Any one with experience with monolithic cement slabs or/and radient heated cement

[kutter52]

Calc, Allowing the warm slab to lose some heat below to the surroundings may indeed keep it from freezing and may save me from grief. And, I 'll save a grand on poly foam. If my tubes are closer to the top the top will emit most of the heat anyway. I don't think I have much water to deal with so gravel probably won't tap that much heat and I 'll stay toasty.

 

[CalG]

My first radiant slab (Kitchen Dining) has the PEX in a "thin slab" second pour over the first slab with a slip sheet between. No insulation under it all, but the grade needed to be built up with coarse gravel.
The heat comes up quite quickly. Not more than 10 minutes pass before the warmth can be felt on bare feet after the thermostat calls for hot water. quite nice on a frosty winter day.

Put your insulation money into keeping the cold from approaching at the sides. The ground is always trying to stay about 50F, it's the **** winter air that get's so cold ;-)

 

[kutter52]

Sounds good to me. Is it beneficial to put insulation around the perimeter of the footer portion of the slab?

 

[NewfieDan]

My BIL put insulation around the perimeter of his slab on his "man cave"(half garage,half bar). He has wall mounted fan heaters. His floor is warm enough to easily be comfortable in bare feet. Even in the middle of winter.

The insulation around the perimeter IMO is worth it.

 

[CalG]

Sounds good to me. Is it beneficial to put insulation around the perimeter of the footer portion of the slab?

YES! Down two feet , and out 2 feet if you can.

https://encrypted-tbn3.gstatic.com/...cS4dceVwx2hRF55Y5L72wX5uQOAZVgykwuj6EqWDx2jEt

http://oikos.com/esb/43/foundation.gif

 

[jgibbens]

Calc, Allowing the warm slab to lose some heat below to the surroundings may indeed keep it from freezing and may save me from grief....

You don't want to do that. Insulating the perimeter 2' down will keep the ground under the slab from freezing. The International Energy Conservation Code has guidelines for insulating a heated slab. For my area, it's R-15 along the perimeter extending 2' down and R-15 under the slab (I think) 4' in. I'm putting R-15 worth of XPS under the whole slab.

 

[blucoondawg]

If you don't insulate under the slab you will be wasting far more money in lost heat than you are saving on insulation, if I weren't going to insulate the slab I wouldn't run the in floor heat I'd just hang a reznor for occasional use when I'm in there working on something.

Concrete of any thickness is prone to cracking, thicker will probably be more forgiving of a bad prep job but it will still crack. The rebar keeps it from spreading, I prefer to lay rebar rather than wire in the slab, if using fiber I would use steel too because fiber can still crack. A 4 inch slab is plenty for a shop/garage unless you plan to bring in really heavy stuff, the slab with a grade beam is designed to float, it is not harmful to anything that it is above the frost line. All concrete will likely crack but it shouldn't ever spread if it's major cracking and settling then it wasn't compacted properly.

 

[SPIKER]

I did mine in multiple steps.

1. grading soil so water runs away from barn.
2. roll tamp and water in often & use for some time if possible otherwise tamp even more, used levels and story boards to get height proper & floor level prior to rest going in.
3. perimeter is thicker & wide to help support edges & hold cracking down.
4. I did insulate under slab in a layered affair of poly/plastic vapor barrier, 3/4" polystyrene, foil bubble bubble poly reflective insulation, and a final layer of HD 3/4 foam. Seams were taped on all poly, & foam layers to seal out radon/water.
5. laid out mesh on 1.5" plastic chairs and ran 2000ft of 1/2" PEX-AL-PEX (dont skimp on the PEX only stuff for in floor heating.) I have 6 loops in mine all under 300 feet per loop as required and with as few of bend & no overlapping runs.
6. tie it all up good using wire ties
7. Poured 6000 PSI Mix with Fiber sept 7 2011 & with 0 cut lines ~5" at thinnest area and 12" at edges. DONT cut where the PEX is at no point in that as if it does crack w fiber & mesh it will not go far why create a line to damage the pex>?
8. let it dry 12 hrs while keeping it damp then start flooding the slab to keep it hydrating slow for extra hard slab. then enjoy!

Mark

 

[CalG]

An interesting comparison of sub slab insulation
Request Rejected

 

[kutter52]

An interesting comparison of sub slab insulation
Request Rejected

I saw that. It concluded the more thickness of polystyrene foam the better for stopping heat flow downwards. I don't thick I can afford over 2" but that would work fairly well.