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Heat loss through the ground floor of a two-storey house typically accounts for about 10% of total heat loss. For a one-storey house, the figure is about 15%.
However, if a house is well insulated everywhere except for the ground floor, the percentage will be higher. The amount of ground floor heat loss depends on the type of soil. Houses built on wet soils tend to lose more heat through the ground than those on dry soils. Also, detached houses tend to lose more heat through the ground floor than terraced houses because their ground floors are exposed on all sides. So, if you plan to live in a bungalow built on damp soil, floor insulation merits special attention.
The 2002 Building Regulations, Part L, Conservation of Fuel and Energy, recommends that the U-value (a measure of the rate of heat loss) of the ground floor in new houses should be no greater than 0.25 W/m 2 K. Compliance with this requirement can in most cases be achieved with the insulation thicknesses given below (indicative only).
Thicknesses greater than these can reduce heat loss further and are recommended.
The insulation thickness depends on the material used.
Typical thicknesses for different house types would be:
Detached two-storey house 103 mm
Detached bungalow 90 mm
Semi-detached two-storey house 90 mm
Mid-terrace two-storey house 60 mm
This insulation should cover the full floor area, not just the perimeter as was sometimes done in the past. Note that for houses with irregular perimeters, (e.g. extensions), greater thicknesses may be required. Also, if under floor heating is to be used, an additional 30 mm or so can help to avoid
increased heat loss from the warmer ground floor.
In existing houses, a relatively simple way to reduce heat loss through the ground floor is to lay a carpet with foam backing or a foam underlay. Both carpet and underlay should be `wall-to-wall'.
Sealing of gaps in the ground floor will help to reduce draughts, and also radon levels in houses with radon problems. Gaps commonly exist at skirtings, at cracks in the concrete slab, and at service (pipe work) entries.
Sealing may be done as a DIY job using Acrylic or silicone sealants for small gaps. The more flexible silicone sealants are better for gaps where
shrinkage, settlement, or expansion/contraction may occur.
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Expanding (polyurethane) foam for large gaps.
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Polymer-modified cement mortars for large gaps in areas which are to be walked on.
Suspended timber ground floors require sub-floor ventilation to avoid dampness and wood rot. Ventilation openings to the sub-floor space should not be blocked.
The insulation methods illustrated below for new houses may also be implemented in existing houses. In some cases this would be disruptive and costly, but if work needs to be done on the floor anyway, this is a good time to consider an insulation upgrade.
New house examples of insulated ground floor constructions are given
below. Other configurations are also used. In all cases, the insulation manufacturer's instructions should be followed.
Concrete floor with insulation under slab, cavity walls
The insulation material chosen should have a high moisture resistance and compressive strength. Rigid insulation board is positioned on the damp proof membrane, which is laid over sand blinding on hardcore. There should be no gaps between the insulation boards.
A strip of insulation is placed vertically at the slab perimeter to minimise thermal bridging via the inner block work leaf. Extending the cavity wall insulation down to the level of the horizontal floor insulation, and/or using insulating blocks between the wall insulation and floor perimeter insulation, will further reduce thermal bridging.
Radon is a naturally occurring radioactive gas, which enters buildings from the underlying soil, and in some areas can accumulate in a building to such a concentration that it is deemed to be a health hazard. The 1997 Building Regulations, Part C, requires that a radon barrier be provided under new
dwellings in parts of the country worst affected by radon. This barrier must bridge the cavity in cavity walls. Care is required in design to ensure a good radon seal while avoiding thermal bridging at floor-wall junctions.
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