Hi illinigirl,

The standard insulation used under basement floors is rigid insulation (XPS -- extruded polystrene). It comes in 2 inch thick often blue pieces of styrofoam. Dow manufacturers it among others. It is not spray foam.

We used it in our build -- we are in southern Canada. The labour to lay the rigid insulation sheets was not high. The guys had to dig away a bit more gravel so the basement ceiling level was not compromised. They used the red tuck tape to attach the sheets to each other. This allowed the concrete flat workers to pour the floor with wheel barrow loads.

I am not sure yet what a difference it will make as we are partway through our build. We have hydronic in-floor heat that was just turned on for the basement level. The floor is wonderfully warm (mostly due to the in-floor coils). We were required by code to place the rigid insulation because of our planned in-floor heat. Otherwise, I understand, the heat from the coils would not just heat upwards and instead would have been wasted heating underground?

We did place the rigid insulation around the perimeter of our our unheated garage. I will go and step on the parts of the garage floor that have the insulation vs the parts that don't to see if I can detect a big difference and report back.

Our architect spec'd for this rigid insulation under the basement slab. At the time, we had not planned for the in-floor heat. In other words, he felt it was a good practice even without in floor.

The rigid insulation sheets are not cheap. We paid $25.00 a sheet (each one is 2 feet by 8 feet). We are in Canada where everything seems pricier. You can calculate the area needing coverage and see how much your builder is charging for labour vs material to help you decide if the price is reasonable. Don't forget the tack tape is also not cheap.

Best of luck.

Carol

We had our basement floor insulated as well...it's styrofoam sheets designed to carry the weight of a concrete floor. Not sure what difference it's made as you can't rip it up and try it first without and then with. However, we did finish off our lower level and the floors do feel reasonably comfortable and not bitter cold like a basement floor might. We don't have in-floor heat. In fact, with the lower level bermed and southern facing with lots of windows, we neither heat nor cool the lower level (though we have facility to) and it maintains a temp of between 63 and 78 all year long.

Thanks Carol,
I would be interested in hearing your findings. My builder said that typically the extra insulation that we asked for would be for those putting w the in floor heat like you but that would run us an extra 15k.

Annie what part of the country are you located?

illinigirl,

I went to the garage to test the insulated parts vs the non-insulated parts of the garage. Sorry, but it was hard to be sure if there was a difference. The part that was insulated may have been slightly warmer than the non-insulated part. On the other hand the insulated part is beside the outside wall, so one would expect it to be colder. In other words, the insulation MAY be offsetting the difference.

One observation that I can be more sure of is the basement floor. Our project was an addition. When we added the in floor heat, we removed the old concrete floors and re-poured this time placing in rigid insulation and the coils before the pour. Given they only turned on the in floor heat a few weeks ago, and we lived with new concrete floors a full year without the in floor being turned on, I do feel that the basement floor prior to functional in floor was slightly warmer than it had been previously. In other words, I believe the rigid insulation was making a difference in the rooms where full rigid insulation was used under the concrete.

Sorry my observations cannot be more conclusive.

Carol

This post was edited by OntarioMom on Sat, Nov 2, 13 at 16:37

We are in so. New England.

I put 2" XPS below the slab without heated floors. The cost of the heat was not worth the expense for minimal gains in my build. The floor does maintain room temperature much better as it creates a thermal break with the cooler earth below. It will feel cooler to your feet since anything cooler than your body temp will feel cold, but its certainly warmer than a non insulated slab. It is currently non a finished basement yet, but friends that have been down there with socks have commented on how much warmer the floor was compared to their home and assume its heated.
Obviously I am always an advocator of putting as much insulation and air sealing in your build as possible, so it would be worth your expense. You can only do it once at this point.

For an in depth discussion about this very issue, see this exchange of views in the Journal of Light Construction forum.

If you have a potential for radon in your area, I would recommend putting the pipes in now while the floor is open. We did and are glad we did...we did have a radon problem, made use of the pipes and now our radon reading is very low.