Underfloor Heating: Retrofit Costs and Limitations

Whether it works in an existing house depends almost entirely on the floor construction and the insulation beneath it.

Wet versus electric

Wet systems circulate warm water through pipes embedded in or beneath the floor. Higher installation cost, considerably lower running cost, and suitable for whole-house heating.

Electric mat systems are thin, cheap to install and expensive to run. They suit single rooms — a bathroom or a kitchen — as comfort heating rather than as a primary heat source.

Choosing electric for a whole house because installation is cheaper is a decision paid for repeatedly in running costs.

The insulation question

Underfloor heating radiates in all directions. Without insulation beneath the pipes, a meaningful proportion of the heat goes downward into the ground or the void below.

In a solid floor without insulation, retrofitting heating without addressing that is poor value. Doing it properly means excavating and rebuilding the floor build-up, which is disruptive and expensive.

Suspended timber floors can be insulated between joists from below where there is access, which is considerably easier.

Floor height

Retrofit systems add height. Traditional screeded systems add substantially; low-profile overlay systems add less but still change levels.

That affects door heights, thresholds, stair bottom risers and skirting. Every doorway needs the door trimming, and level changes between rooms need managing.

This is the practical constraint that most often rules retrofit out in an existing house.

Floor coverings

Tile and stone conduct heat well and are ideal. Engineered wood is generally suitable if specified for underfloor use and installed with attention to moisture content.

Solid timber can move with the heat cycling and is often not recommended. Thick carpet and heavy underlay insulate the room from the heat you are paying for — total thermal resistance of the covering needs to stay low.

Control and running

Underfloor heating responds slowly. It suits steady background heating rather than rapid boosts, and controls should reflect that with longer, gentler schedules.

Zone control per room allows heating only occupied areas and is worth specifying at installation.

Because it runs at lower flow temperatures, it pairs well with a heat pump and improves that system's efficiency considerably compared with radiators.

Where retrofit makes sense

During a full renovation where floors are coming up anyway. In an extension, where the floor is being built from scratch and can be insulated properly.

In a single room being refitted, particularly a bathroom, where electric mats are quick and the running cost is modest for the area involved.