What Is Airtightness?

March 19, 2018

While the Code for Sustainable Homes is no longer relevant, it is still important to achieve a reasonable level of airtightness to improve the energy-efficiency of your new home or self-build business property as well as the comfort of your family or colleagues. This will save you money while encouraging sustainable living.When warm air leaks out of a building and cold air finds its way in, the benefits of improved insulation levels and energy-efficient heating systems are lost. Building Regulations (for England and Wales, and Northern Ireland) has set a mandatory requirement for airtightness to ensure reasonable standards are achieved. Within this, you must subject samples of your newly built dwellings to a pressure test in order to measure and confirm its airtightness on completionWhy self-build airtight properties?Did you know home energy use is responsible for approximately 27% of UK carbon dioxide emissions?  Did you know poor airtightness can be responsible for up to 40% of heat loss from buildings?The level of airtightness a building achieves is measured as air permeability: m3/h.m2 at 50 Pa. That is, the quantity of air (in m3) that leaks into or out of the building per hour divided by the internal area (in m2) of the building fabric at 50 Pa. An easy way to look at is: the lower the value achieved the more airtight the building. For example, a dwelling that achieves 3 m3/h.m2 at 50 Pa is better than a dwelling that achieves 7 m3/h.m2 at 50 Pa. Approved Document L1A of the Building Regulations (England and Wales) currently sets a reasonable limit for airtightness of 10 m3/h.m2 at 50 Pa. So, if your self-build demonstrates this value or lower it is considered airtight – and will be more energy-efficient and more comfortable by extension. Did you know less energy is needed for heating (and cooling) more airtight buildings but more controlled ventilation is needed to ensure the comfort and safety of their occupants?  This means that when building your own home with airtightness in mind, you must also provide adequate ventilation. The Feeling Homes team likes to say: “build tight – ventilate right!”What causes infiltration and exfiltration?Leakage of air through cracks and gaps in the dwelling’s fabric is called infiltration while air leaving those cracks and gaps is called exfiltration. The amount of infiltration and exfiltration will be affected by the design and quality of your self-build’s construction as well as wind speed/direction. These contributing factors are considered more here:

  • Wind – wind blowing against your dwelling will cause pressure differences between the inside and outside. Air is drawn into the dwelling through gaps on the windward face (infiltration) and leaves the dwelling on the shielded face (exfiltration).
  • Air buoyancy – warm indoor air is more buoyant than cooler outside air (buoyant air rises by convection). This rising effect draws in cooler air from outside (infiltration), which will be felt as a cold draught inside. The rising effect increases the pressure inside the dwelling, which pushes warm air out of cracks and gaps (exfiltration). It is worsened when it is very cold outside and warm inside.
  • Condensation – warm moist indoor air drawn out through gaps in the dwelling’s fabric may lead to interstitial condensation on cold surfaces in the fabric. Interstitial condensation is damping that will occur when warm moist air penetrates the walls, reaches the dew point and condenses into water.

We hope this explains the importance of considering airtightness when designing and planning your self-build. Please get in touch with our self-build consultants for a free no- obligation quotation today and they will get back to you as soon as possible.