Floor Insulation

What is floor insulation?

Insulating a floor involves adding an insulating material beneath the floorboards, thereby reducing heat escaping through the floor into the ground. Approximately 15% of heat is lost through a house via this route. Insulation also acts to prevent draughts coming up through the floorboards. In addition the household should also consider insulating the gaps between the skirting boards and the floor, which also helps in reducing draughts.

Floor insulation is most commonly done when putting a new floor in place, but most floors can be retrofitted with insulating material, and this will make a large saving to your overall heating bill.

Insulating a suspended timber floor

A suspended timber floor, where the floorboards sit upon wooden joists can be insulated in one of two ways:

1. Access the floorboards from underneath

If you can easily access the underneath of the floorboards (potentially if you have a cellar), then you can simply insert rigid insulating board between the joists and use battens to hold them in place (if required). Using a saw you need to cut the insulating board to the exact width of the joists, if you have done this correctly then you can use a wooden mallet to gently tap the insulation into place and it should stay where it is so you may not require the battens.

Insulation

If when you have cut the insulation, the width of it is much smaller than the width of the joists, then obviously it will simply drop out of the joists, so in this instance you will need to hold them in place. The easiest way to do this is to push the insulation in place and hold it there using battens that you will need to attach to the insides of the joists as shown in the diagram.

2. No access to the underneath of the suspended floor

Unfortunately if there is no access to the underneath of the suspended floor then the job is made more difficult because the floorboards need to be removed prior to insulating. In this situation wool insulation is used as opposed to the rigid insulating boards.

The first step is to lift the floorboards up, to expose the joists. This will probably mean moving furniture off the floor, and it is worth marking where the floorboards go (either drawing a map or numbering them from a corner) if the floorboards are irregular sizes to ensure they fit back as before!

Once you have exposed the joists, you may notice that one or two may be rotting so these can be replaced if appropriate. Next a net needs to be spread over the whole of the floor space ensuring that it sags substantially between each of the joists. This sagging net acts as a ‘hammock’ in which the insulation sits.

The next step is to lay the insulating wool between the joists, normally by running it parellel to the joists. It is worth trying to purchase wool with a width similar to the joists, although all insulating wool can be cut to size very easily to ensure a nice snug fit. As is the case for loft insulation, the thicker the wool, the better the insulation will be, however you definitely do not want the insulation touching the ground beneath the suspended timber floor so make sure the net does not sag to that extent. Installing floor insulation should also act as a draught excluder stopping cold draughts enter the home through the floor. We do recommend using sheepwool insulation (despite being more expensive), because it is more breathable than the man made insulating wool and it is also a 100% natural material that can be handled without protective clothing.

Once the insulation is in place between the joists, the floorboards need to be relaid over the joists to complete the job.

Insulating solid floors

Many homes are built with solid floors, most of which are made from concrete. These can either be insulated as and when they need replacing, or they can be specially targeted for retrofit insulation. You will however, lose a few inches from the height of the room, as the floor will now sit slightly higher.

Insulating concrete floors cannot be done with the wool used for wooden floors, as this is soft and would compress as people walked across it. Instead sheets of polystyrene insulating board are used (such as Celotex 50 /100 mm insulating board). Polystyrene is easy to cut and drill holes in, but has high compression resistance; it also has excellent insulating properties.

First lay a damp proof membrane over the entirety of the solid floor. Next, lay the polystyrene insulating boards together across the whole floor so there are no gaps, and join them together using glue or insulating tape. Then to finish, lay chipboard across the whole floor (connecting pieces to one another using the tongue and grove connection) and glue into place.

Carrying out floor insulation

Not all home insulation work has to be carried out by a professional. It is fairly simple to do if you are a DIY enthusiast, and all the materials are available from your local DIY store. A roll of the fibre (7m long x 1m wide) will cost about £15, and the insulating boards for concrete floors are approximately £20 (2.4m x 1.2m). Obviously the wool fibres and insulating boards come in different thicknesses and therefore provide different levels of insulation, so please speak to your local DIY store representative for more detail. Filling in the gaps between the skirting board and the floor with a Silicon Sealant costs about £20.

Professional installation of floor insulation is obviously more expensive, so it is worth checking with a variety of local firms to get quotes.

If you do decide to carry out the insulation work yourself, be sure to take appropriate safety precautions, like wearing protective clothing as things like the glasswool are an irritant, so should not be handled with bare skin.

Also it is not necessary to insulate floors on the second or third floor if the rooms below are heated. If the room resides over a basement or a garage for instance you can use a similar process to the wooden floor guide above, by simply stapling the wool between the joists (if you can access them).

Benefits

    • Filling the gap between the skirting boards and the floor can save about £25 and be recouped within the year (assuming a £20 material cost).
    • Since the insulation will slow the movement of heat through the floor, the home will feel more comfortable and warmer in the winter, but cooler in the summer months.
    • You can reduce your carbon footprint by about 240kg per year by installing floor insulation and almost 100kg if you fill the gaps between floor and skirting boards.
    • According to the Energy Saving Trust, installing floor insulation underneath a wood surface saves about £60 per year, which would mean a payback of 2 years based on recouping the material costs (approximately £100).

Limitations

    • If you are wishing to installing the floor insulation as a DIY project, you will need to move furnishings and potentially remove carpets and floorboards.
    • Additional costs may add up, if once you remove the floorboards, you discover that some are rotten, therefore the overall insulation costs may be higher than previously predicted.

Cost

    • Professional installation costs start at £770 but are dependant on the size of the floor space installation.
    • DIY Cost for wooden floor insulation cost approximately £100 and it will cost about £20 to buy the materials to fill the gaps between the floor and the skirting boards.