insulating a suspended hollow core floor

JEG

hi would any of you have any experience with pouring gypcrete on radiant tubing that is stapled to 1'' Styrofoam on a hollow core floor. I am told if I do that, then the gypcrete would crack because the Styrofoam couldn't support that 2'' overpour

hot_rod

All concrete cracks, gypcrete not as much.

What is the final floor covering? Hopefully a floating floor that would accommodate any hairline cracking.

Typical 15 or 25 psi foamboard can handle the weight of concrete and loads it will see in a home. It is used under 4 and 6" slabs all the time.

JEG

The flooring is a vinyl floor covering , I was told if I wanted to use insulation the overpour would need to be concrete (minimum 3 to 4'') versus the original 2'' gypcrete overpour. The ''concrete'' idea would take up way to much head room.

Jamie Hall

Depends a little on what you mean by "styrofoam". If you are referring to the rigid pink or blue stuff (polystyrene), you should be fine. If, however, it's the white stuff that comes apart in pesky little beads, no.

At least one manufacturer makes a specialty version of gypsum concrete intended specifically for radiant heating.

It's worth noting that in that type of application, gypsum concrete is very nearly as strong as fiber reinforced regular concrete, and will distribute point loads (such as small feet of heavy appliances) just as well. What you can't do -- but you can't do it with a regular concrete overpour either -- is put a load such as a structural column on it.

JEG

I am planning on using a closed-cell, lightweight, expanded polystyrene and hermetically sealed with an advanced metallic polymer facer on each side, with a compressive strength of 12 psi. Btw thanks for the quick reply.

Jamie Hall

Hmm. Well, that's a little lower strength than I would like to see. You might find some compression and cracking from heavier furniture with that (a good rough country rule of thumb: if a reasonably strong (not weight lifter!) person can pick up one end of it, it should be OK. To put numbers on it, that foam under 2 inches should be OK up to about 150 pounds per leg for furniture. 25 psi blueboard would handle 300 pounds).

JEG

Thanks, now that you said that, I would go with a 30 psi blueboard that I found, for a little more cost though

JEG

What would you say to no insulation in my application. the main floor is suspended hollow core with floor heat upper and lower level the basement will be r10 but I'm being told I don't need insulation on the main level.

Jamie Hall

Depends on whether you want more heat in the basement. If you do, no insulation would be fine -- you would have some heat going into what would be effectively a radiant ceiling.

I'm much happier with the stronger blueboard!

Zman

Keep in mind that radiant heating just radiates the heat. It knows nothing about gravity and which way you want the energy to go. Insulation will direct the heat upwards. It would also be advantageous to take the high mass slab out of the equation. The system will be more responsive and the space more comfortable if you insulate.
What do you know about the design live/dead load and deflection design of the existing structure? You really need to understand your structural limitations before proceeding down this path.

JEG

The building is an 8 plex with precast concrete walls with 6'' styrofoam sandwiched in a 12'' thick concrete panel, the attic will be sprayfoamed with 4 inches, my heating designer said he cant design a system for the main floor without insulation.

Zman

Sounds like a cool structure. Do you know if they sandwiched insulation in the floor?

JEG

We've put up a few of these buildings on the farm here, it's a pretty solid building. No they don't sandwich insulation in the floors, the floors are prefab with 4" pipe sandwiched in-between to make them lighter and more affordable

hot_rod

I would look into one of the dry over the top systems like the Roth panel. I agree you may want to disconnect the radiant from the mass of the structural slab. While not much, the 1/2 foam would give you some thermal break and a much easier to control radiant system.

JEG

I've looked at that option but the precast floor has a 1/2'' prestress bow so it has to be gypcreted to make it level

hot_rod

Maybe gyp first to get it level, then a dry system. You can feather gyp down fairly thin, it original intent was floor leveling, sound and fireproofing.