Heated Concrete Floors
Concrete is one of the best floor materials you can heat. The slab has serious thermal mass, so it soaks up warmth from the tubing or cable inside it and radiates that heat back slowly and evenly. No hot and cold blasts like forced air. The floor stays warm underfoot, and once the slab is up to temperature it holds there for hours. That same mass is why a heated slab wants insulation under it and a good vapor barrier below that. Get those two layers right and the system pays you back every winter. Skip them and you are paying to warm the dirt.
This is the heat, insulation, and moisture spoke of our larger concrete floor guide. If you are still deciding how thick to pour, start there first.
Hydronic vs electric radiant heat
Two systems dominate, and they work in very different ways. Hydronic runs warm water from a boiler through loops of flexible PEX tubing cast into the slab. Electric runs current through resistance cable or mats that heat up directly. Here is how they stack up.
| Factor | Hydronic (water) | Electric (cable/mat) |
|---|---|---|
| How it works | Boiler heats water, pumps it through PEX loops in the slab | Current runs through resistance cable, heats the slab directly |
| Best for | Whole-house heating, large areas, new construction | Single rooms, bathrooms, kitchens, spot heating |
| Install cost | Higher upfront; needs boiler, manifold, pumps | Around $10 to $20 per square foot |
| Running cost | Usually lower per square foot, especially on gas | Cheap for small areas, pricey as a whole-house source |
| Heat-up time | Slow, often hours; likes to run steadily | Fast, roughly 30 to 60 minutes |
| Retrofit-ability | Hard; best designed into a new pour | Easier; thin mats can go over an existing slab |
The short version: if you are heating one bathroom, electric almost always wins. If you are heating a whole house or a big shop and you are pouring fresh concrete anyway, hydronic usually costs less to run and moves more heat. Family Handyman notes that hydronic systems "generally deliver more heat at a lower operating cost than electric systems" once you get past about 200 square feet.
Why under-slab insulation matters
Concrete does not care which direction heat travels. Without a rigid foam layer underneath, a big share of the warmth you pump into the slab sinks straight down into the ground instead of coming up into your room. You end up heating the soil under the footing, which is a bill with nothing to show for it.
Rigid foam board fixes this. Most builders use extruded polystyrene (XPS) at around R-4.5 to R-5.0 per inch, or expanded polystyrene (EPS) at roughly R-3.6 to R-4.0 per inch. XPS resists moisture better; EPS holds its rated R-value long term and costs less. Building codes in colder climates (zones 4 through 8) require a minimum of R-10 under a slab, about two inches of XPS. For heated slabs, many codes push that insulation down to the footing depth or at least two feet along the edges, since the perimeter is where heat escapes fastest.
If your existing floor is uneven before any of this goes in, our floor leveling guide covers the options.
Vapor and moisture barriers
A heated slab needs a vapor barrier even more than a cold one. Warming the concrete drives moisture upward through the pores, and if there is nothing stopping ground moisture from feeding in below, you get a slab that is constantly wicking water and wasting heat to evaporate it. That moisture also wrecks glued-down finish floors on top.
The standard fix is a sheet of polyethylene under the slab. The American Concrete Institute calls for a minimum 10-mil poly with a permeance no higher than 0.3 perms for best protection, though 6-mil is a common minimum for heated structures. The usual detail puts the vapor barrier in direct contact with the underside of the slab, with the rigid foam below it, so the concrete cures against the plastic and stays sealed from the ground. Overlap seams by at least six inches and tape them. A cheap layer that protects an expensive floor.
Installing radiant heat in a new slab vs retrofitting
The cleanest way to heat concrete is to build the heat into the pour. Crews lay the insulation and vapor barrier, roll out the PEX tubing or electric cable, tie it to the rebar or wire mesh, then pour right over the top. One builder on the TractorByNet forum described his basement job simply: he pressure-tested the PEX at 50 pounds, confirmed no drop overnight or during the pour, and poured the slab straight over the loops. He said he would "love it" and told others to seriously consider it.
Retrofitting an existing slab is different. You work on top instead of tearing up good concrete. Thin electric mats or low-profile hydronic panels go down, then a self-leveling compound or thin topping locks them in. It raises the floor height an inch or so and adds cost, but it warms a cold slab without demolition. Electric is the friendlier retrofit, because water systems really want to be designed into the original slab.
Cost of a heated concrete floor
Numbers vary by system and region. Electric radiant runs roughly $10 to $20 per square foot installed, reasonable for a bathroom but steep for a whole house. Hydronic costs more upfront (boiler, manifold, pumps) but typically wins on running cost across a large area, especially where natural gas is cheap. For a full breakdown of materials and labor, see our concrete floor cost guide.
How this ties into pouring a floor
Radiant heat is a decision you make before the truck shows up, not after. The insulation, the vapor barrier, and the tubing all go in before you pour, and the slab thickness has to account for the tubing sitting inside it. Read our guide on floor thickness and how to pour so you size the slab correctly around the heating loops. When you are ready to order material, run your numbers through the concrete floor calculator to get volume and bag counts without guesswork.
Sources
- Family Handyman: Radiant Floor Heat Systems, Electric vs. Hydronic
- WarmlyYours: Electric vs. Hydronic Floor Heating
- Today's Homeowner: Do You Need Rigid Insulation Under a Concrete Slab?
- Stego Industries: Where to Place Under-Slab Insulation, Above or Below the Vapor Barrier
- TractorByNet forum: Radiant Floor Heat Question (real user experience)
- HomeAdvisor: How Much Does Radiant Floor Heating Cost?
Frequently asked questions
Can I add radiant heat to an existing concrete floor?
Yes. Electric mats or thin hydronic panels install on top of the old slab, then get covered with a self-leveling compound or thin topping. Electric is the easier retrofit. Casting new water loops into an old slab is not practical without breaking it up.
Do I really need insulation under a heated slab?
For a heated slab, yes. Without rigid foam below, a large share of your heat drains into the ground. R-10 (about two inches of XPS) is the code minimum in cold climates, and the perimeter matters most.
How long does a heated concrete floor take to warm up?
Electric reaches temperature in about 30 to 60 minutes. Hydronic slabs are slower and can take hours, which is why they usually run steadily through the season rather than cycling on and off.
Is hydronic or electric cheaper to run?
For small areas, electric is fine. For whole-house heating, hydronic usually costs less per square foot, especially with a gas boiler. Electric as a primary whole-house source gets expensive fast.
What thickness of vapor barrier should go under a heated slab?
A minimum 10-mil polyethylene sheet at 0.3 perms or lower gives the best protection, though 6-mil is a common minimum. Overlap and tape all seams.