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Old 11-15-2010, 03:01 PM   #16
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retrofit slab insulation in finished basement... worth the trouble?


OK... I did finally get some information... and the answer (regarding the insulation) can be found at...
http://www.radiantpanelassociation.o...cfm?pageid=420

Basically, the answer is.. it depends on where you are, and what type of soil. An example given was in Colorado... the savings from insulating the slab (given at 1BTU/Sq ft) for a 1000 sq ft. basement was about $14 per year. Given the cost of insulation (32 pieces of 2" foam running $25/piece) is ~$800, it would take you over 50 years to recoup the costs. However, if you live in Alberta (CA), the costs could be as much as $260 per year, meaning you'd get your money back pretty quick.

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Old 11-15-2010, 03:41 PM   #17
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retrofit slab insulation in finished basement... worth the trouble?


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I have to run 140 degree water for my first floor set-up because it will be under 1.5" oak floors... •• I wish I knew more so as to debate or verify that. Wood has a poor R value, so I don't see how the temp could drop 65 degrees through it. Apparently it can drop from (very approx) 120 to 85 in concrete a couple inches thick, so maybe it will drop that much through wood. Without real knowledge, I'd have to default to your info, though. Where did you get those numbers?

RE: the thinkness of the foam, my real reason for wanting to keep it to a minimum is that headroom... •• That is a valid concern, esp if tall people will be there. I think code is 7'4", but that may feel funny to some folks.

...what do I really gain from an additional 2" of foam? •• Again, I think (THINK) you'll find the slab will take on the temp of the soil because the heat will be running through it relatively quickly. The extra R from the foam will retard the rate of the flow and the concrete will stay closer to room temp. That is just my reasoning; no facts on that.
See above, and post any more info you discover. later. j
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Old 11-15-2010, 04:00 PM   #18
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retrofit slab insulation in finished basement... worth the trouble?


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See above, and post any more info you discover. later. j

I got all the math from "modern Hydronic Heating" by Siegenthaler. I think this is a MUST if you want to build a radiant system that actually works as advertised. Unfortunately, he doesn't say ANYTHING about insulating in below-grade slabs (although he does mention 2" insulation for on-grade heat systems).
The more time I spend on this project, the more I realize very (very) few people know how to do this properly. I can't even find a contractor that *actually* knows how to do this properly in my area. I spoke to a contractor in the So. Illinios area, and when I asked him who I could trust to help me on my project (after giving him details of my proposal)... He told me I probably knew more about this than any contractor so I should just do it myself! It seems a lot of people just "wing it", and it works. They probably don't realize that there are a lot of ways of screwing it up, that may result in 1- insufficient heating (i.e., throwing $$$ in the drain for a warm floor that doesn't radiate heat), 2- over-heating and wasting energy, or 3- damaging your expensive materials because you didn't do all the calculations. How many people working on these projects calculate the head loss through their system and buy the proper circulating pump rather than defaulting to the Taco 007? As evidence of #2 I will state that my home had a 140K BTU boiler installed 10-15 years ago by a professional (well-respected) local company (for the previously-existing gravity-fed radiant system). I thought there was something wrong with the boiler when it was set to 170 degrees, but never got higher than 105. Turns out my heat loss for my house is like 40K BTU/Hr. That means when the boiler turned on, I was wasting >3x the necessary energy, and my gas bills were eye-popping. It's not an isolated phenomenon- my neighbor had the same boiler installed by a different company- with a similarly-sized house.
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Old 11-15-2010, 04:05 PM   #19
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retrofit slab insulation in finished basement... worth the trouble?


Also....
I have to run 140 degree water for my first floor set-up because it will be under 1.5" oak floors... •• I wish I knew more so as to debate or verify that. Wood has a poor R value, so I don't see how the temp could drop 65 degrees through it. Apparently it can drop from (very approx) 120 to 85 in concrete a couple inches thick, so maybe it will drop that much through wood. Without real knowledge, I'd have to default to your info, though. Where did you get those numbers?
Wood actually has a pretty high R-value. But it is a terrible conductor/emitter, meaning it will absorb most of the energy you throw at it. Oak is even worse. That's why ideally you would start with a new floor that is concrete/gypsium. But if you have nice, 100-yr old hardwood floors you'd be crazy to rip them up.

RE: the thinkness of the foam, my real reason for wanting to keep it to a minimum is that headroom... •• That is a valid concern, esp if tall people will be there. I think code is 7'4", but that may feel funny to some folks.

...what do I really gain from an additional 2" of foam? •• Again, I think (THINK) you'll find the slab will take on the temp of the soil because the heat will be running through it relatively quickly. The extra R from the foam will retard the rate of the flow and the concrete will stay closer to room temp. That is just my reasoning; no facts on that.

See the link I provided 2 posts above...
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Old 11-16-2010, 10:34 PM   #20
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retrofit slab insulation in finished basement... worth the trouble?


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Originally Posted by gbwillner View Post
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.
All the textbook reading in the world won't help you if you don't know your actual ground temp under the floor. I'd guess that it's about 60 degrees, so that would be about a 30 degree difference.

As for the contractors you mention later on, and their inability to give you numbers over the phone: You can't be serious. You can't expect a profesional to give you all their hard earned experiences without either visiting the site or without proper compensation. BTW, calculating the heat loss by hand/form may not be compliant to code in your area, I know it's no longer accepted here.

Quote:
Originally Posted by gbwillner View Post

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.

We install 2" foam all the time under basement floors w/o loosing any headroom at all. You merely go down with the foam instead of laying it on the existing subgrade. This is one f the reasons I responded in the first place, because you mentioned waterproofing and a very minimal amount of "gravel". My suggestion would be 4" of free draining stone under the slab, tied into a proper interior drainage system, followed by poly and insulation. For what it's worth, dry clear stone does a great job of insulating as well, do to the free airspace it creates.
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Old 11-17-2010, 12:05 AM   #21
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retrofit slab insulation in finished basement... worth the trouble?


Whilst working on my grapple for my skid steer (the mutha is DONE!), I had this thought on the concrete insul/no insul deal here. I have not apparently effectively articulated my point, so how's this sound? I keep mentioning "cool" and you keep mentioning "R value". I think we are not on the same page here, so see if this gets my point across. If the dirt were a different temp than it is, it would still have the same R value, because it is what it is, structurally. Agreed? Let's say it is 75 F. If it's R value were then 0.01, would you insulate? I sure would not. And if the concrete-dirt had an R value of 90, but it was 20 degrees, you'd insulate, right? I'm not looking at its R, but am looking at its temp. Make sense? Am I reasoning correctly? By the time we get done figuring this out, you'll be done and I'll need a new Mac.....

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