I run Skinner Customs out of Rigby, and every fall somebody in Idaho Falls asks me the same question: is heated concrete worth doing, and what actually goes into it?
The heat part is the easy part to picture. Warm water moves through tubing in the slab, the slab warms up, and the floor gives that heat back to the room slowly. What people do not picture is that almost everything that decides whether a radiant slab works well happens before any concrete shows up. Get the layers under the slab wrong and you spend the next twenty winters heating dirt.
Here is how a radiant floor heat concrete install goes together on our jobs, in the order it happens.
The slab is the radiator, so it has to be built like one
A normal slab has one job: hold up whatever sits on it. A radiant slab has two. It still carries the load, and now it is also the thing storing and releasing your heat.
That second job is why the sequence matters. All of the mass in that slab is going to get warm. If there is nothing between the warm concrete and the cold ground under it, a good chunk of what you paid to heat goes straight down into the subgrade and out the perimeter. The perimeter is the worse of the two, because slab edges sit right against outside air.
So on a heated slab we are not just prepping for strength. We are prepping to keep the heat pointed up into the house.
What goes under a heated slab
Bottom to top, here is the stack.
Compacted gravel base, four inches minimum. More lifts where the soil is questionable, and we have plenty of that around the valley. This is the same base I would build under any slab, and it is still the part that decides whether the slab cracks in year three.
Vapor retarder. You do not want ground moisture migrating up into a slab you are about to heat, and you do not want it in a finished basement either.
Rigid foam insulation under the slab and at the edges. This is the layer people try to cut, and cutting it is how a radiant floor ends up expensive to run. The code is direct about the edge condition. The 2018 IRC, which Idaho adopts as the Idaho Residential Code, says in Section M2103.2.1 that a thermal break of asphalt expansion joint material or similar insulating material has to be provided where a heated slab meets a foundation wall or another conductive slab. That clause exists because without it your heated slab is bolted to a cold concrete wall that runs straight to the outside.
Steel. Rebar, number 3 or number 4 on an eighteen inch grid. Not wire mesh. On a radiant slab the steel does double duty, because the tubing gets tied to it and that is what holds the tubing at the right height while we pour.
Tubing. Laid out on a plan, tied to the grid, and run back to a manifold. Spacing is not a guess. Tighter loops go where you feel it most, like along exterior walls and under glass.
The pressure test is not optional and you should watch it
Once that tubing is tied down and before a truck ever backs up, the system gets pressurized and held.
The same code section is specific about it. Section M2103.4 reads: piping or tubing to be embedded shall be tested by applying a hydrostatic pressure of not less than 100 psi, and the pressure shall be maintained for 30 minutes, during which the joints shall be visually inspected for leaks.
Read that again, because it is the single most important half hour of the whole job. After the pour, a leak in that tubing is a saw and a jackhammer and a very bad week. Before the pour, it is a fitting.
We keep the line pressurized through the pour for the same reason. If something gets nicked while concrete is moving, the gauge tells on it immediately instead of six months later.
Pour day on a heated slab
Three things change once there is tubing in the forms.
Nobody stabs the slab. No rebar stakes driven blind, no digging a shovel down to grade to check depth. Every man on that crew knows where the loops run.
Thickness gets checked constantly. The tubing has to end up buried with real cover over it, and the slab still has to be a full four inches minimum where it needs to be structurally. A thin spot over a loop is both a weak spot and a hot stripe you will feel with your feet.
Mix and finish do not get relaxed. Four thousand PSI minimum, same as any slab we pour in this climate, because freeze and thaw cycling here is punishing and a lower mix just costs you years. Broom finish outside, hard troweled inside where flooring is going down.
Control joints still get cut on a real plan. On a radiant slab the joint layout gets drawn with the tubing layout in front of you, because a saw cut in the wrong place is a cut loop.
Curing and the first winter
Concrete needs to cure on its own schedule, and the temptation with a brand new heated slab is to fire the system up and start drying it out. Do not.
Let it cure. When the system does come up for the first time, it comes up slowly, over days, not all at once. A slab is a big piece of rigid material and heating it fast puts stress into it that shows up as cracking later.
Same idea on the other end of the year. Radiant is not a system you cycle hard the way you would forced air. It works best held steady, because the slab takes hours to respond by design. That slow response is a feature in January and it is the reason people who have lived with a radiant floor rarely go back.
What it costs
I am not going to put a number on radiant in a blog post, because there is not an honest one to give. Tubing layout, loop spacing, insulation thickness, slab size, whether the heat side is being run by a plumbing or mechanical contractor, and what the slab is carrying all move it. Anybody who quotes you a heated slab off a phone call is quoting a stranger’s job, not yours.
What I can tell you is what does not change. The base prep, the insulation, the thermal break at the edge, the steel, the pressure test, and the mix are the parts worth paying for, and they are the parts a cheap bid quietly leaves out. On plain flatwork my floor is eight dollars a square foot for a reason, and a heated slab has more under it than plain flatwork does, not less.
Send me the plans and what the space is for and I will tell you what the concrete side takes.
Frequently asked questions
Can you put radiant heat in an existing concrete slab?
Not in the slab itself. Once the concrete is down, the tubing would have to go above it in a new topping pour or an overlay system, which changes your floor height and your door and stair details. Cutting loops into a finished slab is not a repair anybody should sell you.
How thick does a radiant slab need to be?
Four inches minimum for a typical residential floor, thicker where trucks, trailers or equipment park on it in a shop. The tubing needs real concrete cover over it, so thickness gets watched closely during the pour rather than checked at the end.
Does radiant heat crack a concrete slab?
Not on its own. Cracking comes from bad base prep, missing or undersized steel, joints in the wrong place, or heating a green slab too fast. Built right and started up slowly, a heated slab behaves like any other slab.
Do I need insulation under the whole slab or just the edges?
The edge is where the code language is most direct and where the heat loss is worst, but on a heated slab I want insulation under the field too. Without it you are warming the ground under your house all winter.
Who installs the tubing, the concrete contractor or the heating contractor?
It depends on the job and who is carrying the mechanical design. Either way somebody has to own the layout, the manifold and the pressure test, and that has to be settled before the steel goes in, not the morning of the pour.
Talk to somebody who has poured them
Radiant slabs are unforgiving of shortcuts and very good when they are done right. If you are planning one in Idaho Falls, Rigby, Rexburg, Ammon or Shelley, get the concrete side scoped before the layers under it are already buried.
Need concrete work in Eastern Idaho? Call Zach at Skinner Customs at (208) 390-5794 or start at skinner-customs.com.
Content produced in partnership with S4 Media, marketing partner of SwagerBuilds LLC.
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