The first concern should be how much insulation is under the slab. I would not trust the contractor regarding the efficiency of the installation; he probably has no idea what it is.

Your two questions are indeed the big ones. Joanna, stick to your guns.

A heat break can be researched by using "thermal insulation" as your key words. Wikipedia has good information on this. You need any sheet or liquid-applied substance that physically separates in order to isolate the two layers. As you will find out on Wikipedia, any means to trap air in minute spaces is a good means to insulate thermally, so hard foam is a good heat insulator, and you may find some 1/4" or 1/2" thick. Some flexible sheet-rolled insulators are 1/8th" and 1//16th" thick. I believe Noble has one.

Any product known as a membrane may work well to isolate against heat transfer, but almost none are ever sold on that basis. Market demand is high for sound isolation membranes (not a concern in your basement!) and for membranes against water, moisture, vapor barriers, hygrostatic pressure, etc.

Some of these membranes will also do a fine job of insulating against heat transfer. Hope this helps so far. Just to confirm to you that heat transfer into Mother Earth is a Big Thing and a Constant Permanent Loss, and that height considerations are also significant. After you have scoured the market for whatever you can get your hands on, post again for comments on what might work best.

If you can see foam under parging around your foundation walls, you have a well insulated exterior wall. What thermal break might be under your foundation, only your builder and his employees know for sure.

Since you have mentioned height as a big concern, you will want a thin isolator and electric cables. Heat is heat is heat, and any means to produce heat is all the same in the grand scheme of things, as prices of various sources of energy go up (or down) relative to the other types, from one year or decade to the next. In my opinion you can disregard the nominal or apparent cost savings difference between heated water and heated electric cable. Heating water has other costs too since it makes you buy bigger heaters / boilers.

David

Thin insulation barriers are not very useful in retaining heat. See: www.energyideas.org/documents/Factsheets/PTR/UltraCBF_rFOIL.pdf If there's no XPS under the slab and you don't have headroom to spare, you'll be spending a lot of money heating the ground underneath the slab.

True. There will be heat lost to the ground. Nonetheless, do not conclude there is no point at all in having a layer of insulation: any break is better than no break at all. Think of wearing one layer of light clothing as protection against the cold: better than bare skin. Any small R (e.g. R2, R3) still directs more heat upwards than not. To place radiant heat directly on the slab means more and more heat is lost as floor temperature rises, not just 50% but a far greater fraction.

"thin" , "not very" , "useful" -- terms undefined. A separation layer is better than none. Not to overthink this, just to drum it in. "Something" is more than nothing, and this is the significant point, not whether the R factor appears low or high. Once your floor temperature rises, you want to prevent sending most of it into the ground, which is what happens when there is no insulation / isolation / separation at all.

Note that the definition of R rating testing does not allow one to distinguish between heat transfer occurring by conduction or by radiation. In many circumstances this is not critical. If you want to get into this level of detail, it will take a few dense paragraphs. Suffice to say that R ratings will be misleading and inaccurate for this application so do not rely on the apparent differences between R numbers. They are all small and that is OK. They are almost meaningless in this situation of a slab-floor separator with a thickness of from 2 to 16 mm.

It is obvious that with electric cables you have more room for a thicker insulation layer.

HTH
-david

Does anyone have a detail? Once the slab and ground have warmed up the system would perform fine since the temperature of the earth is not as low as the outside air in a cold climate. But if the heat was only used occasionally it might take a long time to warm up the room. 1/2" or even 3/8" of Styrofoam would probably be fine under a plywood underlayment that has grooves for the radiant tubing.

How much moisture is in the ground? Ground water. Damp earth is never "warmed up", as it conducts heat... Dry sand is better but still not great. I found this in a government agency study on a web site, a few years ago. Your first question is the best but the thread starter may not have an insulated slab so don't discourage him or her. It IS still possible to have heat on an uninsulated slab.

Yes a 1/2" is a good thermal break. Don't think things will be workable "once the slab and ground have warmed up ..." as this encourages thinking about warming up the surroundings.

-david

"thin" , "not very" , "useful" -- terms undefined.

These comments were made in conjunction with the referenced study of a brand of bubble-foil-bubble insulation promoted for use with in floor radiant heating.

That study concluded that:

"The insulating value of this product varies depending on what other insulation it is combined with and where it is installed on the slab. At best, its insulating value was...a maximum possible R-value of 1.5 to 2.0. This maximum is unlikely to be achieved in an under-slab application and impossible if the air gaps are not maintained."

In certain common installation situations, it yielded a negative R value.

Similar disappointing results on this type of popular thin product are reported here too.

Wow you guys, thank you so much for the responses. I have printed out the thread so I can process it mentally and figure out where to go in my research. The more I learn the less I know was never truer for me than this "little" project. But I am determined to make it work, and I am certain I will forget all this trouble the first time I walk barefoot across my heated floor and enjoy my warm toes.

I'll probably have more questions shortly, however!

Again, thank you all.

Joanna

The temperature of the earth under a building is usually close to the temperature of well water which is often about 60 degrees. If the design temperature for a room is only 8 degrees higher, it doesn't take much energy to do that but in order to speed the heat up cycle (assuming the heat is not on constantly) even a thin layer of insulation like 3/8" styrofoam will help but even a layer of wood underlayment designed to accommodate the heating tubing might be enough or a plastic waffle underlayment that supports a wood floor system. I would design the floor system first and then see if I needed additional insulation. Where the insulation is needed most is at the perimeter of the slab either horizontally under the slab or vertically against the perimeter foundation which is now required by all modern building codes so it is most likely already in place.

Here is a link that might be useful: warmboard

Laminate doesn't conduct the heat from radiant heat very well. Instead of transferring the heat into the room, it'll direct it down into the slab. Without insulation below the slab, it't gonna take a LOT of heat moving through the PEX to get the laminate warm enough to actually be the room's heating system. And most of that heat will be lost to the slab, as it will accept the heat transfer more readily than the laminate will, especially since laminate will require a thin foam layer for vapor barrier and cushion that will also act as insulation retarding heat transfer in the upward direction. Tile would be another story, as it conducts heat readily, but again, without the slab insulated below, the heat loss into the ground will be cost prohibitive.

In this situation, the best heating source would probably be peremeter baseboard heating of some type.

I don't know how anyone can comment on where the heat will "go" without knowing the method of installation of the heating tubes. Are they set in poured gypsum, grooved plywood underlayment, or some other support system? Designing one aspect of a system without addressing the whole is usually a waste of time.

I have tile, stone, cork, laminate and carpet over my radiant floors.

On stone and tile, the warm floor is very noticeable with bare feet. On the laminate, somewhat noticable, but doesn't call a lot of attention to itself. With cork [plank type], barely noticeable, and with carpet, not at all noticeable. I think the radiant still enhances the comfort factor since it prevents the flooring from removing heat from your body, and there's no heat units to decorate around or cold spots.

If your floor was going to be stone or tile it would be one thing, but given the rather extensive challenges in your particular home, the high costs and low ceilings, I wouldn't go with radiant unless you anticipate that this will be daily use space. For a game room used a couple times a week or guest rooms used a couple times a month, I personally don't think it would be worthwhile.