Rugs and underfloor heating

What happens under the rug

Underfloor heating emits through the surface of the floor, at low temperature. Standard EN 1264 caps that surface temperature at around 29 °C in the occupied zone, a little more in the perimeter zone and in bathrooms. That is not much, and it is what makes the system sensitive to what is laid on it: it has no reserve of power to set against an obstacle.

Lay down a rug and you create a local thermal resistance. The heat that was rising freely is held back: it builds up in the screed, the underside of the rug rises in temperature, and the room receives less. If the control works on air temperature, the system compensates by heating more, which warms the underside of the rug further still. And if a floor sensor happens to be under the rug, it reads a value that is not that of the rest of the room.

None of this is dramatic in itself. But the rug now lives permanently on a warm, dry surface, heated from below, when it was knotted and has aged on a cold floor.

Thermal resistance adds up

The rule is published and simple: above underfloor heating, the thermal resistance of what covers the screed should not exceed 0.15 m²·K/W. That is not a value per layer, it is a total. The floor covering already has its own — tiling has almost none, a wooden floor has a real one — and the rug, plus its underlay, are added on top.

The order of magnitude can be worked out in two minutes: the resistance is the thickness divided by the conductivity, R equals e over λ. For a wool textile, trapped air included, λ is around 0.04 to 0.06 W/(m·K). Measure the total thickness of the rug, pile and foundation, in metres.

• Short pile, 5 mm: R of the order of 0.08 to 0.12 m²·K/W, acceptable on its own, already borderline over a wooden floor
• Medium pile, 10 mm: 0.17 to 0.25 m²·K/W, above the limit on its own
• Long pile, 20 mm: beyond 0.3 m²·K/W, the rug becomes a blanket
• Felt or foam underlay: often 0.05 to 0.10 more, sometimes a good deal more
• Open-mesh anti-slip grid: negligible contribution, which is the whole point of it

These values are orders of magnitude, not certified measurements: density and method of manufacture make them vary. They are enough all the same to settle most cases, and they show something you do not expect — the underlay, chosen without a thought, often weighs more in the total than the rug itself.

Wool that dries out

Wool is hygroscopic. At room temperature and 65% relative humidity, it holds of the order of 15% of its weight in water, bound inside the fibre. That water is not an accident: it is what gives wool its suppleness and its resistance to repeated flexing.

A heated home falls in winter towards 30 or 35% relative humidity, sometimes less, and the water content of the wool drops towards 7 to 9%. Over underfloor heating, the phenomenon is made worse from below: the underside is heated continuously and the drying happens from the bottom up, unevenly.

Wool that is too dry loses its spring, builds up static electricity and breaks more easily when flexed. None of this shows in one winter: it shows on a rug that has stayed ten years in the same place, whose pile no longer stands back up.

Shrinkage of the foundation

The most concrete problem, though, is not the wool, it is the cotton. The warp and weft of a hand-knotted rug are almost always cotton, and cellulose contracts as it dries. A rug dried from below, over a long period, sees its foundation shrink — and not everywhere in the same way, because the temperature of the floor is not uniform and the edges often extend beyond the heated zone.

The result is seen around the edges: the selvedges lift, the sides ripple, a corner turns up, the rug no longer lies flat. People blame a manufacturing fault or a rug distorted in washing; it is often prolonged, asymmetric drying. On an antique piece, the cotton turns brittle and gives way under flexing, and you find splits along the lines of strain. That is reweaving: it can be repaired, but it is better avoided.

One honest qualification, because it changes the conclusion: these effects are gradual and cumulative. No rug is ruined in a single heating season. It is ten years in the same place, on a warm floor and in dry air, that make the difference — and those are ten years that can be spent otherwise.

What suits, and what does not

What suits: a short pile, wool, a cotton foundation rather than a glued backing, bearing in mind that it is the foundation that will need watching, a small total thickness, a hand-knotted rug. Flatweaves — kilims, soumaks — are the best candidates: a few millimetres, no intermediate layer, negligible resistance. If an underlay is needed, it must be open: an open-mesh grid holds the rug without forming a continuous layer.

What does not suit: long pile and shaggy rugs, which are insulators before they are rugs; underlays of rubber, solid foam or thick felt, which double the thermal resistance and whose components can migrate and mark a warm floor; tufted rugs with a latex backing, whose adhesive ages faster in the warmth and crumbles; and silk, the most sensitive of all to dryness, which turns brittle and loses its lustre for good.

Two laying rules go with this. Do not cover the whole heated surface: a covered area emits almost nothing, and a free margin around the rug lets the system work. And glue nothing down: the rug must be able to be moved and turned over, and a rug glued to a heated floor will never be checked on the back again.

The particular care these rugs need

Three things really make a difference. Watch the relative humidity with a hygrometer, and hold it between 40 and 55% through the heating season. Below 35%, a humidifier does more for your rug than any care product — and incidentally for your wooden furniture.

Turn the rug two to four times a year, by a quarter or a half turn. That spreads the wear, the exposure to light, and above all the drying from below, the only genuinely new factor here. A rug that is turned barely distorts at all.

Vacuum both faces more often than usual. Dry air and convection carry the fine dust down to the base of the pile, where it acts as an abrasive between the strands at every step. Over underfloor heating, a rug looks clean for longer than it is. Vacuum under the rug as well.

And one thing never to do: dry a washed rug on the heated floor. The drying there is violent, one-sided and uncontrollable — the surest way to obtain in one go the shrinkage that would take ten years to set in. A washed rug is dried flat, in controlled conditions: with us, in under twenty-four hours. See wool rug washing.

What to check before buying

Before buying a rug for a room heated from the floor, take one measurement and ask four questions. The total thickness in millimetres, measured and not judged by eye. The declared thermal resistance, in m²·K/W: a professional seller should be able to give it, or at least the thickness and the composition so that you can do the calculation. The nature of the backing: no latex, no rubber, no glued tufting. And the recommended underlay, whose resistance is added to the total and which is always forgotten in it.

Then check on the floor side: the maximum surface temperature planned by the installer, and the position of the floor sensor if there is one, so as not to lay the rug over it. An infrared thermometer costs little and allows a real check: take the temperature of the bare floor, then that of the floor after three or four hours under the rug. A large difference tells you that the rug is too insulating for that room — before the foundation tells you instead.

One last point, which is not technical. A valuable antique piece has no business on underfloor heating in continuous use: the comfort it brings is that of a more recent and finer weave, but the risk cannot be undone. Keep the heated floor for a kilim or a short pile, and put the antique piece in a bedroom.