Garage Floor, Insulated and Heated!

How great would it be to work in a garage in the middle of winter, and for it to be warm? Pretty great, right? That’s what I wanted, and achieved. In this post I’ll discuss the details that went into making my garage floor (trust me it’s interesting). This post is a supplement to a video that you can find here: Garage Floor Video The video or blog post can be absorbed independent of each other, but together help share all the details. Enjoy.

After the foundation walls were up, my excavator and I backfilled inside and out. I did this in lifts and compacted in between. So now we have a dirt floor. Just pour a slab, right? Nope. I’ve got to overcomplicate things a bit. But let me share why I’m not just pouring a slab which is done in almost every situation out there.

I knew that I wanted the garage to be heated, but I didn’t want to spend a lot of money every winter. Most garages I’ve been in (both residential and commercial) loose heat very quickly and are not heated 24/7 like a house. Most have a large air heater of some type that is turned on when you walk in and turned off when you leave. These things are huge, loud, and suck down fuel faster than a jet airplane. I wouldn’t necessarily have a problem with this except I was never comfortable in that environment. The hot air would be in a layer around your head with cold drafts at your feet. This meant that if you got on a creeper to work under your car you were still in below freezing air temps, but stand up and your head is in a thick layer of 90-degree air. My feet would freeze. My hands would also freeze because of handling all the cold tools. It’s terribly uncomfortable and nauseating. Oh, and heaven forbid you do a woodworking project. The wood will warp overnight with the changes in temps and humidity. Nahh, not for me.

So how do you make a comfortable space, with garage doors opening and closing, and high ceilings? With a radiant floor. Radiant floors are warm and there is no stratification (separation of layers ie. Hot air and cold air). Radiant floors are also silent. Their perfect for a garage. On the downside, radiant floors have a huge lag in time. Don’t expect to walk in, turn it on, and be warm 30 minutes later. The hot water in the tubes has to heat the concrete before it can heat the air. When operating a radiant floor just keep the air temperature constant 24/7.

But isn’t that a huge waste of energy? Maybe, it depends. At work, the shop has to be kept at 74 degrees to keep people comfortable. At home, we keep the air temp 68. I figured I would keep the air temp in the garage at 65 to be comfortable. It turned out that I’m comfortable with the garage air temperature at just 60 degrees. This was a surprise, but works great. The floor is always warm so my MRT (mean radiant temperature) is high. (I’ll do a post in the future on MRT and link to it.) Bottom line, if you keep the garage a constant temperature with a radiant floor, then you are comfortable at a much lower air temperature than you might expect. The lower the air temperature, the lower the rate of heat loss to the outside through the walls and ceiling. Thus, you don’t actually have a lot of heat loss through the shell.

Back to the construction:

We had the walls up and dirt is backfilled in the middle. The dirt is an irregular surface so it would not work to lay the foam insulation on top of it. I needed a smooth and level surface. I ordered three trucks of “flowable fill” from my local ready-mix supplier. I can remember the conversation with the manager. He described this product as self-leveling because the additives coat the aggregate making it act like ball bearings on each other. As the trucks were pouring I could see right away this was not self-leveling. I needed to jump in and begin screeing. It was a futile effort. The dried result was wavy. I ended up screeing a thin layer of sand to finally make the finished surface.

A radiant floor is warmer than the rooms air temperature. So, doesn’t the ground suck up half the BTU’s you are pumping into it like a massive heat sink. It normally would, and does in many cases, unless you insulate.

In my garage, we insulated between the ground and the concrete slab using “Geo Foam”. Which is just a brand of high-density EPS foam. EPS is the most environmentally friendly of the foam family, but it has the lowest R-value per inch of thickness. This is why we used a total of 14 inches of foam (two layers each 7 inches) for an R-value of 56. That is pretty insane when you think about it. Most homes don’t have any insulation under the slab. If they do they normally max out at 2” of XPS for an R-value of 10. But having an insulation value of 56 means nearly every BTU I pay for will heat my garage, not the dirt under it.

There is a vapor barrier between the foam and the concrete. This is standard 6-mil poly. I tried to buy some super thick stuff originally, but it was all a special order and pretty expensive. Maybe if I was building below the water table I would have forked over the dough, but I just didn’t think it necessary with my situation.

The tubing is ½” diameter PEX with an oxygen barrier. I bought a 1,000-foot roll and un-rolled in down my driveway to cut it into four equal length pieces. I then zip-tied these to the rebar grid in the floor. The tubing is 18 inches on center. There are a couple things to note here. If you asked a plumbing expert to do a heat loss calculation for you, he/she will say you need the tubing closer spaced, and more total length. They want a lot of “distribution” so the BTU’s can be transferred at a faster rate. This is only necessary if you have an inefficient building. Our garage is super insulated so our heat loss is at a very slow rate. The distribution system only needs to supply heat at a rate equal to the heat loss of the building.

By the way, our tubing is probably even more length then needed. I can say that because I was able to bring the garage up to 85 degrees in the middle of winter; no reason other than seeing if I could.

Let’s talk about the concrete slab itself for a moment. I ordered the concrete with a plasticizer added to increase workability without the need for adding water. (water weakens concrete). There is also a grid of reinforcing bar half inch thick. (#4 Rebar) The grid is 18” on center and is in the middle of the concrete slab where it belongs. And the slab is five inches thick. Bottom line, this is one hell of a slab. I wouldn’t want to be the guy to bust this up in the future. I wanted such a strong slab in case I decide to add a lift in the future.

The slab is also the thermal mass for the garage. The mass helps balance out highs and lows in temperature. This means that during the swing seasons (fall and spring) I don’t need any heating or cooling. It’s pretty neat. In the future, I hope to add a solar thermal system and the concrete mass will become a thermal battery.

Now that it is done, what would I do differently?

Instead of laying the tubing on top of the rebar, I would have preferred in under. I could have laid down 6x6 welded wire mesh on the floor. Then I would have tied the hydronic tubing to this instead of the rebar. I would have then filled the tubing with water, and pressurized it. Then laid rebar on top of the tubing.
After backfilling I would have used gravel instead of trying “flowable fill”. Check out my video for more details on this.
Half the building is our garage for cars, and half is the shop. I had the whole slab poured level (great for shop) but it allows water to puddle. In winter the cars have snow on them which melts. I wish I had a slight slope just where the cars are parked.

What would I do again?

Insulation! I’ve got no regrets about putting down a super thick layer of insulation. The foam insulation went down fast (just 4 hours). It requires no maintenance, no service, and is there for the life of the building. It just sits there and does its job. I don’t think I could ever regret making the thermal envelope better.
Strong floor. I haven’t had any cracks form in the slab. I was expecting some shrinkage cracks, but I guess with the plasticizer and low water even that has been eliminated. I don’t have a lift right now, but I do have a jack and jack stands. I’ve raised my cars up without any plywood under the jack or stands and they don’t leave any marks in the floor.
Radiant heat! All I can say is that if you are going to pour a slab, put some tubing in it. You won’t regret it.