Underfloor heating in period and listed homes: stone floors, lime and consent

Old houses are often cold at floor level, and radiators can look out of place against panelling, stone and original joinery. Underfloor heating hides the heat source and warms the room evenly at low water temperatures, which suits slow-responding solid walls. It can also be the moment to deal with a damp or failing floor.

The risk is treating a 200-year-old house like a new build. An impermeable floor can push ground moisture into the walls, and lifting historic flagstones or boards without consent can lead to prosecution. This guide covers consent, solid stone and earth floors, limecrete, suspended timber floors and how to run the system once it is in.

Do you need listed building consent for underfloor heating?

Usually, yes, if the work disturbs historic floors or fabric. In England, listing protects the whole building including the interior, and consent is needed for any change that affects its character. The Royal Borough of Kensington and Chelsea's guidance gives removing floorboards as an example of smaller works that need consent.

• It is a criminal offence. Carrying out works that affect a listed building's interest without consent is a criminal offence. Both the owner and the person doing the work can be liable, with fines of up to £20000 in the magistrates' court or unlimited in the Crown Court, according to RBKC.
• Interiors are covered even if not described. Withers' legal briefing notes that the Planning (Listed Buildings and Conservation Areas) Act 1990 does not separate internal and external works, and that a list entry saying "interior not inspected" does not exclude the interior.
• Record what you lift. Cotswold District Council advises making a record before lifting historic boards or flagstones so the floor can be relaid the same way.
• Building Regulations may apply too. Cotswold also notes that lifting and replacing a whole floor may bring in Building Regulations.

Talk to your local conservation officer early, before you order materials. Consent rules and procedures differ across the UK, so check with your own local planning authority.

Why is concrete and a DPM a problem in an old house?

Because old solid floors were built to let moisture out. GreenSpec explains that traditional solid floors are breathable: moisture rising from the ground evaporates through the floor. When that floor is replaced with an impermeable slab and membrane, ground moisture collects under it and is drawn up into porous footings and walls instead.

The result can be damp at the wall and floor junction, salts drawn into the masonry, failing decoration and decay of timbers or earth walls. That is why heritage bodies and councils recommend breathable floor build-ups for historic buildings. Cotswold District Council strongly recommends limecrete with vapour-permeable insulation such as foamed glass when an old floor is replaced, though it notes limecrete is not suitable everywhere, for example where the water table is high.

How does limecrete underfloor heating work?

The pipes sit in a lime screed on top of a limecrete slab, which sits on a deep bed of loose foamed glass insulation. The whole build-up lets moisture pass while keeping heat in. One lime flooring supplier's technical guide gives these typical build-ups:

Build-up	Insulation (recycled foamed glass)	Limecrete slab	Limecrete screed
Slab and screed	150 to 450mm or more	100 to 150mm	50 to 100mm, at least 70mm with UFH
Slabless	150 to 450mm or more	None	90 to 120mm

The total depth is the insulation, slab, bedding and floor finish added together, so these floors usually mean digging out. GreenSpec warns that digging below the foundations can destabilise the building and suggests two or three test pits by the walls first to find the foundation depth.

The usual sequence, from GreenSpec and that lime flooring supplier, is:

1. Dig out and level. Check foundation depth first.
2. Geotextile. A breathable membrane on the prepared ground.
3. Loose insulation. Foamed glass or expanded clay. GreenSpec gives foamed glass a lambda of about 0.075 to 0.08W/mK against about 0.1W/mK for expanded clay.
4. Limecrete slab. Cast and left to cure, around three weeks in GreenSpec's description.
5. Pipes. Clipped or tied to a geogrid, then pressure-tested by the heating engineer before the screed goes on.
6. Lime screed. Laid over the pipes and protected from drying too fast.

If radon is present, a membrane will likely be needed. The supplier puts a radon barrier above the foamed glass.

When can you turn the heating on in a limecrete floor?

Not until the floor is at least 28 days old, and then only slowly. The supplier says to commission the heating only after 28 days, and after any floor finish has dried or set. Start at the lowest flow temperature and raise it by about 1 to 2°C every couple of days until you reach the target.

That is far slower than a typical cement screed start-up. Lime gains strength slowly as it carbonates, so give it time. Set the controls so the system cannot run hot in the first heating season, for example by capping the flow temperature at the manifold. Our guide to underfloor heating temperature settings explains the options.

Which floor finishes suit a breathable heated floor?

Finishes that also let moisture through. GreenSpec advises avoiding glazed ceramic tiles, cementitious adhesives and grout, and rubber carpet underlay, because they seal the floor.

• Stone flags and clay tiles. The supplier says these can go down after about 28 days, once the slab has started to carbonate, bedded in lime mortar or lime-based adhesive. Relaying original flags is possible but takes care.
• Timber. Wait until moisture content is 2 to 3% or less, usually at least three months. The supplier's check is to tape down 1m² plastic sheets for 24 hours and look for condensation. Lay timber on battens or a support system rather than gluing it down.
• Carpet. Only once the floor is fully dry, on a breathable underlay such as jute, hessian or hemp.

Can you fit underfloor heating in an old suspended timber floor?

Yes, often without lifting much at all. In a period house with a cellar or a void below, pipes can be fitted between the joists from underneath with aluminium spreader plates, leaving historic boards in place. Where boards must come up, mark and record each one so they go back in the same position.

• From below. Fit-from-below spreader plates clip to the underside of the boards. See fitting spreader plates from below.
• From above. Lift boards, fit plates and insulation between the joists, then relay. Our guide to underfloor heating in joisted timber floors covers both methods.
• Insulate below the pipes. Without insulation under the plates, much of the heat goes down into the void.
• Keep the void ventilated. Cotswold District Council advises dealing with any damp first. Do not block airbricks that keep the timbers dry.

Cotswold also points out that simple draught-proofing between floorboards is a low-cost way to cut heat loss, which helps any heating system.

Which heat source and controls suit a period home?

A low flow temperature suits both the floor and the building. Lime floors and old solid walls respond slowly, so the system works best running steadily at a low temperature rather than in short hot bursts. A heat pump does this well, and so does a boiler with the UFH on a mixing unit. See underfloor heating with a heat pump.

Use pipe with an oxygen barrier, such as PERT-EVOH pipe, or multilayer pipe, as the pipe will be very hard to reach once the lime screed is down. Pressure-test before screeding and keep the test certificate with the consent papers.

MaxHeatUFH supplies the kit but does not install. If you want a vetted installer for an old or listed property, send us the details through our installation enquiry.

Frequently asked questions

Can you put underfloor heating in a listed building?

Yes, with listed building consent where the work affects historic fabric, such as lifting floorboards or flagstones. Listing covers the interior, and unauthorised works are a criminal offence. Speak to your conservation officer first.

What is limecrete?

A concrete-like slab made with natural hydraulic lime instead of cement, usually laid over loose foamed glass or expanded clay insulation. It lets moisture move through the floor, which suits old solid-walled buildings.

How deep is a limecrete floor with underfloor heating?

One lime flooring supplier gives 150 to 450mm or more of foamed glass, a 100 to 150mm slab and a 50 to 100mm screed, at least 70mm with UFH, plus the floor finish. Most jobs need digging out.

Can I relay the original flagstones over underfloor heating?

Often, yes. Record and number them before lifting, then relay in lime mortar or a lime-based adhesive once the floor has started to carbonate, after about 28 days.

How long before limecrete underfloor heating can be switched on?

One lime flooring supplier says at least 28 days, then start at the lowest flow temperature and raise it about 1 to 2°C every couple of days.

Can I use tiles and tile adhesive on a limecrete floor?

GreenSpec advises avoiding glazed ceramic tiles and cementitious adhesives and grout, because they stop the floor breathing. Use breathable clay or stone bedded in lime.

Is underfloor heating suitable for an old house with no damp-proof course?

It can be, but deal with the cause of any damp first and use a breathable floor build-up. Get advice from a surveyor who knows traditional buildings before you choose the floor.

Sources: RBKC: listed buildings explained, Withers: listed buildings, it's what's inside that counts, Cotswold District Council: historic buildings, floors, GreenSpec: limecrete floors for old buildings