I am building a detached garage slab-on-grade 24x32 feet. I have foam board insulation down and 3/8 rebar on 18 inch center grid. I have one 300 foot roll of 1/2 inch pex tubing and my goal is to heat the garage to 40 to 45 degree F. If I were heating this space to room temperature I know I would need more tubing. Can I do this with one 300 ft circuit and what are all the components that will eventually need to be purchased to complete the job? Getting ready to pour concrete soon. Thanks for the help!

What insulation and construction methods are you using in the walls and ceiling?
Windows and doors? R-values?
Are you insulating the exposed slab edges, or is the foam board just under the slab?
What is the thickness and type of foam board?
How thick will the slab be?
Did you use a vapor barrier?

Where is this located? That might affect the temperatures some.

Eventually, you will need something to make hot water with. You know.....a boiler or heat pump or whatever.
And, maybe, a circulator pump.
Among other things.

Is there some reason you can't spring for one more roll of PEX tubing? You can get there with a poorly designed system, the question is just how much energy (and money) will you be wasting.

oh so close to doing it right,
Foam... real insulation, just add pipe and H20
750 square feet would be well served with 500 more feet of pipe, (300+ 250+250 foot loops). Then should you or the next guy want to turn up the heat and tunes it would respond,
500 foot more in 2 250 foot loops will do it. 500 feet of barrier pipe cost about $130.00 delivered.
Cant stuff it in once concrete is poured.
Dan

Well I am doing this on a tight budget and I probably wont even have the heat up and running the first winter. The slab is 5 inches thick, one foot around the edges, and is entirely above grade for now. So the ground will not be pulling heat from the edges only the cold air. The foam board is 1 inch thick extruded polystyrene over a vapor barrier. The 2x4 stick building will eventually be insulated but not in the begining and the location is Morgantown, West Virginia.

So the ground will not be pulling heat from the edges only the cold air.

Doesn't the air get even colder than the ground in Winter?

Air temperatures will rise and fall. The ground radiates heat to space...always. We insulate the perimeter to fight the frost and the natural migration of heat from the conditioned space to the cold exterior.

A can of beer will get colder in ice water much faster than in the freezer because of the mass effect. This concrete pad will for the beginning literally sit 1 foot above the surounding ground and will for awhile be uninsulated and I am currently looking for a second 300 ft roll of pex tubing. So 2 300ft loops. I could use some advice on mixing valves, expansion tanks, manifolds, circulators thermostats, boilers and such.

The 1 foot thick edge will suck a lot of heat out of the 5" slab. Having foam board between will help heat loss.

ChrisJ

Uninsulated around the edges in the beginning but will have the 1 inch foam underneath. The concrete pad sits on top of an elevated platform made from railroad ties filled with stone that lays all over my property and capped with compacted gravel. I can't bear to look at ugly exposed foam around the concrete pad edges. I will find a way later to bury and insulate these edges of the pad where I can hide the foam under something

I don't understand what you mean by the mass effect. Is the mass of the can of beer different in ice water than it is in air? The reason the can of beer gets colder in the ice water than in the air is that the thermal conductivity of water (0.58)is ~200X better than the thermal conductivity of water (0.024)

Uninsulated around the edges in the beginning but will have the 1 inch foam underneath.

That's basically an uninsulated shell. It's going to run hot and cold. It seems as if your only consideration is to keep it from freezing right now. The 1" of foam underneath, which I'm pretty sure is less than needed in your area is not going to do much for you. You will probably need 10kBTU - 20kBtu per hour to keep it in the forties during the colder weeks. That's about 5 kWh on electric heat, less, of course when the temp comes back to the thirties or forties.. You can go through the cost and effort of setting up the radiant system, but much of the heat you put in that slab is going to be substantially wasted, particularly through the edges.

So I should get about a 25,000 BTU boiler and on cold days it will cost me about $11 per day or under a thousand for the winter because it wont be that cold all the time. I can live with that the first year. But I will eventually insulate the concrete edges and the walls and ceiling of the structure and that cost should come down considerably right? Any of you experts want to continue to help me design the rest of this thing? I am building this myself and my wife is really counting on me to get something up for her to park her car in out of the weather for the winter. I know more insulation under the slab would be better but I could just not believe the prices for the foam! Way high! I feel the foam board is substantially more than some of the bubble wrap some people were trying to tell me to use at 84 lumber and other internet sitess. I am looking at it and think it will help alot.

Posted By NH Homeowner on 09 Sep 2011 11:05 AM
I don't understand what you mean by the mass effect. Is the mass of the can of beer different in ice water than it is in air? The reason the can of beer gets colder in the ice water than in the air is that the thermal conductivity of water (0.58)is ~200X better than the thermal conductivity of water (0.024)

Didn't you mean "air" on that second number?

(And yes, thatzit.)

I could just not believe the prices for the foam! Way high!

What did they want for it?

I just paid $26.50/sheet for the 2" Foamular 150 (pinkboard) which is 83 cents per square foot.

2" Foamular 150 (XPS) is about R-10 which is generally appropriate if you need to put a heat barrier on a slab for radiant.  Some places need to go with more than that.

http://www.lowes.com/pd_304090-210-304090.0_4294858106_4294937087_?productId=3122447&Ns=p_product_prd_lis_ord_nbr|0||p_product_qty_sales_dollar|1&pl=1¤tURL=%2Fpl_Insulated%2BSheathing_4294858106_4294937087_%3FNs%3Dp_product_prd_lis_ord_nbr%7C0%7C%7Cp_product_qty_sales_dollar%7C1&facetInfo=

Formular150 is a 15 psi product and the 250 is 25 psi, astm c578 type x vs. type IV. I was not aware that 15 psi was acceptable for under concrete slab. I may just go deeper into credit card debt and spring for enough to get 2" under slab it has been bothering me all night and it almost seems that I have to do it more suitably in order to continue to get the advice I am seeking on here. I had always planned to eventually insulate the edges and walls and ceiling dont worry about that, I am just going to do it as money comes in rather than credit. Can anybody run the additional inch through a calculator and give me an idea what my heat savings would bw by stepping up to 2" under concrete rather than 1" or R5 vs. R10?

I was not aware that 15 psi was acceptable for under concrete slab.

Yup. You are pouring a minimum 4" slab with reinforcing, aren't you?

I have to do it more suitably in order to continue to get the advice I am seeking on here

It is GREENBuilding Talk. Code minimums are usually a starting point....

give me an idea what my heat savings would bw by stepping up to 2"

Roughly speaking, you cut it in half. In the first scenario, where you just want to keep the interior above freezing, the wastage doesn't seem like much, but when you start using the slab as a radiator, the amount is HUGE.

You are correct, it should have been air. Sometimes the fingers don't type what you want them to.