When should underfloor heating be installed?

Waterproofing a shower

Underfloor heating is installed near the end of the build, just before the final floor covering goes down. The walls should be lined, all gib stopping and sanding complete, and all painting finished. The one piece of sequencing that is not flexible is waterproofing: if underfloor heating is going into a shower, the waterproofing must be done after the heating is installed.

Where does underfloor heating sit in the build program?

Late, and for a practical reason. The heating cable is laid on the prepared subfloor and screeded over, and from that moment until the floor covering goes down, the screeded surface and the heating cable underneath is vulnerable. Every day it sits exposed to other trades is a day something can be dropped on it. Installing it after all the other trades have completed their work keeps that window as short as possible.

The sequence Heatwell works to:

1.   Walls lined, gib stopped and sanded.

2.   All painting complete.

3.   Floor prepared — old coverings, glue and adhesive removed, surface clean, free of dust and paint, and free of cracks, holes and sharp protrusions.

4.   Wall prep done: 25mm holes drilled through the wall nogs, a 50mm wide and 25mm deep section removed from the base plate, and a draw wire run from the thermostat flush box to the bottom plate.

5.   Underfloor heating cable installed, fibreglass mesh laid, screed poured.

6.   Screed dries — allow 24 hours, longer in cold conditions.

7.   Final floor covering laid.

8.   Electrician connects and commissions the thermostat.

Does underfloor heating go before or after waterproofing?

In a bathroom, if underfloor heating is going into the shower, waterproofing must be completed after the underfloor heating. Not before, and not at the installer's discretion. The heating has to be in and screeded, and the waterproofing goes over the top.

If there is no underfloor heating in the shower, underfloor heating can be installed either before or after waterproofing in a bathroom. Heatwell's preference is still to install the heating first and waterproof afterwards wherever possible. Where waterproofing has already been done, a waterproofing patch is needed where the cold tails pass up through the wall.

In a laundry or a kitchen the same principle applies: if waterproofing is required it is preferable that underfloor heating has been screeded in beforehand.

The screed will need 24 hours to dry. In cold conditions it takes longer, and a heater in the area helps. No floor covering can go down until the screed is fully dry. Moisture trapped under a floor covering can cause many problems including shrinkage, delamination and lifting, and it is avoidable simply by waiting.

Does the floor need to be level before underfloor heating cable is laid?

Yes. In some cases floor-levelling compound will need to be installed 2 times. First to level the floor and second to bind in the underfloor heating cable. If the floor is not level first the underfloor heating will be inconsistent. Colder in areas where the cable is buried deep. Warmer in areas where the cable is shallow. If you are going to do it, do it properly.

What has to be planned before the cable goes in?

Anything that will be fixed into the floor needs to be planned and allowed for before the underfloor heating cable is laid. Shower screens, doorstops, anything screwed down. Drilling into a finished floor without planning risks drilling into the heating cable, and risks a lengthy and expensive repair job. Remedial work on a finished floor also voids the warranty. If the position of a shower screen is known, say so before the cable is laid.

What does the main contractor need to manage?

Two things, both small and both easy to lose.

The kOhms reading is written on tape attached to the cold tails at the flush box. The flooring installer and the electrician both need it, so it has to still be there when they arrive.

The photographs. Heatwell's Best Practice requires a minimum of four photos from the installer and two from the electrician. They are the record of when the cable was intact, and they are what makes a warranty claim possible if something goes wrong later.

Can underfloor heating be installed in winter?

Yes. Screed drying takes longer in the cold, so allow more than 24 hours and use a heater in the space.

How long does the installation itself take? 

Heatwell are professional installers. We have the experience and we have all the necessary equipment A typical bathroom only takes Heatwell 60 to 90 minutes.

Installing a cable DIY will take longer. If you are going to install the cable yourself, allow 2 to 3 hours for a typical bathroom.

What happens if the cable is damaged after installation? 

If you think there is even the slightest chance of cable damage, test the resistance in kOhms immediately! A kOhms test only takes 5 minutes. A damaged cable is easily fixable by an experienced electrician if the damage is found before the final flooring is installed. Finding damage to a cable after the final flooring has been installed will result in a costly and lengthy repair.