Radiant slab perimeter - insulate or isolate or both?

[Testocules]

Hello All,
Hopefully I have the right forum here - could be heating/AC or could be flooring.

A 'pre-slab-pour' question on how to finish insulate/isolate the perimeter foundation walls of my small radiant garage floor...

Scenario...

-garage is part of house - attached on 1 side wall and also the back wall, other side wall is the end of the house, and the front wall is obviously fronting to the exterior as well.
-garage is only 12' wide X 18' long (wish I had more!)
-floor will be heated by a single zone pex run.
-floor will have a recessed 'pit' in the center that will house a 7' X 2' motorcycle lift.
-floor will reinforced with rebar 12" on square grid pattern (pit will be reinforced with rebar tied to floor rebar).
-concrete will be 5.5" thick for pit pad and 5" - 6" thick for the balance of the floor.
-concrete will be 4500 psi with fiber, appropriate slump.
-base is well compacted sand/gravel (it's been there for years and I've plate tamped it even more for extra measure).
-under slab from base up will be 6" of 3/4" crushed stone (compacted), 10 mil vapor barrier (tape sealed), 2" eps rigid 250 foam, rebar on chairs to center the reinforcement to the middle of the slab.
-pex will be tied to rebar on 8" centers (also centered mid-slab).
-the garage has 2 foundation walls exposed to the exterior and 2 foundation walls that are separation walls between heated interior basement spaces.
-2 exterior walls will be insulated to a depth of 4' below the slab with 2" eps rigid foam.
-2 interior walls are against heated basement space so will not be insulated.
-I'm in Southern Ontario, Canada, where we see winters similar to upper New York and Michigan (hence the radiant!).

Question...
For the perimeter walls, I realize it's beneficial to insulate the perimeter walls (particularly those exposed to the exterior) to create a thermal break and reduce heat transfer from the slab to the foundation walls. I also realize that there may be a need for an isolation joint around the perimeter of the slab to allow for expansion/contraction (maybe?). I'm uncertain whether I need both for my size of slab and conditions, or if I'm fine with just insulation beveled to the top of the slab, or if I should stop the insulation at the bottom of the slab and put 6" or so of isolation board on top of the insulation board where the slab would otherwise meet the foundation wall.
I've read that some people 45 degree bevel the top of the eps against the foundation wall and pour to the top of the bevel to minimize the amount of exposed foam around the perimeter of the slab. Some stop the foundation wall eps at the bottom of the slab then put 1/2" isolation board from there up to isolate the slab from the foundation wall (may also act as a bit of an insulator?)
I can easily put isolation board on the 2 interior walls as they won't be insulated...does this act as enough expansion/contraction isolation for the whole pad and size of the pad?
What might you recommend I do around my 2 exterior walls?...beveled eps only, eps and isolation board together, or isolation board only sitting on the top edge of eps?

Sorry for the long read, just want to paint the clearest picture I can - hope I did but can certainly answer any questions

Thanks for suggestions based on your own expertise, experience and/or learnings.

 

[rburke65]

It isnt going to get any easier or more cost efficient than doing both right now.

 

[Testocules]

Absolutely true.
I'm good to do both, was just trying to minimize the gap around the perimeter...I'll be running 2" rigid foam up the foundation walls, if I add 1/2" isolation board I'd be out 2.5"...could bevel the foam to minimize the gap, and put a run of 1/2" isolation board against the bevel. I wasn't sure how much expansion my 12' wide slab would do...and I don't believe the rigid insulation would compress much but maybe it wouldn't have to?

 

[gnpenning]

Welcome. You did a nice job outlining what your doing.

What's your location? Freeze thaw cycles?

I'm not sure that it matters in this situation. When I did mine I just used foam. My garage door comes down on the foam and it gets driven over every day with a 3/4 ton diesel. I have had no issue's at all for 7 years.

Other as you mentioned loosing some space I see no other down side to doing both if you wish.

 

[Testocules]

Thanks gnpenning.

I'm in southern Ontario Canada so we can get some fairly cold winters...minus 20 Fahrenheit wouldn't be out of the question. With the floor being radiant, I'm thinking it would maintain a relatively consistent temperature over the whole year, summer and winter so hopefully not too much expansion/contraction.

I've already put 2" eps against the foundation wall (it reaches down to 42" below the slab height...and I'm thinking of going with 1" thick eps above it, 5.5" high which will be the thickness of the slab, then 1/2" of expansion joint material sandwiched against the 1" eps...this would give me a bit of a thermal break between the slap and the foundation wall (R-5 for the eps, plus whatever bit the expansion joint material might offer).

I'll likely do 1/2" cement board at the bottom of the garage walls to prevent any moisture issues wicking up drywall, and I can use a 1" thick 'baseboard' for a total of 1.5" thickness, the same thickness as the combined eps and expansion joint material...so it should cover for the most part.

At the garage door, I'm thinking of separating the slab and the apron with 1/2" expansion joint material only, no foam, so I don't end up with a wider gap which potentially could become worn away/fill with dirt etc. over time. Less R value though.

I'm likely overthinking it a bit but I plan to only do this once so I want to use the best methods.

Thanks for reading my post and offering your feedback.

 

[MagKarl]

You're over thinking it in my opinion. We have to make compromises, I would not want to see a big gap or any foam if it were mine, even if I was wasting a bit of energy. Concrete shrinks as it cures, I'd pour the slab up against the stem wall and apron.

 

[gnpenning]

With out a thermo break to the outside pad heat will be pulled from the building to the outside and you will be heating the great outdoors. Think how A/c works. I made that mistake back in 87 and will never do that again for any reason.

Testocules, if you have never had radiant before I've found in the fall to turn the boiler on sooner rather than later. You are creating a thermo mass. It takes less effort and energy to heat a 60 degree slab to 67 or 68 than it takes to bring up a 50 degree slab. And its not just the slab that is coming up to temps it everything else. Remember radiant is not a like forced air where when its cold you just turn up the thermostat. It's a set and forget.
Once it's up to temp it just maintains the temp. Find the temp you want and leave it alone. I set mine to 67. The master bed a little less. The garage stays at 58, remember your working in it, and anytime you bring a vehicle in the engine heat will add some heat as well for awhile. In mine it offsets the cold steel.

Hope you have a boiler that will preheat your water heater as well. I have never ran out of hot water.

Don't over think it but watch out for things that might pull or leak heat.

You will love it when you get it dialed into your tastes.

 

[Testocules]

Thanks gentlemen...your help is appreciated.

I think I may take a hybrid approach...meaning I'll still incorporate a thermal break, and expansion material, between the pad edge and the exterior foundation wall...although the combination of each won't be the full 2.5" wide...I'll reduce the thickness of the foam to 1" (compromise a bit on the r-value but still retain a thermal break), and include 1/2" of expansion material. This gives me a bit of insulation where needed, a bit of expansion material if needed, and a small enough gap that I can cover it with the combined thickness of the 1/2" drywall and 1" baseboard. I'll still have 2" of eps 4' down the foundation wall, and 2" of eps under the slab, so hopefully that will help contain a reasonable amount of heat for the space above.

I haven't had the opportunity to enjoy a heated floor in my past but am certainly looking forward to it based on all the comments and reviews in the forum.

As a newbie to this 'luxury', I appreciate the suggestions for operation as well...I like the idea of set and forget and the science behind it makes sense in terms of 'maintaining' a temperature rather than 'cycling' a temperature range. It bodes in favour of me spending more time in my garage if the temperature is reliably comfortable as well

 

[Flail]

I did both under floor insulation and 2" rigid foam at the wall that went from the top of the slab to the top of the footing. I painted, then attached 2x2x1/8" angle iron to the walls with the horizontal surface inline with my intended slab finish and the vertical portion of the angle going down the wall below the slab top. The wall footing foam was tucked under the horizontal portion of the angle. The angle iron served two purposes. 1) it protected the foam at the edge of the slab. 2) it provided a dead-nuts edge for the concrete finishers so I could make the floor drain the direction I wanted rather than relying on the finishers guessing what I wanted. I thought it was cheap insurance (about $1/foot for the angle) to insure my concrete drained correctly especially after all the time I invested in grading, vapor barrier, underslab foam, Pex for heat and rebar.

 

[Ed Devinney]

Beveled 2 inch rigid foam going up here in Virginia - temps not near as cold as you. Same plan, though - cover the gap with wall and trim.