, but this was not the place for our skills - too much heavy equipment and man hours for that.

Have fun down there.


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## concretemasonry (Oct 10, 2006)

CarlsonRower -

You are just like 95% of the people around grossly over-rate the amount or even the need to insulate the floor area more than i minimal airspace.

Even if it is -10F, outside the temperature of a soil and slab in place is in the range of 55F and is constant. If you condition the space and cool, insulation of the floor reduces the thermal benefits of the floor in the winter. Most people assume the soil at the floor level varies closelt with the exterior temperature. - By the way, the theoretical required frost depth is not necessarily related to the theoretical frost level.

A little insulation may help if it is not carpeted.

I do not have an exact answer regarding the suitablity of the 2" new concrete, except that the successful jobs I have seen usually are thicker than 2", have wire reinforcement and are poured over a slip barrier the prevent any cracks in the old slab from telegraphing through.

Dick


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## astersia (Jan 13, 2008)

In case my comments added to confusion about insulating under the floor, the purpose of the insulation is to provide a thermal break between the
("55 degree") soil and the new floor. The soil should stay cold, while the floor needs to warm up. Unless there is a thermal break (we used 2 inch rigid EPS) the heat will be wasted in warming the soil, rather than the concrete floor. As I understood it at the time, there have been many disputes when the thermal break was inadequate and the floor underperformed.


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