Air Tightness
The Building Energy Rating (BER) assessment takes into account the air tightness of your home and how effective your home is at preventing unwanted cold air from entering through cracks and gaps and also stopping heated internal air from escaping. The more air that is travelling through the fabric of your home the more expensive it will be to heat. The main areas of leakage (excluding open doors and windows!) are around windows and doors, points where services such as water, electric cables etc enter the home and where there is a physical hole in the building such as ventilation holes.
The measure for air tightness is cubic metres of air lost per square metre per hour (m3/hr.m2). To measure this a blower door test is performed which creates a standard pressure difference between the inside and the outside of the building and then measures the changes taking place in the readings over a period of time. A blower door test will not be required during your BER assessment as the assessor will input a standard value based on the construction type of your home. If you carried out additional work above standard to make your home more airtight it is advisable to carry out your own blower door test and give this information to the assessor.
An air tightness standard has been a recent addition to the building regulations which designers and builders have to comply with. At this point in time however the maximum permitted air loss is relatively high (10m3/hr.m2) so it is expected that this will be reduced in future updates. This level would be achievable through standard construction with standard levels of workmanship. The classic leaky old house that we have all come across would have values as high as 30m3/hr.m2. An efficient home built to a high quality standard but naturally ventilated (windows, vent holes) could achieve less than 5m3/hr.m2. A home that has air tightness as a key component of its design and construction including a heat recovery ventilation system (which changes the air in your house regularly) should have negligible air loss (less than 2m3/hr.m2). Homes like this would require a very minimal level of heating.
Our advice is that if you are building a new home keep air tightness in mind during your design and ensure the construction is properly supervised. For example, features such as bay & dormer windows are more difficult to seal. Your architect can offer advice on this and other areas such as how to best incorporate utility services into your design. A vapour membrane/check will be fitted to the areas where air can escape such as around windows & doors, dormer & bay windows, joints and timber stud walls. This should be fitted by professionals and you should warn follow on trades to ensure it isn’t punctured.
If you are in an existing house and you are trying to achieve an improved Building Energy Rating we would advise that you discuss this in advance with your BER Assessor to see what impact it would have on your rating. We would certainly advise that you do all you can to minimise draughts around windows and doors to make the home more comfortable to live in and to reduce your heating bill. You should carefully evaluate any other options which can be difficult and expensive to complete unless you are carrying out a full renovation. It should also be noted that it is not advised to block up ventilation holes unless you have installed an alternative ventilation system.
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