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Flooring R-Values and Radiant Performance: Why Carpet and Pad Throttle a Heated Slab, What Each Floor Covering Does to the Water Temperature, and How to Keep It Under R 1
Flooring & Tile
Flooring R-Values Over a Heated Slab: Every Covering Is Insulation, and Carpet Is a Lot of It
A heated slab pushes heat up through whatever you put on it, and the warmer the slab has to be to get the room warm, the more fuel it burns and the closer the floor gets to the temperature where wood and vinyl start complaining. The thing that decides how warm the slab has to be is the R-value of the floor covering. Tile is nearly nothing. Wood is a little. Carpet and pad is a wool sweater over the whole heater. I have designed and poured heated slabs for thirty years and this is the number I argue about most with clients who want broadloom in the basement bedroom.
The table
| Floor covering | Typical R-value | What it means for the slab |
|---|---|---|
| Bare or sealed concrete, polished | 0 | The slab is the floor. Maximum output, lowest water temperature. |
| Porcelain or ceramic tile, 3/8 in., in thinset | 0.2 | Nearly transparent to heat. Add about 0.1 for an uncoupling membrane. |
| Natural stone, 1/2 in. | 0.1 to 0.2 | Same as tile. Denser stone is slightly better. |
| Glue-down LVP, 2 to 3 mm | 0.1 to 0.2 | Almost as good as tile for heat; the sheet caps the surface at 27 to 29 C. |
| SPC click with attached pad, 5 to 6 mm | 0.3 to 0.5 | The pad and the air film under a floating floor are most of it. |
| Engineered hardwood, 1/2 in., glued | 0.5 to 0.7 | Wood is about R 1 per inch. Fine in a tight house. |
| Engineered hardwood, floating on foam pad | 0.9 to 1.1 | The pad adds R 0.4 and the hollow sound for free. |
| Solid hardwood, 3/4 in., on sleepers or plywood | 0.9 to 1.2 | The wood plus the plywood plus the air under it. A joist-radiant question, not a slab one. |
| Laminate, 8 to 12 mm, with foam underlay | 0.9 to 1.2 | Fibreboard core plus the required underlay. Usable, not good. |
| Carpet, low pile, over thin dense pad | 1.5 to 2.0 | The best carpet case. The slab works noticeably harder. |
| Carpet, medium, over 1/2 in. rebond pad | 2.5 to 3.0 | What most people buy. Output drops by about half at the same water temperature. |
| Thick carpet over felt or thick foam pad | 3.5 to 4.5 | A rug on a radiator. The system was not designed for it and it shows. |
Area rugs count. A 9×12 wool rug over a glued engineered floor is R 2 on that patch, which is why the floor under a thick rug sometimes shows a line where the heat built up.
What the R-value does to the water temperature
Heat flows from the warm slab to the cooler room in proportion to the temperature difference and in inverse proportion to the resistance in the way. Double the resistance between slab and room and you need roughly double the temperature difference to move the same heat. In a house that needs, say, 15 BTU an hour per square foot of floor on a January night, which is about what a well-built ICF house needs, a bare slab at 27 C does it. Put tile on and nothing changes. Put glued engineered on and the slab under the wood runs a couple of degrees warmer to deliver the same 15, and the water in the tubing goes from about 35 C to about 38 or 40 C. Put medium carpet and pad on and the slab has to run 6 to 8 C warmer under the carpet than it would under tile, and the water heads for 50 C.
That is the whole problem with carpet in three ways. The boiler or heat pump runs hotter, which costs fuel and, for a heat pump, costs efficiency fast. The slab under the carpet is now hot enough that if the carpet ever comes out and wood goes in, the wood is past its sheet’s 27 C cap on day one, and the warranty with it. And the room is still slow to respond, because the carpet sits between the heater and the air. The hydronic radiant cost page covers what the system costs to install and to run; the floor covering is one of the biggest levers on the running part.
The two numbers, written for a designer
On our slabs the water goes in at about 40 C and the floor surface runs 30 to 37 C, which is what thirty years of heated floors under tile, vinyl and engineered have been built on. The flooring sheet says 27 to 29 C under wood or vinyl, and the warranty is tied to it; the designer has to know both numbers and the homeowner has to know which one they are living by. Those numbers only stay close if the house’s heat loss is low and the floor covering is thin. Pick a thick covering and the designer has to raise the water temperature, and the further that pushes a wood surface past the sheet the more you are relying on the builder’s experience and the product instead of the warranty, whatever the room thermostat says. A floor sensor or a mixing valve on the zone is how you know the number instead of guessing it. The radiant compatibility article gives the rule by floor type.
That is why the heat loss comes first. A room-by-room heat loss tells the designer how many BTU each room needs, the floor covering tells him the resistance, and the two together set the tubing spacing and the water temperature. The calculations we use on our own houses come from OntarioHeatLoss.ca, and the radiant design page explains the design process. Order of operations: insulate the house, choose the floors, do the heat loss, then size the system. Most radiant disappointments did it backwards.
Where homeowners go wrong
- Carpet in the basement bedrooms “because it is warmer.” The slab makes the floor warm; the carpet makes it feel warm while blocking the heat. A low-pile carpet on a thin radiant-rated pad, or a glued engineered floor with a washable rug, both do the job better.
- Floating a floor that could have been glued. The pad is R 0.4 and the air under it is more. Over a heated slab that is free heat left on the table.
- Mixing coverings on one zone. Tile in the kitchen and carpet in the family room on the same loop means one is too hot or the other too cold. Different coverings want different water temperatures, which means different zones or a mixing valve, decided at design.
- Changing the floor after the system is set. Pulling carpet for engineered on a zone designed for carpet puts the wood well past anything the sheet allows. The water temperature has to come down and the heat loss has to be rechecked.
- Ignoring the rugs. A thick rug under the dining table is a hot spot for the floor under it. Thin rugs, breathable backing, no rubber mats on wood.
What the better choice costs
The usual trade is carpet versus glued engineered in a basement bedroom. Carpet and pad supplied and installed, mid grade, is $5.00 a square foot; glue-down engineered is $12.75. A 12×14 bedroom is 168 sq ft: $840 against $2,142, a difference of about $1,300 per bedroom. Glue-down LVP at $7.00 splits it at $1,176. Against that, the carpet zone wants water 8 to 10 C hotter for the life of the house and leaves you with a slab you cannot put wood on later without re-engineering the loop. The basement flooring page is the longer version of what we put in our own basements, and the flooring cost calculator prices the rooms in any mix.
Questions people ask
What is the R-value of carpet and pad?
About 1.5 for a low-pile carpet on a thin dense pad, 2.5 to 3 for an ordinary carpet on 1/2 in. rebond, and 4 or more for a plush carpet on felt. All of it sits between the heated slab and the room.
What is the maximum R-value for flooring over radiant heat?
The radiant industry’s rule of thumb is to keep the total covering under about R 2, and under R 1 if you want the system to run cool and efficient. Our own rule is R 1: tile, vinyl, glued engineered. Anything thicker is designed for, zone by zone, before the slab is poured.
Does hardwood block radiant heat?
A little. Half-inch engineered glued to the slab is about R 0.6, which raises the needed water temperature by a few degrees over tile. In a well-insulated house that is nothing. Floating it on a pad roughly doubles the resistance.
Can I put a rug on a heated floor?
Yes, a thin one with a breathable backing. A thick rug or a rubber-backed mat traps heat and can push the wood under it well past the sheet’s cap while the rest of the floor is fine. Most hardwood makers mention rugs in the radiant fine print for this reason.
Why does my radiant floor feel warm but the room stays cold?
Too much insulation over the slab for the water temperature it was set at, usually carpet and pad. The floor surface is at the top of its range and the heat is not getting into the room. Either the water goes up, with the fuel bill, or the floor covering gets thinner.
Pick the floor, then size the system
The radiant flooring compatibility checker flags coverings that fight the slab and tells you the conditions for the ones that work.
Sources and further reading
- Keeping the Heat In: Natural Resources Canada: basement floors, insulation under slabs and the moisture that comes through concrete.
- Uponor Canada: hydronic radiant tubing and controls; their design manual is where the surface temperature and floor R-value tables come from.
- OntarioHeatLoss.ca: the stamped heat-loss design that sizes a radiant system to the floor covering on it.
