Insulation for thin slab over suspended hollow core

cg_3

What do you guys suggest for insulating under a radiant heat that has thin slab cement poured over hollow core floor? Customer, the nicest customer a guy can have, is building cement home. Has walkout basement and the main level is a cement hollow core floor. He using thin slab cement vs gyp. Because the hollow core panels are bowed for structureal purposes, the center floor is 3" higher than the outer edge. Most of the basement is living space. He will finish the basement ceiling with drywall hung 12-18" from hollow core floor. We both rather insulate on top of the floor vs under for quicker response. No room for foam at center of house. Would you guys use Insultarp or a foil bubble type insulation? Please educate me on what will work best in this application.

hr

Sounds like

he will end up with an uneven thickness slab above due to the camber of the panels? Do they figure this will "decamber" as loading is added? Is any movement is possible you will want a slip sheet, cleavege membrane. NobelSeal CIS (crack isolation sheet) is one brand.

So it sounds like at the center you will have a slab 1-1/2" thick, and 4-1/2" thick on the edge?

How much "on top" dimension could you give up for insulation. Think I'd go with 1" foamboard for true R value and consistent level-ability.

Better check with the gyp manufactures for that thickness over foam or insulation. A hybrid concrete mix or polymer blend might be better. JLC this month has a good article on thin polymer mixes and applications.

Get a blessing from whoever supplies the overpour. IN WRITING

Might even consider Isoynene sprayed on the botton, for a second layer of protection.

What's the budget look like

hot rod

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cg_3

Thanks, hot rod - more info

Thanks for responding. I've just started following "on the wall" the last month or so when I get a chance and I especialy agree with and value YOUR opinion and experience.

more background: he is DIY General contracting his dream home. He is DIY to insure it gets done right & for fun I thingk. Taking his time and spending the money to do it right. To answer your budget question, he will spend what ever it takes to do it right within reason. One of the reasons that makes him a great customer! I work for company that supplying him the material. I've installed the copper & boiler, munchkin, myself on side. I plan to send you pictures when I'm done to get approval or to change where necessary. I too want to do my part right! So he is laying tubing and conctracting pour over himself.

my bad. talked again to customer. only 1" camber from center to edge. He has allowed 3" for high point and 4" for lower point. he can go higher but will mess up doors.

He wants to use cement vs gypcrete. I sent him the cement "potion" recipe from RPA basics book from '99 pg. 82. He will have his supplier follow. He's also talking to MI concrete association.

The 1" camber will still be there after the pour. He has purosely over built structural. he has a huge I beam down center, oversized to allow for cut outs etc.

Good point about the bond breaker. We thought the insulation, foam, bubble whatever would accomplish the same. does it? do you need bond breaker when pouring over cement as aposed to plywood? That question has been haunting me.

Can we use diff size foam board? thicker on edges? Is foam better than foil bubble. Whats your thoughts on foil bubble for this application as well as in general like staple up?

Hope you're still out there and your response is greatly appreciated. Thank you in advance.

Earthfire

overpour

I would use 1 in. styrofoam (blue, pink or green). That would serve as the slip sheet and use a pumpable concrete formula with small agregate and fiber. 2 in. of concrete will handle any residential load applied to it in an overpour like this. The 1 in. foam would also serve as a stapling base for the tubing. Do not use the foil backed sheating insulation in underslab applications as the foam absorbs moisture over time and looses its insulating powers. Also several of the bubble foil manufacturers have told me not to use bubble foil combo in slab to slab or slab to ground applications as they are designed to work with an air gap. As it was explained to me ,when in dirrect contact with the concrete and soil they are a heat sink that moves heat, with an air gap, as under joist, the aluminum component reflects the radiant energy. In installations where there was concern with traffic on under 2 1/2 in. concrete over Styrofoam, we have used light weight wire mesh (galanized chicken or livestock wire)attached on top of the stapled tubing. Make sure you run air pressure test on your tubing (we use 80 to 100 psi.) during the pour in case one of the concrete guys cuts the tubing with a rake.

cg_3

Thank you Earthfire. I have another question not related to this customer. We primarily deal with car wash industry nationaly. Bread & butter is de-ice packages. Common: one has old self serve car wash and existing de-ice is swiss cheese or doesn't exist at all. Many guys are pouring a 2nd slab on top to advoid the tear up. QUESTION: do you need a bond breaker in this application?

cg_3

Thank you Earthfire. I have another question not related to this customer. We primarily deal with car wash industry nationaly. Bread & butter is de-ice packages. Common: one has old self serve car wash and existing de-ice is swiss cheese or doesn't exist at all. Many guys are pouring a 2nd slab on top to advoid the tear up. QUESTION: do you need a bond breaker in this application?

Earthfire

Depends

If the old slab does not have any major cracks no, but an isolation membrane is not a bad idea to stop any cracks in the old slab from telegraphing to the new pour

hr

I think the

foam would be a safer bet, more of a track record under concrete. Suppose the bubbles break, or lose pressure?? I think the foam board or tarp would be an adequate slip sheet. Contact DOW, they must have info for this exact design.

As to the camber, you could use 1" around the edge, 3/4 on the hump. The more consistent the thickness of the over pour the better the final product.

Concrete tips. You want 3 times the aggreate diameter above the tube. So you would want a pea gravel mix. However the smaller the aggreate the less strength and more shrinkage.

I think I would beef up the fiber mix. Generally it is mixed 1-1/2 lb to a yard. Consider 3lbs/ yard, this is what the concrete counter top builders use to reduce shrinkage cracks as much as possible.

You are wise to involve the local batch plant. There are hundreds of admixes to use with concrete for special applications, they are the ones to stand behind the product!

Also try any of the online concrete sites for expert opinions, unlike mine .

I just make stuff up as I go

hot rod

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