Green Homes Grant | Floor insulation (for a suspended floor)

What it is

If your house has a suspended timber floor on the ground floor, then a layer of insulation can be held in place under the floor between the joists. This is called Under Floor Insulation (UFI).

The first step is to remove any carpet or floor covering so that you can access and lift the floorboards to see if your home could have UFI. Before carrying out any work, a thorough inspection of all the joists and underfloor timbers should be carried out by an approved professional for signs of damp, rot or insect damage, and appropriate repairs carried out if required.

There are several different insulation materials and products which can be used, including mineral wool, rigid boards and foam and professional advice should be obtained on the appropriate thickness of each of these to be installed in your home. Regardless of which product is selected it is important to ensure that no gaps are left in the insulation coverage, especially between the end joists and the floor perimeter wall.

If you have an unheated cellar beneath your house, you can fit the insulation from underneath without removing the floor covering and floorboards.

Is it right for me?

If the ground floor of your house is built on wooden joists with a gap underneath, then floor insulation is possible. If there are any damp problems affecting the floor, or any signs of rot or insect damage, then you will need to get the underlying problem solved and any repair work carried out before you can insulate your floor safely.

The gap beneath a suspended floor should always be well ventilated, and this becomes particularly important when you insulate the floor. Usually there will be a number of air bricks fitted in the outside walls beneath the floor level, and these should never be blocked off. If you do not have any air bricks supplying ventilation to the void below your floor, these will need to be added at the same time as the insulation and professional advice should be obtained on the appropriate location and number of these. If these are not installed there is a risk that moisture can build up and condense on timbers in the floor, increasing the risk of damp and rot.

Having UFI usually means that the room contents will need to be removed to provide access. You may want to consider doing this at a time when the room is empty for other reasons, such as when you are replacing the carpet, fitting a new kitchen etc.

You should usually fit insulation on the ground floor only, plus any other floor that is over an unheated space, such as a garage, or a walk way. There is no need to insulate any floor that is immediately above another heated space, such as the intermediate floor, which is the floor between the ground and first floors of a house.

If your ground floor is a concrete slab with no gap underneath, then you have what is known as a solid floor. This can be insulated, but the process is completely different and can be a lot more disruptive.


  • How to get it
  • You should use an approved professional installer to fit suspended floor insulation.
  • You can find professional installers to insulate your suspended floor by using the following link.
  • The installer will generally provide all the materials.
  • Other things you may want to know

UFI is not currently very common in UK houses. However, because it is an effective way to reduce heat loss from your house and reduce your fuel bills, all modern houses must now be built with floor insulation. UFI is therefore a good improvement option for most existing homes.

Installing UFI will reduce the air temperature in the void under your home. This will not be a problem unless you have existing damp issues in the floor or if you do not have adequate ventilation under the floor. Professional advice should be obtained to check if you have any existing damp issues and also identify if you require any additional ventilation (air bricks) adding to the floor void.

In uninsulated floors heat loss is relatively uniform and so usually there tends not to be any cold spots or condensation risks. However, if UFI is added and gaps in the insulation layer are left, i.e. an area is left uninsulated for some reason then you will create new cold spots in these gaps. This may occur if for instance there was some built in furniture or bathroom or kitchen units and so access to these areas was limited. There is a risk that these new cold spots will pose a condensation risk, especially if they are in areas of high humidity, such as under baths, showers and sinks. It is therefore advisable to always insulate 100% of the floor.

A similar problem occurs around the perimeter of floors especially if floor joists are located very close to the wall and there is very little room available in which to place insulation. Missing out these areas will cause a cold strip at the floor perimeter which can again become a condensation risk, especially as corners are always the coldest parts of rooms.

In summary, UFI can be an effective means to improve the efficiency of homes. However, unless UFI is installed carefully by an approved installer it could cause condensation and increase the risk of rot in floors.


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