Insulate concrete basement floor?

zanp said:

I am finishing a basement that has a poured concrete floor and block walls. I am insulating the walls w/ rigid foam and covering this w/ drywall.

Do I need to insulate the poured concrete floor or can I simply install an appropriate basement flooring on top of the concrete? If so, what type of insulation? I don't have much room to work with (just over 7' to ceiling).

Thanks.

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This is the kind of question I like to send folks to Buildingscience.com for advice, as I think they are probably the best on how to do that kind of thing.

My personal tendency would be to say that you'd want some kind of insulation or that floor will be mighty cold and hard. If you NEVER have water issues, there is some stuff called "Dri-core" that is less than an inch thick, comes in a sort of tile - like form with tongue and groove edges that you can put down as a sub-floor. It has a layer of plastic dimples on the bottom that put a little space between the sub-floor and the concrete to allow water vapor or soil gases to dissipate, and an upper surface of OSB to give a smooth and reasonably warm substrate for floor covering. I think there are other comparable products that also have some insulation built in. This gets the floor covering away from the concrete, which will make it far more comfortable.

Gooserider

+1 on the DRIcore.

http://www.dricore.com/en/eIndex.aspx

Code, I believe, requires an R-10 in the floor which is 2" of foam. I put the foam down on the old concrete floor and then poured 3" of new concrete on top of the foam for a 5" taller floor with R-10 on my latest remodel.

You don't NEED insulation on the floor. Prior to my remodel, this concrete floor had carpet glued right to it for about 35 years.

Code questions aside, the biggest reason I can see for doing insulation of some sort under the floor is to provide a "thermal break" between the floor covering and the concrete... While it depends somewhat on the planned use for the space, if it will be any kind of "rec room" there are fairly good odds that you will end up with people sitting / lying on the floor, especially if there are kids involved. Concrete is a very good heat sink, and will make the floor very uncomfortable to be on as it will make the surface cold. Adding even a small amount of insulation, or an air gap will greatly increase the comfort level.

Gooserider

I offer an additional vote for some form of insulation, for reasons beyond comfort (especially given your location in a place that can be warm/humid): warm, humid air could cause condensation on the floor (which will stay cool because it is in contact with the soil underneath). With bare concrete, the condensation will re-evaporate; with some form of floor covering laid down, the floor covering may absorb/hold moisture, either degrading the covering or providing habitat for microbes, or both.

It's always good to insulate wherever you can, but the return on your investment will be less with the floor than with the walls, since so much less heat is lost that way.

The floor is a special case though, because you always need to envision the damage and needed repairs, if the basement should ever get wet. Some flooring materials can (supposedly) handle it, but many cannot.

yep as i type my feet are on the concrete in my basement office. it is cold. sometimes to cold for the feet. my house was put up in 1952 i had no choice. my ceiling is to short now never mind insulation then new floor. it's beyond my why houses were built with a 7 foot basement ceiling. what kind of cost would it have been to give 1 more foot. end of rant.
there is a advantage to this cold floor. in the summer when it's warm upstairs i come down here sit put my feet on the floor and give it a few minutes and it seems to cool the whole body down a few notches

Plagued by this question last winter. We do get a small amount of moisture along the floor on the back wall of the area we wanted to finish. We also wanted full 3/4" solid pine plank flooring (18" wide from one of our trees). I went with Dricor. It is a floating floor, allows for some degree of expansion/contraction, provides a vented vapor barrier between the cement and wood planks, and provides a surface to screw the planks to the Dricor without penetrating the plastic dimpled concrete contact surface of the Dricor. I used the square drive siding screws, drove the heads just below the floor surface, very small heads, and almost unnoticeable.

All I can say is that after one full year, no issue of any kind. No splits on any of the plank flooring. Time will tell if adequate moisture protection. So far very satisfied.

I had ceiling height issues when we went to change the garage under to a bedroom on a raised ranch.

Squeeked by with thin nailing runners , closed cell foam , thinnnest ply allowed and then Pergo . Didn't want laminate, but anything else was too much height.

The only place I've seen insulation under a floor here is on a slab.
But basements have to have interior drains to a sump pit.


I've had tile and also carpet squares glued to bare concrete floor. The tile was cold on bare feet and those thick circular carpets help.

Depending on the type of basement, sometimes insulating the perimeter is the most important. Especially a walk-out where the basement floor comes in close contact with a outside wall. Otherwise a full basement should have 6-7 feet of dirt backfilled around the permitter helping insulate. Saying that, cement floors are cold and depending on what kind of flooring your are intending on using may or may help insulate to a comfortable level

I finished my basement in a house I owned 3 homes ago and put the carpet (heavy padding) directly on the slab. It was freezing during the winter - but nice in the summer. No water issues. Worked fine.

Saw a TV show recently (holmes on homes). They put down a foam board (like exterior foam board) glued, taped all the seams, and then put down 5/8 tonge and groove plywood (pneumatic nailed into the concrete) I believe. Looked like a solid set up. Gonna add to the expense - but if you are gonna be living there and enjoying the space...... spend the money and do it right.

My downstairs, on slab, is going to get foam, sleepers, foam in between the sleepers and then plywood.

Are you going to tapcon the sleepers down to the slab?

I was not as I was going to essentially frame out the floor, like a wall but with the 2x4s on the flat. I have to do the walls to so i figured nothing will reall move once the subfloor is on...

If you do use rigid foam sheets for insulation, TAPE THE SEAMS so that condensation does not form underneath at the seams and cove areas.

I used platon, added a layer of insulation and 5/8 subfloor. I think that system cost me about the same as dri-core. I'm happy with the results.

http://www.systemplaton.com/flooring.html