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Draughtproofing: aiming for air-tightness

patio doorsPatio doors let the light in, but can be a source of draughts and discomfort.

A substantial proportion of the energy used to heat the interior of a house, perhaps as much as 20-35%, can be lost by uncontrolled leakage of air through gaps and cracks in the building fabric - popularly referred to as draughts. Hot air escapes, and is replaced by cold air from outside. This not only causes discomfort to the occupants, but also makes heating systems work harder, wasting fuel and money, and increases the risk of damp air penetrating the building structure leading to its gradual deterioration.

Just how a building is leaking air depends on its design, construction and maintenance, and will vary from case to case. However, there are many common pathways via which air escapes. Another factor is how the occupants use the building. It's not much use spending lots of time and money on draughtproofing a building if windows are left wide open for long periods 'to let in the fresh air'. Hence, installing effective means of ventilation should go hand in hand with efforts to achieve air-tightness, to ensure occupants' comfort and safety.

Where to look and what to do

There are several common routes by which air escapes, and which you can fix relatively easily and cheaply.

Floors

Suspended timber floors generally have many gaps where air can leak into the unheated void beneath - between floorboards (less so with tongue-and-groove Pipe and skirtingboarding), at junctions with walls, and where holes have been made for pipework, such as heating pipes. Use an appropriate sealant, ideally in a sealant gun, to seal the perimeter around the skirting and any other gaps before laying carpet or other floor covering. Alternatively, lay hardboard sheeting over the floor to cover the gaps; do not use plastic sheeting to cover timber floors due to the risk of damp and subsequent rot.

Solid ground floors also are prone to develop gaps and cracks, which should be repaired and filled. Any gaps around skirting should be closed with sealant.

Windows and doors

Ensure that any gaps around window or door frames are sealed, and that casements, sashes and lights close properly. Apply draughtproofing strips to seal gaps around opening windows and doors. An exterior mastic can be applied to seal the perimeter of window and door frames on the external wall.

Double doors, such as patio doors, are often responsible for letting the heat out due to a faulty closing mechanism or failure to seal properly. Check with the installer if this is the case. Also, seal the the keyhole over the winter - masses of unwelcome cold air can enter through just this small gap.

Loft hatchLoft hatch

An ill-fitting and poorly insulated loft hatch provides warm air with an easy escape route into the roof space. Make sure its insulation is up to scratch and fit draughtstripping.

Fittings

Poorly sealed light fittings can provide a pathway for air to escape, into the void above the ceiling or into the roof void. The sameLight fitting applies potentially to any wall or ceiling-mounted fitting, such as spotlights, fans, switches and sockets. Attention to detail is the key.

Service pipes

Service pipesGaps in external and internal walls are inevitably created by the myriad of pipes, flues and cables that service the property. The bathroom and kitchen are especially likely to suffer draughts due to these. Again, deployment of the sealant gun can help cut those annoying draughts out completely, although you may have to delve behind kitchen units or beneath baths and sinks to reach the offending orifices.

Fixing the fabric: getting serious

Any major renovation of a house provides an opportunity to fix defects in the constuction that impact the building's air-tightness. These include:

  • Check mortar joints are sealed and repair gaps or cracks in masonry.
  • Ensure dry-lining sheets (i.e. plasterboards) are correctly sealed at junctions with ceiling and floor, and between adjacent sheets.
  • Block redundant fireplaces and insert vent. Cap the chimney.
  • Check that the ends of floor joists are sealed so they do not break the air barrier.

Always remember, improved air-tightness must be matched by more efficient and controllable ventilation.

Measuring air leakage
Air leakage, or air permeability, into or out of a building can be measured, and there are standards of air-tightness to which new houses must conform. The minimum acceptable air permeability for new homes in England is currently 10 m3/h/m2 (cubic metres of air per hour per square metre of external surface area) at a reference pressure difference between the inside and outside of 50 pascals (Pa). However, the best designs can achieve values much lower than this; best practice standard in the UK is 3 m3/h/m2, but even this is far exceeded by European PassivHaus designs, which can have values as low as 0.75 m3/h/m2.