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Old 10-30-2008, 05:50 PM   #1
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Insulating a concrete floor


I am going to pour a new concrete floor in my basement. Is it better to insulate under the concrete pad or insulate on top of the concrete pad? I am planning on putting tile down. I am thinking it would be better to insulate under the concrete to keep the concrete closer to room temp. I'm looking for some insite on the situation. Insulation type? vapor barrier? stuff like that.

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Old 10-30-2008, 05:57 PM   #2
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Insulating a concrete floor


I'd put as much foam under the slab (up to 2.5") as you can, Foamular 250 for example, with a 6 mil vapor barrier under the foam. Make sure you get the foam that's rated for under slab...it's not all equal and it's not all rated for under slab install.

As a side thought...while you're at it...why not run pex for future radiant heat? The pipe by it self will not be that much more...

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Old 10-30-2008, 09:50 PM   #3
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Insulating a concrete floor


Since this is a basement, I'm assuming the basement floor is well below the frost line. The temperature of the earth should be fairly constant year round - somewhere around 50 degrees. You won't get much value out of insulation at that point. If it were slab on grade construction in a cold climate I would absolutely recommend it due to the possible temperature differential.

I do agree that underfloor heating would be ideal for a basement - especially with tile floors.
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Old 10-31-2008, 09:03 AM   #4
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Insulating a concrete floor


I have a walk out basement. So about 2/3 of the basement is below the frost line. I would like to put tile where the doors is, which will be the coldest part of the basement.
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Old 10-31-2008, 09:34 AM   #5
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Insulating a concrete floor


Given your additional information, you make the case for in-floor radiant heat....check it out more before ruling it out.

While Stubborn1 is correct about the ground being 50 degrees (or there about) I have to disagree about the value of the insulation. More importantly the insulation will provide a thermal break between the ground at 50 degrees and the conditioned air at 70ish.

Make sure you have really thought through everything including the drainage under the slab, and I personally really like a heavy vapor barrier under the insulation, it keeps moisture levels down, and the concrete will not wick moisture from below. Some say vapor barrier and insulation is over kill...I say for what it costs at the time...why not? No damage can result from having it there....and it's way cheaper to install before the concrete is poured.

How much of this are you planning on doing yourself?
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Old 10-31-2008, 12:42 PM   #6
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Insulating a concrete floor


I am doing all the work myself. Its the only way I can afford it, how ever I want to do it right the first time. I am not planning on living in this house for ever, so I want to do it right but not spend money where I don't need it.

It is an older house, 1940's. The concrete that was in it was never done properly. It was patchy, 1/2" in some spots, 4" in other spots. So I took it all out. I dug it down 6". I put a tile in and a sump tank. Then I put about 3"-4" of gravel down. The tile has been in for about 2 weeks and there is no water in it. It is a pretty dry basement for an old house. The walls sweat a bit but thats about all.

Could I put a vapour barrier under the whole pad and only insulate about a 1/3 of the pad that is on the basement door side of the house.

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Old 10-31-2008, 01:05 PM   #7
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Insulating a concrete floor


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Could I put a vapour barrier under the whole pad and only insulate about a 1/3 of the pad that is on the basement door side of the house.
You "could", I wouldn't how many square feet are you talking about? If you're going with the in-floor heat...then do the whole thing.
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Old 10-31-2008, 03:31 PM   #8
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Insulating a concrete floor


when i dropped my basement apartment floor, this is the layering i applied:

1. @ least 2" gravel
2. 6 mil vapor barrier
3. 1.5" of dow blue foam insulation (3 x 0.5")
4. 6 mil vapor barrier again
5. wire mesh tied to anchor bolts epoxied into the footing
7. concrete (approx. 3")

this is in washington, DC. the floor is below the frost line but i insulated it anyway. the foam cost approx. $600 for the whole place.
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Old 10-31-2008, 05:01 PM   #9
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The basement is about 800 sq/ft

amakarevic, did you have to put down a thin layer of sand, or something like that, to prevent the stones from poking holes in the first vapor barrier?
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Old 10-31-2008, 05:18 PM   #10
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Insulating a concrete floor


My 2 cents worth is that 4 feet of insulation on the walkout end would be more than adequate. Unless you are in the far north, frost is unlikely to travel more than this!
Of course, if you are considering radiant heat in the slab with ceramic tile, then go for the insulation under the whole shootin' match.
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Old 10-31-2008, 06:39 PM   #11
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I like what Skippy is saying. 2.5" of polystyrene is a good Idea. The radiant heat is great but expensive. You are going to need a boiler to heat it.
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Old 11-03-2008, 10:00 AM   #12
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Originally Posted by Wildie View Post
My 2 cents worth is that 4 feet of insulation on the walkout end would be more than adequate. Unless you are in the far north, frost is unlikely to travel more than this!
Frost, while it is a factor, it's the least of the concern when considering a below grade living space. It's all about providing the thermal break between the conditioned space and the ground. I can't tell you what difference it makes, unless you've experienced it. Couple the vapor barrier with the insulation, and assuming there's measures in place to remove ground water...it becomes a great living space.

Here's a decent article...
http://www.radiantpanelassociation.o...cfm?pageid=420
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Old 11-03-2008, 10:05 AM   #13
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Originally Posted by Illinois G.C. View Post
I like what Skippy is saying. 2.5" of polystyrene is a good Idea. The radiant heat is great but expensive. You are going to need a boiler to heat it.
Welllll....not necessarily...I've heard that it's possible to provided the hot water heat with a water heater. I'm looking for someone that has real world application. I've read postings on the net...but never had discussions of anyone with experience.

In my house I have a Weil-McLain boiler that does all the heating for the house...domestic hot water, forced air heat, and in-floor radiant. I'll be putting up a shop with in-floor heat, and am considering using a HW heater there...I gotta do more research.....
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Old 11-04-2008, 06:26 PM   #14
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Insulating a concrete floor


Dear Oneida,
You've gotten lots of advice some of it good. Some of it is utter nonsense and based on some sketchy understanding of thermal engineering. I don't want to start a huge fight but take a look at this paper http://www.toolbase.org/PDF/DesignGu...dFPSFguide.pdf
Then you decide if it is correct to put insulation under a slab floor. Short answer: heated space NO. Unheated space, Yes. Long answer where are your cold bridges and can you isolate them in a retro fit. is your basement wall insulated? Are you in a frigid place? Is your perimeter drainage and backfill correct? Can you excavate your walkout side 16 inches deep to install insulation and drainage? How do you feel about Volatile Organic Compounds released by Polystyrene board? Have you considered finished concrete? There are many products out there, some toxic and some quite non toxic. Have you checked out the health hazards associated with solvent based ceramic tile cement? let us know what you decide.
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Old 11-04-2008, 07:34 PM   #15
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The basement is about 800 sq/ft

amakarevic, did you have to put down a thin layer of sand, or something like that, to prevent the stones from poking holes in the first vapor barrier?
no, that is absolutely not gonna happen.

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