Airtightnes Design Guide
Design Guide

Air Leakage and U-Value Performance

The smallest crack in a plasterboard or unsealed overlap in a vapour control layer, which is unavoidable using traditional building techniques, will lead to what is referred to as ‘convection’. A small 1mm wide gap over an area of 1m2 in the inner building fabric like the plaster board will lead to a reduced U-value. It is important that reliance on workmanship and cross over trades are reduced as research shows that in cold winter conditions the difference between a perfectly sealed vapour control layer system and a small 1mm continuous gap in an area of 1m² can reduce the U-value performance by a factor of 4.8.

This means that the U-value cannot be achieved if the structure is not airtight and therefore you would require increased energy to heat the house. It is important to remember that air tightness doesn’t increase U-value, however it does provide the perfect conditions for the intended calculated U-value to be achieved.

Common Air Leakage Path

There are a vast number of possible air leakage paths in a building which need to be considered for Airtight design such as:

  • Gaps around windows and external door frames.
  • Areas around extractor fans, cooker hoods and boiler flues.
  • Gaps around electrical services, sockets, light fittings, built-in spot lights, antenna cables.
  • Partition junctions such as ceilings or external walls.
  • Holes through walls such as gaps in the mortar joints or missing joints on the inner leaf block work.
  • Cracks in plaster board.
  • Cuts and holes in the vapour control layer for service penetrations.
  • Gaps around loft hatch frames or the seal between the hatch and frame.
  • Gaps around central heating and soil pipes, plumbing pipes behind sinks, baths and toilets.


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