Biggest slab size with radiant heat

[Cd_r3105]

I'm building a detached garage with a radiant heat floor, and currently deciding on the design of it. Initially I was thinking of pouring 8" of concrete on top of 2-inch 25-psi insulation foam boards, with reinforced steel mesh and chairs to lift the PEX about 2 inch up from the boards.
However, I'm thinking that with that design, the room would take too much time to heat up and 8" might not be supported by the foam boards anyway. I'm now thinking of pouring 2 layers of concrete (3" and 5" or 4" and 4") and sandwich the insulation boards in-between with the mesh and chairs on the upper concrete layer. What do the experts here think?

 

[benwah]

Here in Gunnison County concrete is typically poured 4"-5" thick over 2" Polystyrene board, then mesh, then the PEX is tied to the mesh, then rebar is layerd over. IDK how other parts of the country do it, but this is how most of us do it.

 

[ConCretin]

deleted - double post

 

[ConCretin]

With all due respect, your plan seems unnecessarily complicated and expensive. Four or five inches or concrete is more than adequate for all but the heaviest loads and two placements will add to the time and expense. If it's a mono slab you can just thicken the edge to support building loads.

I prefer to staple the PEX to the rigid board insulation and support my reinforcing above it. You could install a layer of cheap mesh first and tie the PEX to it if you want to get it a little higher in the slab.

You want your reinforcing as high as possible without risking interference when cutting your control joints and you definitely want your PEX out of harms way. I've found this approach works best to accomplish both goals.

Give my Guide to Floor Slabs in the link below a read for some additional thoughts. Good luck and congrats on the new shop!

 

[Cd_r3105]

Thanks for your comment. The heaviest load for my would be my Toyota tundra or similar kind of truck, 5,480 lbs. Did you mean to include a link?

 

[Cd_r3105]

Also it's not a mono slab. There's foundations with footings for the walls.

 

[ConCretin]

Thanks for your comment. The heaviest load for my would be my Toyota tundra or similar kind of truck, 5,480 lbs. Did you mean to include a link?

There's a link in my signature line. It's titled "The Concrete Underground Guide to Floor Slabs".

A slab on grade can span short distances and 'spread the load' but it isn't really structural and depends on the ground below to support the loads placed on it. A four inch slab on a solid base is absolutely fine for vehicular loads. With that said, it's not very costly to thicken it an inch for insurance, a possible lift or just to fit everything in. It can get a little congested with radiant especially if you want some extra concrete under the PEX.

 

[haugy]

I did a 6" slab on mine. 15 mil vapor barrier, then foam board underneath. I put my #4 rebar on 1.5" chairs so with the rebar and chairs the top of rebar was about 2" into slab. I then tied my pex to the rebar. I have safely 2.5"+ below the top of my slab before I get to pex. My 6" slab is more than enough for most loads, including a fork lift and heavy trucks (empty) as well as for a 2-post lift.

It's your shop, but 8" is major thick, and if you put your pex at the bottom of that, you'll never get the benefit of the heat in my opinion. Some places you can call and they'll configure your layout and depths of pex for you based on your design and need.

 

[Feralghoul88]

8" is major thick, and if you put your pex at the bottom of that, you'll never get the benefit of the heat in my opinion.

You clearly do not understand the way radiant heat works.....

 

[Snowfighter]

I’ve done all my houses and garages with radiant heat my current new garage/car shop pics are attached 5” of concrete is plenty. It all falls back to your sub base. The concrete will only be as strong as what is supporting it.

 

[haugy]

You clearly do not understand the way radiant heat works.....

Neither do you apparently.

There are too many variables we don't know. Floor size, will the building be insulated, will it be heated as well? What size pex is he going with? What size boiler and pumps?

I said he won't get the benefit. The benefit being a heated floor to either maintain a shop temperature above freezing, or have on demand heat at your desired level. At 8" thick, he'd have to have very large pex and boilers to get that thing up to temperature in a day. So he either knows exactly when he'll be out there, or he'll have to leave it on all the time. At which point that becomes very inefficient at those oversized numbers.

Again too many unknowns. So go ahead and tell me I don't know. That's fine. There's a million ways to skin this cat. I look at it in terms of functionality, efficiency, and simplicity. Sure you can make 8" heated. But it'll cost you. In terms of money, time, and effectiveness.

 

[SBAG]

I went with (36x48 ft - still in progress) of 2 in foam, pex stapled to the foam, fiberglass rebar on chairs, and six inches of concrete. I definitely wanted the pex stapled to the foam and my installer was in agreement. This lets you drill in maybe 3-4 inches without worrying about hitting a pipe.

 

[jblnut]

Neither do you apparently.

There are too many variables we don't know. Floor size, will the building be insulated, will it be heated as well? What size pex is he going with? What size boiler and pumps?

I said he won't get the benefit. The benefit being a heated floor to either maintain a shop temperature above freezing, or have on demand heat at your desired level. At 8" thick, he'd have to have very large pex and boilers to get that thing up to temperature in a day. So he either knows exactly when he'll be out there, or he'll have to leave it on all the time. At which point that becomes very inefficient at those oversized numbers.

Again too many unknowns. So go ahead and tell me I don't know. That's fine. There's a million ways to skin this cat. I look at it in terms of functionality, efficiency, and simplicity. Sure you can make 8" heated. But it'll cost you. In terms of money, time, and effectiveness.

Isn't one of the largest benefits of in floor radiant heat to be consistent heat throughout the space ? Leaving it on all the time is sort of a requirement for that. It sounds like the OP is wanting to vary the temp based on "the room would take too much time to heat up" so let's ask the questions you don't know the answers to.

Hey OP !!! Do you intend to only turn the heat on when you plan to work out there or leave it on all the time ?

I have a 54x72x18 farm shop with a strong 6" of concrete and I leave the heat on all the time. We frequently have gatherings in there over winter and if I raise the temp from 58f to 67f it will heat up in 12hrs. My 120k btu boiler ***** the propane down like crazy the entire time but it responds fairly quickly.

OP if you do intend to leave the heat on all the time and aren't afraid of the added expense of all that goes with in floor heat I'd say go for it. Even if you want to pour 8" of mud in true GJ overkill fashion it'll work just fine. It just won't be as responsive if you want to change the temp in a matter of hours. If you ever purchase a payloader or want to back a fully loaded gravel truck in there you'd be covered and won't fall through the floor lol

 

[TractorJeff]

I sometimes have 3 Bulldozers up to CAT D6 size in my Shop at one time for Service on 6 inches of Concrete.
Never Cracked! It's all in the Base!

 

[mike93lx]

An 8" slab for a tundra. Lol.

Thin that way, way down