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Old 11-13-2010, 08:55 AM   #1
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retrofit slab insulation in finished basement... worth the trouble?


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.

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).

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...

Any advice would be greatly appreciated. Thanks!

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Old 11-13-2010, 10:40 AM   #2
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retrofit slab insulation in finished basement... worth the trouble?


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1. My original idea- Polyethylene sheeting over excavated floor, then 2" gravel, then 3" slab with radiant heat inside.
Ayuh,... I'd skip the gravel, 'n use 2" of rigid foamboard...

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Old 11-13-2010, 11:36 AM   #3
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retrofit slab insulation in finished basement... worth the trouble?


If there is any chance the original dirt was loosened, remove it, replace w/ gravel, and compact it. Gravel is a good capillary break and will compact; dirt may or may not. Radiant floor heat REQUIRES insulation under it; 2" is probably OK for your area, and most people recommend XPS instead of EPS. PassivHaus uses as much as 14" of EPS, though; dunno why EPS. 3" of concrete may or may not be enough by the time you have wire or rebar, then the PEX. Since I don't know for sure, I would run the "normal" 4" slab, but a good concrete place may tell you 3" is fine. I'd just hate to see a section of PEX sticking up at the surface as the concrete is hardening.... Poly on top of gravel, XPS on top of that. Stapling the PEX to the XPS is common, but some folks attach the PEX to the wire; don't know which is more common, but above the wire is, IMO, more risky of it popping out. Do NOT staple in the bends of the PEX. j

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Old 11-13-2010, 12:24 PM   #4
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retrofit slab insulation in finished basement... worth the trouble?


We use a similar method, although an inch of sand covers the hardcore or gravel to prevent a sharp stone piercing the poly. The pipework is put on top of the slab and covered with a 3 inch sand/cement screed. Insulation is also placed around the perimeter nowadays.
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Old 11-14-2010, 07:52 AM   #5
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retrofit slab insulation in finished basement... worth the trouble?


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Originally Posted by gbwillner View Post
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!
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.
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Old 11-14-2010, 08:17 AM   #6
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retrofit slab insulation in finished basement... worth the trouble?


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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.

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.

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?
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Old 11-14-2010, 01:29 PM   #7
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retrofit slab insulation in finished basement... worth the trouble?


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So why do most people use 2" foam?
Yes, because their delta T is not as small as yours apparently is. What is your soil temp, anyway?
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Old 11-14-2010, 03:52 PM   #8
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retrofit slab insulation in finished basement... worth the trouble?


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. 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".
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Old 11-14-2010, 07:34 PM   #9
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retrofit slab insulation in finished basement... worth the trouble?


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Originally Posted by gbwillner View Post
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?
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Originally Posted by jklingel View Post
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. 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".
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.
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Old 11-14-2010, 10:30 PM   #10
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retrofit slab insulation in finished basement... worth the trouble?


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. I think my numbers came out at about 102 degree water, to give a floor temp of about 70-72 degrees. Jomama: Yes, I was surprised when we looked at the blue foam at HoDePo, too. I assumed the more expensive 1.5" was because of the volume of 2" that is sold here, but I dunno. The stuff here is typically 25 psi; I was advised by an engineer to use the 60 psi under a garage slab, and I think that is also OK under footers. I know: that probably makes folks squirm a little, and I have not completely crossed that bridge, either, but I am further researching putting foam under footers. But, that is all another thread somewhere; let us not diverge too much. later. j
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Old 11-14-2010, 11:02 PM   #11
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retrofit slab insulation in finished basement... worth the trouble?


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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.

My basement water temp will be 90 degrees, and 140 degrees for the first floor.
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. The floor/soil R-value is like 47.
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Old 11-14-2010, 11:03 PM   #12
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retrofit slab insulation in finished basement... worth the trouble?


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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.
My water will be 90 degrees in the basement, and the pipes will not be under the slab but rather in it. The entire slab will function as a heat emitter.
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Old 11-14-2010, 11:05 PM   #13
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retrofit slab insulation in finished basement... worth the trouble?


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Yes, because their delta T is not as small as yours apparently is. What is your soil temp, anyway?
My delta T for that calculation was 60 degrees. Maybe an underestimation... not sure. I'm not sure what the soil temperature is 5 feet below grade. I would guess 55-60 degrees.
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Old 11-15-2010, 12:48 AM   #14
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retrofit slab insulation in finished basement... worth the trouble?


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____
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Old 11-15-2010, 02:31 PM   #15
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retrofit slab insulation in finished basement... worth the trouble?


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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____
Thanks for the additional feedback. I have to run 140 degree water for my first floor set-up because it will be under 1.5" oak floors, which are poor emitters (this will be a set up beneath the existing floor). As the heat goes through the wood the surface will be 75 or so degrees.

RE: the thinkness of the foam, my real reason for wanting to keep it to a minimum is that headroom is impacted (OK, cost too). I want to keep as much as possible- if I go 2" then I will have less than 7'4" ceiling height. Then the other problem... I guess I just feel uncomfortable doing something when I don't really know why it's being done. Yeah, the soil is probably cool under the slab, but according to the textbook in acts as a good insulator from the outside (freezing temperatures).. so once you've warmed it up it should stay warm. If, as Seigenthaler suggests, the R-value of a subgrade slab and soil (>2.5 ft) is 41.7, what do I really gain from an additional 2" of foam? the R value is now 51.7?

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