garage slab radiant

archibald tuttle

Planning a large radiant slab in truck/equipment maintenance building. I am following up on on recent posts regarding a thick slab with radiant tube toward the bottom.

The more of this I do, the more insulation under the slab I favor. But this is the first non-residential application to a slab I have tried.

I'm wondering how the potential point loads of heavy vehicles and jacks interacts with the system.

My first inclination is to disturb the substrate as minimally as possible. Put in column footings and supports for car lifts and such on their own footings and isolate those from the finish floor with 1/2" high density isocyanate foam.

Then level and compact the area to receive the radiant slab. vapor barrier (how many mil plastic or other new inexpensive miracle material), put 2 2" layers of pink foam with offset joints, 6" mesh and 8" slab. I tie the tube to the mesh as most folks do. Not too worried about pulling the pipe up higher in the slab as long as the insulation is in place.

Just looking for emperical results for folks who may either have handled these projects in the whole, or who might have put in heat loops for garage/industrial projects handled by other contractors as to what you've seen and how these installs have performed over time under load.

Two other detail areas of interest:

around the edge of the slab. This building will be buried on three sides and on those sides the slab will come up against footing. My current plan following from other threads here is to use 2" high density around the edge of the slab. This is no problem as I plan shelves and benches around these edges so that detail can be covered.

The area that seems the most difficult is how to detail the joint between concrete aprons outside the garage doors on the exposed side. A 2" vertical foam joint seems like a lot. I suppose I could cover it with steel channel slotted over the insulation so I don't get point load damage to the foam separation. Even though the steel would have high conductance, I was hoping that the fact that it was a separate piece, not pouring into place would give a fighting chance not to wick a lot of floor heat out the doors. Or I suppose I could go with 2 or 4" of insulation and form a recess lip in the apron and floor edge, put 1/2" high density on the lip and a 1/2 or 3/4" steel plate to cover the gap or ...???? any other clever ideas for this transition.

Thanks as always.

Any other clever approaches to this detail?

Brad White_185

Brian

Radiant is ideal for a garage- good choice.

Just a couple of factors of note:

Your biggest help in this mix is the compression strength of your concrete. Normal garden-variety concrete has a compressive strength if 2,500 to 3,000 PSI. You can get 4,000 and even much, much higher. The German Autobahn (as opposed to what, the Maldives Autobahn?) uses a mixture over 10,000 even 19,000 PSI I have heard.

Your idea of grade beams or haunches at support points makes sense but then your limiting factor is the compressive strength of your insulation which can be in the 20 PSI range. The distribution of weight typically follows a 45 degree angle of repose away from the point of contact. In other words, using steel plate spreaders will spread out that weight. Depth and rebar mesh...I am not a structural engineer so will stop here and suggest you contact one.

While I am a huge fan if under-slab insulation, I can see insulating the edges to as thick as you can stand to, insulate under the slab similarly but maybe the haunches, if entirely internal and where most weight will bear, be left uninsulated? Presumably those will be toward the interior, the area of least heat loss.

Tom Manton

Slab detail

If this is to be slab on grade I would opt for more perimeter insulation if you are in a cold climate. I also frost protect the slab by placing insulation below grade angling down at about 15 degrees for 2' around the perimeter.

Footings for point loads are a good idea. Small footings 2'x2'would not concern me too much from a thermal conductivity standpoint as long as there aren't too many. My slab is 6" and will take a large point load even over the foam, but footing for a truck lift are prudent ( my 10,000 lift is ok anchored to the floor over the insulation)

As far as the apron goes, as long as you break the conductivity with a .5" expansion joint of foam I wouldn't lose much sleep over it, just insulate as much as you can elsewhere and insulate under the apron outside ( remember that where the doors sit will be exposed to the outside too) No insulation system is perfect, ist all in the details.

Couderay

Subgrade

Make sure your subgrade is up to par, because your concrete is only as good as your subgrade. I would also use rebar instead of mesh,and chair the rebar to your desired heigth then place tubing on top of rebar. Drainage is also important to keep water away from the slab.