Hello everyone! Newby post, I hope I am in the right place and format :)

Moving into a 1972 construction house which was built with a two car garage on the ground floor, but the PO never installed a garage door! Boston suburb ...imagine the heat loss :(

Before I add the garage door, I’d like to pour an additional garage thin slab (17’ x 20’) and embed a ½” or ⅝” PEX for the future hydronic heat zone. I would also like to extend the slab beyond the door by about 24” I plan on a wire mesh for slab reinforcement and anchor base for the PEX.

The existing garage slab is poured into the perimeter foundation and is exactly level with the outside grade. The step up into the living area is approximately 7” The house sits on a hill and no ground water is present (no efflorescence on the cement floor), although on warm and humid days some condensation is evident in the center of the garage floor.

I have a few design questions in brainstorm format:

...minimum thin slab thickness for a garage floor with ability to support a car floor jack?

...vapor barrier under the new thin slab? then the new concrete just floats on the plastic sheet? Is that good? ...or ditch the barrier and bond new concrete to old? Bubble foil sounds great to vapor block and provide thermal insulation, but again, the new thin slab just floats on top of the barrier?

...thermal break around the perimeter? 1/4" closed cell strip or similar?

...need additional expansion joint provision? ...or can the 1/4" thermal break gap do both for 340 ft.sq.?

...thermal break between the slab and the outside: I am thinking to make the thermal break about a foot or so inside the garage (before the door) and then have the exterior skirt effectively start in the garage and then extend by about 24" on the outside beyond the garage door.

I am sure I am forgetting something important, but hope to get better educated from your comments. Many thanks!

Michael

You may want to sand the surface of the existing concrete, then wash it down and etch it.

Use an adhesion promoter on the surface to promote bonding at the cold joint.

The new slab will probably need to be somewhere between 4-6", and steel reinforced to handle a car properly without breaking.

For the exterior shoulder, use rubber expansion joint up against the slab. Then pour and form your shoulder.

You don't want to use a vapor barrier between layers of concrete. It'll just lead to moisture buildup between the two slabs and possible freezing around the perimeter.

Bad idea all together....sorry, I have made this mistake and hope to help you.

You aren't going to heat it all the time....
Radiant floors in garages are nice to lay on but will take about 6 hours to heat up and keep heating for that long after.....a waste of energy....
Every inch counts in a garage when you buy a truck and can no longer fit it in because it hits the opening....
So you could pour a thin slab of a couple inches but....
No insulation will hinder heating and bubble stuff will cause floor to crack....
The thin cement with the expansion and contraction might be a problem with delamination....
A lot of work and cost for what you get....
Just install a mini split in the garage and you can remotely turn in on a hour before going out and have it toasty warm efficient area...

Hope this helps and not costing you a lot.

Thank you for your replies! Good guidance, Diletante. No vapor barrier in this application does make more sense. Thank you for the other pointers also! Newboston: I appreciate hearing about your experience with this! I didn’t mention in the first post, but the garage radiant loop I am considering would be (could be) the radiator heat sink for the excess heat, if any, generated by the DHW solar thermal system (30 collector tubes). The assumption is that I might have extra free heat to divert into the garage loop, and that at least some of the time during the year (spring and fall) I may actually be able to benefit from that heat in the garage. At least that was the initial impulse. I want to do a real heat loss analysis on this space to get better clarity. Looking for a good on-line heat loss calc...

I agree this is NOT the best idea. Even with excess heat, you will still have an excessively thick uninsulated slab. You also would likely need to have glycol in this zone that would add complexity to the system and reduce performance. So it's better to be 100% water whenever possible. We have free DIY heat loss analysis and HR floor heating design calculators on our website.

BTU's are BTU's....thinking that the water coming out of your solar collectors is going to be the same regardless of the water temp going in is not right.

You could divert to garage when house is to hot....that might be what you are thinking?.....

I had a 40'x60' garage in Michigan with R21 insulation that never froze and wasn't heated.