Originally Posted by gbwillner (Post 533498)

Hello all,
OK, I am finishing the basement on my 102 year-old house. It was a cellar, and, after exavating to the footings, I'm doing waterproofing (polyethylene sheeting, sump-pump with perimeter drain), and adding in-floor radiant heat. I am nearly at the point to installing the new slab. However, I just read an article stating that adding insulation under the slab would help with heating costs and comfort. The bottom of my excavated floor is about 5 1/2' below grade, and I live in St. Louis. Here are my options I am considering:

1. My original idea- Polyethylene sheeting over excavated floor, then 2" gravel, then 3" slab with radiant heat inside.

I wouldn't put the poly under the gravel, as it's bound to get punctured.


2. Excavate another 1/2", then gravel layer, then 1/2" R3 rigid foam insulation, then polyethylene sheet, then slab with radiant heat inside (new idea).

As others stated, 2" is usaully the min. You may be OK squeezing in a little less, but the minor cost savings is merely upfront, you'll pay the dividends in doing so in the long run.

What is the consensus of the location of the polyethylene sheeting? Should it go below or above the gravel? Does it matter? Is the rigid foam paneling worth the extra cost and labor? I don't think I will lose that much heat that far below grade...

Over the stone & under the foam is ideal typically.


Any advice would be greatly appreciated. Thanks!

Originally Posted by jomama45 (Post 533968)

If you lowered the basement and obviously plan to spend time down there, I think you owe it to yourself & all your hard work to at least put a few inches of washed stone (good drainage) and and a few inches of foam insulation, because once you go through all the labor of the concrete placement & finish, it's somewhat permanent to add either.

Originally Posted by gbwillner (Post 533983)

So why do most people use 2" foam?

Originally Posted by gbwillner (Post 533983)

Thanks guys. I did the initial heat-loss calculations for the basement without any insulation on the slab and I was not really losing much heat there since the temperature of the soil (5 ft below grade) is only slightly less than the heated room.

No doubt the temperature difference is minimal, but I think you're forgetting to factor in that the hot water flowing through the lines is about 120 degrees. I've never had to deal directly with heat loss calcs, but I don't think you can take the simple approach that you have because the heat source is below the concrete.

Here's the math- for heat loss through a basement slab, where
Q= 0.0024(A)(DT)
where Q= heat loss (BTU/Hr), A= floor area, and DT= change in air temperature from inside to outside ( from Siegenthaler)

I get 1220 BTU/Hr loss through the slab, which is practially nothing. The R-value of the soil between the basement slab and the outside air is roughly 41.7 (1/0.024). Similarly, I'm losing almost no heat in the bottom foot of my walls, where the soil makes the effective R-value 11.3 + 1.13(R(insulation))- where I'm using R10; making the R value in the lower part of the wall almost 23.

So why do most people use 2" foam? Unless they are on-grade installations, which makes the soil as cold as the ambient air?

Originally Posted by jklingel (Post 534219)

Also, since concrete has basically no R value, I think you will find that its temp is closer to the ground than the room's temp, if uninsulated. Try half the basement insulated w/ 2" XPS, and half naked, then tell us how each half feels this winter. :laughing: Kidding. I'd put at least 1" under it, and price the 1" vs 2". When I looked at HoDePo a couple of years ago, 2" was cheaper than 1.5".

Originally Posted by jklingel (Post 534448)

Oh, yes. Jomama reminded me that you are installing hot PEX. Refer to my above statement: In-floor radiant heat REQUIRES insulation under it. P, EOS. How much is what you'll have to calc, but DO NOT put a naked, hot slab against the ground. Again, 2" is typical; I'll be using 6". You should also do a full heat loss analysis, because you may not need 120 degree water.

Originally Posted by jomama45 (Post 534369)

I can't say I've ever seen that before, as I don't but insulation from HD. But last time I looked, they didn't have the higher density, 25 psi foam at any HD's around here. I personally would not suggest pouring concrete on the softer 15 psi. The 25 psi is available through actual insulation supply houses, and to be honest, costs me slightly less than HD's inferior 2" foam.

Originally Posted by jklingel (Post 534155)

Yes, because their delta T is not as small as yours apparently is. What is your soil temp, anyway?

Originally Posted by jklingel (Post 534488)

Some final thoughts, after the bullets. Good luck on your floor. j

"If I have to put insulation I will only use 1/2" if only because I have to. I still don't understand why it is necessary." •• Half inch is worthless, IMO. The soil is cool. Do what you want to, but we warned you.

My water will be 90 degrees in the basement... and the pipes will not be under the slab but rather in it. •• Yes, that is typical. Some folks run the pipe toward the bottom and some like it a tad above center line. Both have their reasons.

•• Just keep these in mind: An 85 degree F slab will produce 35 btu/sf.hr, while
75 degrees will produce 10-14 btu/sf.hr.

Keep floor a few degrees above desired room temp (if well insulated)

140 degrees F is extremely hot for radiant floor heating, some say too hot. If your house is real well insulated, I think that will that will cook you and produce a floor above 85 F.

You may want to read here, too. Good stuff. http://www.pmengineer.com/Articles/C...00f932a8c0____