Who Wants Underfloor Heating?

In the previous two weeks on this blog I have introduced and discussed both overlay and screed underfloor heating systems.

This week we compare the two to help you decide which may be right for you.

Underfloor Heating: Is Overlay or Screed Best?

Construction depth

Overlay systems because of their low profile are ideally suited for situations where the floor level cannot be reduced. However even an overlay system will likely require threshold and door modifications around the home. Screed systems for retrofit properties where the floor level cannot be altered often are a none-starter unless suspended timber floors are present on the ground floor.

Insulation levels

Overlay systems because of their requirement to be low profile have a reduced ability to provide resistance to heat penetrating the floor beneath resulting in precious energy being lost to the floor below. Conversely with greater floor depth the installer of a screed system has the luxury of installing very high levels of insulation to stop heat being wasted.

Thermal mass

Whilst the fibrous board that the overlay system pipes are clipped into has some thermal mass its ability to retain heat once the underfloor heating is switched off will be markedly reduced compared to a screed solution. Simple intuition, let alone physics tells us that by installing a large quantity of screed around the underfloor heating pipes this mass will hold heat better and reduce unwanted floor temperature fluctuations.

Pipe spacing

For efficient low temperature systems pipe spacing is proportional to ultimate efficiency. The ability to minimise pipe spacing and change pipe detail near key building openings is common sense. Screed systems allow this design flexibility but with pre-machined boards this is simply not possible and heat pumps especially have to work hard to operate successfully with overlay systems.

Water flow rates and temperatures

The small bore pies used by overlay systems (nominally 10-12mm) and require water at 45-55oC to create suitable levels of heating during winter. Alternatively screed systems can run at much lower temperatures 35-45oC and their larger pipe diameters (nominally 15-16mm) allow slower water flow to enable better heat transfer to the room.

Cost

The all in costs for the two different systems are stark. Including materials, labour and VAT screed UFH prices come out around £40/sqm. In comparison overlay systems cost around £70/sqm installed.

Summary

When all is said and done the only advantage of overlay systems is the ability to lay a wet underfloor system over a floor that simply cannot be modified or would prove too costly to take up and remove. Given the price difference in the two solution types not to mention the ongoing running costs of an inefficient overlay system there may well be a financial argument in favour of taking the plunge and taking up that floor.

Either way as always I hope you are a little more informed than you were before and can proceed with more confidence to whatever the final solution may be.

photo credit: Jeremy Jenum via photopin cc

Doug Johnson

Doug is a chartered mechanical engineer and has had the pleasure of being involved in the UK renewable sector since 2008. With wide-ranging operational and design experience in both the domestic and commercial sectors across a wide range of recognised technologies he hopes to be able to share his experience to help enlighten and save you hassle along the way.