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Floor insulation

Underfloor insulation being laid between the floor joists of a Victorian home
27 February 2023

Insulating the ground floor of your house is an effective way to keep your home warm and reduce your energy bills.

Floor insulation reduces the amount of heat that travels directly through the floor fabric. If you want to know about draughtproofing – sealing the gaps between the floorboards and along the skirting – that’s here.

Whether you have a suspended wooden floor or a concrete floor, improving your insulation could save you £53-68 a year if you live in a mid-terraced house or semi-detached, and up to £96-121 a year in a detached house or bungalow.

Floor insulation is a significant undertaking. It should only be tacked by a professional builder or a very experienced and competent DIY-er.

What kind of floor do you have?

Almost all UK homes will have one of two types of floor. A suspended timber floor (boards laid over wooden joists), or a solid concrete floor.

These are insulated in different ways.

But in each case, if you are the homeowner, it’s your responsibility to comply with Building Regulations.

Adding extra insulation to your existing floor may include making sure:

It’s a good idea to get advice from a Building Control Officer at your local council before carrying out the work.

Insulating a timber floor

Before you install additional insulation under your floor, make sure any damage from damp, rot or infestation is repaired. You also need to ensure the new insulation doesn’t block any ventilation openings like air bricks.

Solid insulation board (pictured below) or rolls of mineral fibre (like the stuff used to insulate lofts) can be fitted between the flooring joists. If your floor is above an unheated cellar or basement, you’ll need to fit the insulation snugly between the joists and secure in place with netting if required. Plasterboard should then be fixed to the ceiling of the basement to provide fire resistance.

Foam insulation boards are laid between the floor joists
Foam insulation boards are laid between the floor joists

If you have a suspended floor with a small cavity below that’s hard to access, you’ll probably have to take up the floorboards to fit the insulation. However, there are companies like Q-Bot that insulate floors from below. They use a remote-control vehicle – a floor-insulating robot – that surveys the underside of the floorboards, before spraying on insulation.

When insulating a suspended timber floor, a vapour permeable airtightness layer should also be installed. This will reduce heat lost through infiltration but also through the floor fabric.

Insulating a concrete floor

Insulation can be added over an existing concrete slab to make a room warmer and more comfortable.

This is normally in the form of high-performance insulation panels or boards that provide the best thermal performance at any given thickness.

Keeping the thickness of your insulation to a minimum will reduce the likelihood of having to make alterations to door openings, stairs and other fixtures as a result of the increase in floor height.

If your chosen insulation does not come already attached to a layer of moisture-resistant chipboard, you may need to lay a separate deck on top. We reccomend laying a damp-proof membrane underneath the insulation (taking care to overlap any damp-proof course in the external walls).

If an old concrete floor is being replaced, you will likely be required by Building Regulations to upgrade the insulation. In almost all cases this is a job for a professional builder.

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