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

[kutter52]

I'm building a 24 x 36 garage in western PA. It gets pretty cold here so I want to insulate it and heat it economically. My plan is to wind about 1000 feet of PEX 1/2 tubing logically over the mesh prior to the cement pour. I'll using 4 circuits at around 225 feet a piece and entering & exiting the slab in one corner where my boiler will sit. Now it looks like my preferred builder wants to save me some money by using a floating "one pour" single pad where the footer is poured right with it but well above the frost line. He said about 18" thick with rebar in it around the perimeter where the slab will be a more conventional 4" thick over crushed limestone. Now I understand it is a good idea to insulate the slab by pouring on an inch or 2 of high density polystyrene foam which has a compressive strength of about 25 lbs per sq in. I wonder how much heat I will lose by convection into the footer which I don't think wold be a good idea to pour over polystyrene.

Does anyone have any insight for me?

Kutter52

 

[dstig1]

Start around post #110 and read a bit there. I haven't had time to update the thread in ages, but I did a bigger version of what you want to do... Sounds like you are in the ballpark at 1000' of tubing. Consider a tankless water heater as your heat source. Very easy.

http://www.tractorbynet.com/forums/projects/222268-nother-home-different-woods-11.html

 

[MossRoad]

Welcome to TBN! :thumbsup:

 

[Tractor Seabee]

Doing a monolithic placement is done a lot, also to do it in a continuous pour. The most important thing is control joints. Next is compaction of the subgrade. Joints need to be cut to 1/4-1/3 of the slab thickness and not exceed 12 X 12 feet pattern. DO NOT let them talk you into saw cutting the next morning as cracks are already started by then. Yes, all concrete cracks, you want it to crack where you want it not a random pattern. Either trowel in the joints or saw cut as soon as the finisher is done. Forget about wire mesh and use fibered concrete instead. Mesh never gets placed in the proper level in the slab due to walking on it. If you insulate under the slab for your PN temps I would use 4" of polyisocyanurate or urethane. It will carry more loads and better R factor. If heating economics is your goal too little insulation is as bad as no insulation. Also then you need to insulate the footing walls before back filling. Glue it on with asphalt cement or you are wasting your money on it.

I just did a mono foundation/slab 12' X 30' 4 weeks ago. No cracks. We do not have a frost problem here so that was not a problem so I left out the rebar on the footer. Are you sure 18" gets you out of the frost heave zone?

My dialog is based on the latest recommendations from the American Concrete Institute. I was a member back when I was a QC Manager.

Ron

 

[CalG]

I've got a 12 X 36 "addition" on the lower floor. Slab on grade, "Alaskan" style ;-) But I did dig a four foot trench and filled it with stone to assure drainage.

The pex went into the pour located on the wire grid. ( I don't give rebar any credit at preventing cracks, It only holds the pieces together after)_

Rigid foam went down over the hard tamped base (key detail, Tamp wet, then tamp some more) Just stay a foot back from the perimeter. The concrete will act as it's own barrier. But do insulate outside the footer! In fact, I did a "skirting" that extends out 2 feet about 4 inches down. enough to cover, and I drive the tractor over it from time to time clearing snow. No troubles

Hot water is circulated from the furnace.

I did a "thin slab" on the kitchen addition. No insulation under, but it has a traditional tile and stone tile surface.

Both systems work EXCELLENT.

eta

Frost heaving needs two things,

Water and freezing temperatures. take away either, and you can have no issues.

I go for the dug trench, filled with draining stone to keep the water out, then do a bit of insulation .. 1 inch or two of blue board.

 

[blucoondawg]

In northern Wisconsin we get plenty of frost and everyone uses the floating slab for garages, the only time a frost wall is used is on an attached garage. Generally all we do here is remove the top soil and haul in a good fill of sand or a good gravel then pour on that. 2 inch foam is the norm for insulation under slab. Usually a 4 inch slab with 12" around the edges

 

[NewfieDan]

Aa 4" slab will crack. I built a garage in northern Alberta in '99. The slab was 22x26 with a knee wall all around. Similar to a vertical mirror image of what you are building. My slab was a minimum of 5" at its thinnest.

Before the pour I tamped(damp not wet), then tamped again. Then I tamped and tamped again. I built a rebar mat tying in the knee wall to the slab. The rebar was placed in a grid on about 12" grid.

In 2010 I moved to where I am now. After 11yrs there were no cracks. I figure there are 3 reasons why not. We had winter temps regularly down to -40F. Summer temps up to about 90F. The slab was strong enough to put my 6500lb diesel truck on with zero issues.

1- well tamped soil. I can't stress this enough.
2- lots of rebar to help strengthen the pad
3-not the usual 4" slab. Most 4" slabs only the width of a 2x4(actually only 3-1/2".)My slab was thicker than most.

I didn't have the in floor heat like you are putting in. In hindsight I probably would have. My concern at the time was cracking concrete creating a leak. Buddies slabs all had cracks big enough to drop a wrench into.

 

[kutter52]

I plan to buy a small slant fin cast iron tank less boiler fired W gas. My tubes will be only 2 to 3 in down so I 'm scared to death to saw cut to promote cracks. One, I don't want to cut my tubing and two, cracks will separate and possibly break my tubing. I believe my guy said he uses fiber to reinforce and will put the screen in for me to tie my tubing to. I am greatly concerned about the height of the screen so I will use bricks to support it in many places. I almost think it might be a good idea to put a screen on top of my tubing. It would ensure my tubes stayed down and better control their depth.

 

[kutter52]

18 in is well above the frost line as we have 36 in to the frost line. My garage may be floating around my back yard. I don't want it cracking apart though so some serious rebar usage is no doubt paramount.

 

[CalG]

There is really no way that rebar can prevent a slab from cracking.
It can only hold the pieces close together after the cracks form.

Slabs crack in bending, the rebar is placed in the neutral axis. There is no "tension" on the rebar until AFTER a crack is formed.

But ti is nice not to see gaping cracks caused by poor preparation.

ETA

Some will contend that insulating the slab just incourages freezing. That is, the heat loss down into the ground from the heated slab serves to keep the sub soil from freezing.

Insulating the perimeter is probably more significant than the slab it's self. If the base material is kept dry, heat transfer can be minimal.

 

[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.