Heated floor - insulate around lift accomodations?

[dwvcfii]

hi all,

We're doing a 40x80 with heated floor. In the area where the two post lift will be installed, we dug a couple holes we expect to fill when the floor is poured, thus creating two integrated "concrete posts" beneath the lift posts.

The slab is fully insulated in a "bathtub" and a vapor barrier is installed EXCEPT where we've dug down to support the lift posts.

How critical is it to...

1) insulate the concrete in these holes?

2) preserve the vapor barrier in this area?

We obviously don't want these "concrete posts" (which will be exposed to earth as it is right now) to **** heat out of the floor or provide a "release" for all the moisture sitting under the slab, but it will take some extra effort to insulate and seal. Just want to know if we should bother.

Thanks for any help.

 

[Milton Shaw]

Heat rises so you should not loose much from just those lift footings. I would use vapor barrier and put some rebar in to tie the slab together. You might notice them cold if you walked barefooted around the shop but I don't think that would happen most winters. Too much steel splinters, shards etc to ever be barefooted in my shop. I even had metal shavings go through the bottom of my shoes several times.

 

[dwvcfii]

I hoped that would be the case. I believe they're planning to install rebar so I'll get them to extend the vapor barrier down there before they do that. Thanks for the reply.

 

[jumpingryan]

I would insulate around it myself to the same standard as the floor, and use tuck tape for all the joints for the vapour barrier

Contrary to the pervious post, heat is form of energy that radiates... it radiates from hot to cold irregardless of which way is up.

It is hot air that rises.

R

 

[raspy]

Insulating in the dug out areas will give you absolutely nothing as far as heat loss is concerned. Assuming the dug out areas are not directly exposed to the outside along a stem wall or something.

Remember, insulation simply slows heat flow through conduction. Any additional energy that conducts down faster in the non-insulated areas will not be lost. It simply heats the ground under the slab a bit faster than in the insulated areas and, sort of, increases the mass. Of course, this assumes there is no moving water or air flowing through your underslab fill.

Slab edge insulation is the type that prevents heat loss and most of that can be controlled by holding the heat back from the edge of the slab. Also, slabs on a level surface grade can be better than ones on a slope that have a stem wall. I'm also somewhat uncomfortable with structural slabs being fully supported by insulation. Especially ones that can have heavy equipment traveling over them. Insulation is compressible and not designed to last as long as concrete. Good solid compacted ground under the slab is reassuring.

A moisture barrier is usually required by code and is a good idea to prevent higher humidity under the slab from causing efflorescence in the concrete and mold or moisture damage to things sitting on it.

 

[babzog]

^^^ I'd been curious about that too. Foam under a slab. I like the notion of insulating, but, what's preventing that foam from crushing under the weight of the concrete on top and years of heavy vehicles being driven on top?

 

[raspy]

I'm with you.

I consider heat loss through the underside of a slab to be a non-issue, but structural integrity must be the beast we can do, always. I wouldn't bet the integrity of a slab on any of the insulation types I've seen. But with a lot of rebar and some square support areas left uninsulated, the slab could bridge and carry some load directly to the solid ground. This method is sometimes done with cardboard insulation that is designed to "go away" after a time and leave a void. In fact, I think it's called "void forming". But for very heavy traffic, no thanks.


John

 

[badgerboiler]

Insulation is specified just like every other mechanical application. How thick the slab is and the insulation to support it is determined by its intended purpose. To make it simple, insulation is a matter of percentages; the more you cover the less you lose in terms of comfort and efficiency.

Rigid insulation is specified by PSI just as concrete is specified. In the case of XPS the manufacturer writes the spec. so the educated designer or engineer (even a literate layman) may choose the correct rigid insulation for the application at hand. In this case it is slab-on-grade concrete slab with radiant floor (PEX tubing) attached for space heating.

The most important thing about pouring any slab is the substrate. "Verify that substrate is flat, sound, clean, and free of oil, grease, [objectionable air surface voids], [fins], [irregularities], [materials or substances that may impede adhesive bond]."


The common specification for a residential/light commercial slab is 25psi e.g. Owens Corning Formular 250 or equivalent. If you are really worried about "heavy" equipment and shun the help of professionals, Formular can be specified to 100psi read commercial jet hangers...no worries.

You should also ignore those promoting blankets or foil for use as insulation below a radiant slab. Yes you must insulate under a heated slab and yes the perimeter is the most important place to do it, as insulation is all about the temperature differential. When you heat a slab you make the difference between the inside and the outside temperature greater. Moving the tube further away from the outside wall is of little use as concrete has an equivalent R-value of R1 per foot.

Be careful taking advice from your peers.