How can I insulate a solid floor?

15% of a household's heat is lost through the floor so insulating this area is important. A solid floor of stabilised earth or limecrete should have a solid insulation material below it, such as cork, perlite or foamed glass, with recycled polythene vapour check and damp-proof membrane (DPM) below this.

Traditionally, solid floors were laid directly onto soil. This relies on the ground underneath being kept dry, usually by it being higher than the ground outside the building, and by having adequate drainage.

The most common method now used is to have a thick concrete slab laid on a damp-proof course (e.g. a polythene membrane). A layer of polystyrene insulation is then finished with sand/cement screed and tiles or board.

For a low-impact alternative to the above you could look into using recycled aggregate in the concrete (rather then newly quarried material), and perhaps using stabilised earth as the screed. You could also consider using recycled polythene or bitumen for the damp-proof course.

Try to achieve at least 150mm of insulation for a solid floor. Insulation should be placed around the edge of the floor, and the floor finish supported on some sort of rigid insulation. Possible materials include cork, perlite (volcanic glass), lightweight expanded clay aggregate ('Leca'), foamed glass (slabs or granules), fibreboard, mineral wool boards, or plastic foam of some sort. A vapour check layer will normally be required to prevent condensation occurring within the insulation layer.

Another possibility is a hemp & lime (or 'hempcrete') floor. Lime has a much lower environmental impact than cement, so if you can use it place of cement in mortars or concrete you will be reducing the 'embodied energy' of the floor and the carbon emissions from construction. The hemp provides the insulation.

For example, in CAT's recently constructed WISE building, the ground bearing slabs were constructed of Hempcrete (a lime based binder mixed with hemp shiv). The Hempcrete was laid on 200mm of perlite insulation, used in bags. A vapour control/radon barrier was supplied by a recycled polythene membrane. Floating floors were laid onto all floors (including suspended floors), all with a layer of 'Silencio Thermo' grooved softboard and underfloor heating pipes, and finished with a proprietary floating floor with plywood and looselaid, secretly nailed, ash floorboard.

If you are redoing a floor, then you may have the chance to consider underfloor heating. Because it runs at a much lower temperature than standard radiators, wet underfloor heating is more efficient and provides a more comfortable type of heat (especially in larger rooms or those with high ceilings). Underfloor heating should use water heated to about 35 degrees C (compared to 70 degrees for radiators) running through a system of pipes laid in the solid floor (or under a timber floor). It is particularly appropriate for use with heat pumps, as these need to supply low-temperature water to run efficiently.

Electric underfloor heating should be avoided, as you are then tied in to using electricity, which is a very expensive fuel. Wet undefloor heating is a flexible option, as the appliance heating the water can be changed in future.

You can also see some information on the these web sites:

Study at CAT: Our University Courses

Graduate School of the Environment - Sustainability Masters Courses

Have more questions? Get in touch!
use our form or call 01654 705989

Related questions