Around 15% of the heat leaving a house exits through the ground floor. And that does not account for the draughts coming up through the floor. So the simple answer is Yes - it is worth insulating under the floor boards.

The problem is that, unless you have space to get under the floor boards, it means taking the floor boards up.

The illustration above is a typically method of insulating.

In this case the insulation would typically be a quilt type, such as mineral wool, sheep wool or hemp. The insulation would ideally be up to 200mm, or the thickness of the joists. 200mm will reduce the U-value from 3.98W/m2 for an uninsulated floor to around 0.3W/m2.

The advantage quilt has over rigid foams is that it can be cut slightly over-size and “squeezed” in to the gap, thus eliminating draughts. If rigid foam must be used then simply replace the wire lacing with battens nailed to either side of the joist and put mastic around the edges of the insulation to stop the draughts.

Care must be taken to ensure that any air bricks are not covered with the insulation. The air bricks are there to ensure a good circulation of air over the joists to prevent rot. So long as the bottom surface of the joist is exposed and there is plenty of air circulating there will be no problems.

About the author: Tim Pullen is eco-editor for Homebuilding & Renovating magazine, author of Simply Sustainable Homes and founder of sustainable property consultancy WeatherWorks.

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