Radiant in slab new garage build

Here is how I did my 60'x60' with 2x8 stick walls ( 17' sidewall ).

Place the top of the 4' frost wall 6" above the finished floor. this keeps the sill boards high and dry where they belong and an internal concrete 'curb' so if you push something against the wall with a skidsteer or something, the wall isn't affected.

Make sure the backfill inside the foundation is compacted correctly. if you leave this up to a low bidder you are asking for trouble with hollow spots along wall and in corners. do it yourself.

Put down your 2" or 3" of styrofoam on base, and run a 6" strip (or whatever your slab thickness ) of 1/2" dow foam glued to the frost wall. half inch is plenty for a thermal break. I put 1.5" dow against the outside of the frost wall before that was backfilled.

Your internal slab is floating with the exception of at the service doors and overhead doors. Notch the frost wall down 13" or so. Run an inch of dow over top of that wall for a thermal break. In my case I poured my floor half way ( 4" ) to bear on the wall, the exterior floating slab lies on the outside 4". Yes, you will have 5 or 6 inches of heated slab hanging out beyond your OH doors, but your weatherstripping is always on a thawed slab.

Most importantly, make sure you budget for an inch of spray foam into the wall cavity as part of your wall insulation.

In truth, I used regular impregnated expansion joint material instead of 1/2" dow. You seemed more concerned on the thermal break than I was, so I suggested the 1/2" Dow. I was happy with the black expansion joint stuff. I was ( and am ) concerned with where the pressures go when a 60x60 slab is heated and expands inside my foundation walls. I think there is enough initial shrinkage in the slab cure that it doesn't get tight.

Yes the bond breaker is visible. Doesn't cause me a bit of problem. Most of my walls have shelving, workbench, table saws, etc, etc pushed up against them. If you were really anal you could use a zip strip on top of the foam, and fill in that gap with a bunch of grey SikkaFlex sealant for cosmetic reasons.

One other lesson I wish I paid more attention to: Make sure you pitch OUT the concrete edge at the OH doors enough. In my case, in a driving rain, the water sneaks itself under the weather seal and into the shed. Not a huge problem as I have trench drains, but it still irritates me. Also we didn't nip out the concrete far enough to the sides, so I had to diamond grind the floor at the outermost 3" of my OH doors to get them to sit down tight.

Sticking that stem wall 6" above grade prevents a lot of problems in the future.

standard would be 1" and tape the top at a 45 degree to be zero thickness at the top edge. I would do something similar, stopping the slab under the door, and having a small separate pour outside to make the apron. the thermal leakage in that 8" exposed to the outside is not small.

hondaracer2oo4 said:

Hey guys, building a new garage this spring. 28x40 3 bay with full frost walls and poured floor. I am looking to install radiant heat in the floor. I know that I will have to basically make a foam tub to pour the floor in so that I have my underslab insulation and my thermal break to the frost walls. My question is how to do the thermal break to the frost wall? I have seen some drawings of a stepped in frost wall 6 inches below the floor height so that the frost wall insulation will have the stud wall land over it and you will have a fully poured floor all the way out to the stud wall. I think this is doable but what about where the cars pull in through the bay doors, how do I insulated the floor from the frost walls there?

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Assuming your insulating the walls; and ceiling to an R 32 have you compared the cost per square foot of in floor heat
at 12, 16, 18, 24, 32 and 36 inch spacing versus a number of hot water radiators on a single loop system for the three walls and hot water to air ceiling hung heaters on a second loop that would only activate when the doors open the entire system could be heated and used at a much lower temperature.

I always worry about concrete floors in garages especially if they are not scored properly or installed properly with felt expansion joints and or floor drains. The entire slab foats unless it is properly installed with expansion joints and felt or the proper amount of rebar for the entire slabs square area.

The so called "builder" that installed my fathers garage slab incorrectly and it looks like a spiderweb with all the cracks because it was not installed properly with a score slot or expansion joints.

At least with radiators the piping is exposed and you can find the leaks if you know you have them.

Another thing to consider is forced air heat in a large garage like that when comparing the cost per square foot as the registers can be placed in the ceiling and near the overhead doors if desired.

The other advantage is the registers can be closed and only a certain number of them left open by the overhead doors if desired to counteract the loss of heat as quickly as possible.

The thing is that heat is going to leave your garage the minute an overhead door is open and it will take a long time for the slab to recover.

By having new steel or surplus cast iron radiators installed the heat loss is gradual and not massive and the heat in the garage will be back up to temperature sooner as the radiators are the only thing shedding head versus the entire slab.

You could install the radiators along both walls and have more than enough heat for the garage as the radiators are about 150 BTU per square foot of area in heating area for each radiator section and many large surplus radiators are still saved rather than scrapped.



I could see heating the apron on a separate loop if there is an ice buildup in your case but not having floor heat.

hondaracer2oo4 said:

Some others suggested a 45 degree bevel on the foam and then the floor would pour all the way to te wall but you would have a very weak edge. That approach makes no sense to me.

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Oh no, this makes perfect sense. I used 2" on the edge with the top cut at 45. Works great. See, you won't load that edge and even if you do it is quite thick all the way to the razor edge.

Whether or not to use floor heat in a garage is another issue. I never hooked up my 1800 LF of tubes in the 5 years since I built the shop. Woodstove works great and after you insulate very well, it never freezes.

Very pleased with my decision for in-floor, 12" OC, in my 32 x 48 x 14 shop. My Tarm and 1000 gal storage tank (well insulated) also is in the shop. 2" foam under entire floor, and 2" perimeter foam as well. Floor heat is set at constant 61F with a differential of 1F. 35,000 Btuh input into the floor via a mixing valve set at 100F. What I really like is the lack of drafts and the warm floor. I stubbed in plumbing to add an overhead hot water unit heater, but never installed it. Shop temp stays very comfortable no matter what the outside weather conditions may be, from calm to windy, regardless of outside temp (-35F design temp). Floor responds very slowly to temp changes, which keeps very even heat in the shop.

Maybe I was a bit lucky, but after six years not a single floor crack, even hair-line, except in the sawed crack lines, and the cracks that formed are barely visible. I think keeping the floor warm with insulation underneath basically eliminates expansion/contraction and floor cracking.

hondaracer2oo4 said:

All of the responses are great, I am certianlly looking for opinions both ways as well. I figured that you only get one chance to put the radiant in, if I never use it I never use it but I wont regret not putting it in the floor. I plan to heat the garage to 45-50 degrees all the time and place a modine or similar up in one corner for when I want to bring the garage up to temp. What did everyone else do about the thermal break around the garage door apron area? Just lay down 2 inch on the top of the frost wall where the floor will get poured over and up the sides of the frost wall at the edge of the door frame? Same for the man door? Anyone do the thermal break somehow across the apron to separate it from the outside?

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I saw a great detail for that at last week at a conference. it essentially was to continue the perimeter 1" inside insulation right across the door opening under the seal for the OH door. it was cut to a taper at the top, and then had a heavy rubber seal gasket glued on top. the outside apron portion was poured on top of the frost wall separately. the IR camera photos were most impressive. wish I'd done that.

The question of whether to do in-floor or not is the real question. it basically comes down to how you are going to use the space. occasional use space that will be left to freeze, minimal insulation, don't bother. it takes too long to come up to temperature to use it for a weekend. if it has to be kept at a steady 50 degrees all the time, and you have or can do pretty good insulation, in floor is a great way to go.

Karl_northwind said:

I saw a great detail for that at last week at a conference. it essentially was to continue the perimeter 1" inside insulation right across the door opening under the seal for the OH door. it was cut to a taper at the top, and then had a heavy rubber seal gasket glued on top. the outside apron portion was poured on top of the frost wall separately. the IR camera photos were most impressive. wish I'd done that.

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Do you have any pictures or diagrams of this method? I kind of follow the idea but a picture would really help.

Greetings,

Antifreeze is used in heated car wash floors to keep them and
the aprons hot and drying constantly.

If you use the steel or cast iron radiators you will have less work and
the garage will stay warmer longer simply because of the radiators
shedding heat. If you place foil insulation board behind them to reflect
the heat you will be even warmer in the garage.

Running a single pipe heating loop with cast iron or steel radiators is both
cost effective and allows for quicker recovery as the radiators store a
huge amount of heat in the water and even keeping the water at
60 degrees translates into a huge heat bonanza if you place the steel
or cast iron radiators on the three closed walls using foil backed insulation
board and piping is simple as the pipe follows the walls and over the doors
to the modine if used and then back to your boiler.

You can save money and invest in sand mix concrete slab with more
rebar and mesh too to make it stronger. You just have to make sure
you have a felt slab separator every eight feet or so to protect the
slabs from cracking and spidering.

I could see spending money on heating the garage apron as that is where
a lot of ice will build up from opening and closing the overhead doors.

As long as you follow the building code for the climate, snow load and
the local zoning codes you should not have many issues with heating your project.

hondaracer2oo4 said:

Is a heated slab not the most efficient way to keep a space at an even consistent temp?

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heated slab is. and 100,000 lbs of concrete holds a whole lot of heat, and will bring the air temperature right back up when the OH door closes. fair chance the heating loop won't even turn on.

Buzz Saw said:

Do you have any pictures or diagrams of this method? I kind of follow the idea but a picture would really help.

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none that I can share, as they're not mine. the general idea is to do the same bevel up to the slab top, and do a pour inside and one outside. home depot sells a heavy rubber air seal for under garage doors that would cover any gap nicely.

Well, I have never done this on any slab I've poured even though my policy over the last few years is to always install tubing on every one but this problem has kept me awake a few times. If I were to pour another slab (which is highly unlikely) I would lay a rugged piece of angle iron 2 inches X whatever the concrete thickness is under the door. Tuck my foam insulation behind the angle iron which doubles as my form. The 2" dimension covers the form. Then pour the floor. This will give you a nice straight and flat jam for your door seal. Then pour the apron separately. Be sure to pin it well to the top of the frost wall.
When I was a youngster back in the 50s and 60s I worked in my dad's shop which he always kept warm with oil hot air heat but I was always cold. Here it s 2016 and my lower legs and ankles are just starting to thaw out. Remember, it's not how hot but how comfortable. When you open the big roll up door long enough to get a vehicle in, you're only loosing some warm air which is so low mass that it will re-heat before you notice it. You could easily drop the temperature 3 to 5 degrees with radiant and still feel comfortable.
The perimeter foam insulation cut at 45 degrees works well. Just remember to have a good supply of good porous backfill there for your excavator operator to tap and don't turn your back on him. Be sure he brings his compactor and USES IT. Words of experience!

I'm going to use PERT. It's made for radiant floor heat. 1/2" is about $0.35 / foot.

Right or wrong, one thing I do when pouring, is pressurize my loops to 25 psi during and keep the pressure on a few days after the pour. Keeps the tubing from getting crushed and also reduces the pressure on the concrete when heat is applied. No spider cracks here!

hondaracer2oo4 said:

Is a heated slab not the most efficient way to keep a space at an even consistent temp?

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A heated slab is not "efficient". The heated slab is just the delivery device. You can send water to the slab heated by burning 100 dollar bills and it will be the least efficient way to heat your shop that I can imagine.