Tog is a measure of how good a material is at thermal insulation. In simple terms, the higher the tog the better something is at keeping heat in.
If you are technically minded, tog is actually a measure of the thermal resistance of a unit area. The basic unit of insulation coefficient is called the RSI (defined as 1 m2 K / Watt), and 1 tog is one tenth of an RSI, so 1 tog = 0.1 m2K/W.
We hope that’s clear, we’ll be testing you later!
When you put underfloor heating into a room you want as much heat as possible to come through into the room. The last thing you want is for the underlay to insulate the floor and to stop the heat getting through.
The lower the tog the better. In general, a tog of less than 1 will be acceptable.
It is generally easier for laminate underlays to have a very low tog as they are normally quite thin, so a tog of less than 0.5 is a good guideline to aim for.
Carpet underlays are usually thicker because they provide more padding for the carpet. This normally means they also have a tog greater than 1. Specialist carpet underlays designed for underfloor heating (UFH) are available with a tog as low as 0.75 but which still provide good cushioning and underfoot feel.
While the underfloor heating is on it will help to make sure the sub-floor is very dry, so you would think that you can do without a damp proof barrier. However, you need to consider what would happen in the summer when the underfloor heating is turned off - if we have a very wet summer (otherwise known as a normal British summer) then the ground can become very waterlogged. If it is possible for the damp to track through to your sub-floor then you will still need a vapour barrier of some kind.
The very best thing is to check with either the supplier of your underfloor heating system or the contractor who installs it and finishes the sub-floor.