Low Energy House - Timber Floor Insulation - Suspended Ground Floors

The thermal performance of an insulation material relies on the amount of air trapped within it. If that air is kept dry and still then less heat can get into or past the material and the higher its insulating capacity will be

Quilt, Loose Fibre or Rigid Under Floor Insulation

Timber battens and ply board supports, or proprietary netting supports, are constructed between the joists of the suspended timber ground floor. Quilt, loose fibre or rigid board insulation material is then laid on the supports. To prevent air movement through the floor, an airtight breather membrane is fixed to the underside of the joists and turned up at the edges.

Under Timber Suspended Floor Ventilation

The conventional method of preventing condensation beneath suspended timber ground floors is to remove moisture by creating natural cross ventilation between the external walls. While this method is usually successful the continual flow of cold air greatly increases heat loss through the timber suspended floor and encourages draughts at floor to wall junctions, at service penetrations and through gaps between floor boards.

Maintenance of the Under Floor Ventilation

Although draughts can be a problem, it is important to ensure that the air vents under the floor are not obstructed. If the natural ventilation system is efficient the cold side of the timbers will be exposed to continuous air movement and as a result should not be at risk from condensation. A ventilated space of at least 150mm (6 inches) in depth should be maintained between the over site concrete slab and the underside of the timber floor (or insulation).

Draughts Rising from the Ventilated Space Under Suspended Ground Floors

In suspended timber ground floor situations, care is needed to minimise draughts rising from the ventilated space. The recommended way to achieve this, and to protect the new insulation layer from condensation, is to fit an airtight breather membrane underneath the timber joists. The membrane should be turned up at the edges and sealed against the walls with special tape. The fitting of a membrane can greatly improve the performance of the insulation particularly with quilt insulation and loose fibre insulants.

Airtight Breather Membranes Under Ground Floors

Airtight breather membranes are designed to provide protection to the building envelope, restricting draughts and deflecting water. Reflective membranes reflect radiation while allowing moisture to migrate freely and safely to the atmosphere. They are fastened to the underside of the joists, with stainless steel staples or large headed nails, to form a continuous barrier that prevents cold outside air infiltrating the floor.

Because the breather membrane is vapour-open, water vapour generated in the building is free to pass to the under floor space where it will be dispersed to air by cross ventilation.

Breather Membrane Material

Airtight breather membranes are generally flexible non-woven sheet materials made from high density polyethylene. They are spun bonded and designed to provide minimum resistance to the passage of water vapour and at the same time providing an airtight barrier that is watertight.

Materials Commonly Used to Insulate Suspended Timber Ground Floors

Glass Wool Suspended Floor Insulation
Glass wool quilt rolls or batts fitted between floor joists

Rockwool Suspended Floor Insulation
Rock wool quilt rolls or batts fitted between floor joists

Extruded Polystyrene Suspended Floor Insulation
Extruded polystyrene rigid foam insulation boards fitted between floor joists

Rigid Urethane Suspended Floor Insulation
Rigid urethane insulation boards with aluminium foil on both faces

Rigid Phenolic Suspended Floor Insulation
Rigid phenolic insulation boards with aluminium foil on both faces

Sheeps Wool Suspended Floor Insulation
Sheeps wool batts laid between joists

All insulating materials must be fitted in accordance with the manufacturer’s instructions

Target U-value for Under Ground Floor Insulation

Fifteen per cent of heat from a house can be lost through the ground floors. Floor insulation must be installed to achieve a minimum Target U-value of 0.25. The exact specification will depend on the construction of the particular floor and the type of insulation proposed.

Airtight Breather Membranes made from high density polyethylene are also available faced with thin aluminium sheet. They are airtight, watertight with low resistance to the passage of water vapour and they reflect over 90 per cent of radiant heat