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Background What is air leakage? Air leakage is the uncontrolled flow of air through gaps and cracks in the fabric of a building (sometimes referred to as infiltration or draughts). This is not to be confused with ventilation, which is the controlled flow of air into and out of the building through purpose built ventilators that is required for the comfort and safety of the occupants. Too much air leakage leads to unnecessary heat loss and discomfort to the occupants from cold draughts. The increasing need for higher energy efficiency in buildings and the need in future to demonstrate compliance with more stringent Building Regulations targets means that airtightness has become a major performance issue. The aim should be to ‘Build tight – ventilate right’. Taking this approach means that buildings cannot be too airtight, however it is essential to ensure appropriate ventilation rates are achieved through purpose built ventilation openings. What is the impact of Air Leakage? Fabric heat losses have been driven down over many years by the various versions of the Building Regulations and there is limited return in reducing them down significantly further. Airtightness of buildings has only recently been addressed and only for buildings greater then 1,000 m2 in the 2002 edition of Part L of the Building Regulations. The airtightness of the UK building stock has been proven to be poor, which leads to unnecessary ventilation heat loss but also to widespread occupant dissatisfaction. Just to take one example from research, a comparison was made between two notionally 20,000 m3 buildings one with an air permeability of 9.3 m3.h-1.m-2 and the other with an air permeability of 23 m3/(h.m2). The infiltration heat load from the first was 861 GJ p.a and the second was 2,439 GJ p.a. These are highly significant energy differences caused by ‘holes’ being left in the building structure. Such scenarios are no longer acceptable. Why should we test ? Gaps and cracks in the building fabric are often difficult to detect simply by visual inspection. Air leakage paths through the building fabric can be tortuous; gaps are often obscured by internal building finishes or external cladding. The only satisfactory way to show that the building fabric is reasonably airtight is to measure the leakiness of the building fabric as a whole. Air leakage is quantified as Air Permeability. This is the leakage of air (m3/hour) in or out of the building, per square metre of building envelope at a reference pressure of 50 Pascals (m3/(h.m2)@50Pa) between the inside and outside of the building. Measuring Air Leakage Assessment of building envelope air leakage involves establishing a pressure differential across the envelope and measuring the air flow required to achieve that differential. This is normally achieved by utilising variable flow portable fans which are temporarily installed in a doorway, or other suitable external opening. HVAC plant is switched off and temporarily sealed prior to the test and all external doors and windows are closed. The test fans are switched on and the flow through them increased until a pressure of 50 – 60 Pa is achieved. The total air flow through the fan and the building pressure differential (inside to outside) are recorded. The fan speed is then slowly reduced in steps down to around 20 Pa with the fan flow and pressure differential data recorded at each step. The recorded fan flow (Q) and building pressure differentials (p) data allow a relationship to be established. This can be defined in terms of the power law equation: Q =C (p) n Where C and n are constants that are assumed to relate to the geometry of a single opening in the building envelope. The total air flow required to achieve a pressure differential of 50Pa can then be calculated from the equation. This is then divided by the total building envelope area to provide the leakage rate in m3/(h.m2)@50Pa. Good and Best Practice Standards Recommended airtightness standards for a variety of different building types have been established over many years. Airtightness standards up until the introduction of the 2001 Building Regulations were based on an air leakage index in which the envelope area defined as the area of walls and roof. The airtightness of buildings in the Building Regulations are based on air permeability which differs in that the envelope area is defined as the area of walls, roof and ground floor slab. For improved energy efficiency and much better control of the indoor environment better airtightness standards are required than the relatively relaxed standards required by the Building Regulations. The new standards developed for new school buildings, for instance, requires a target air permeability of 3 m3/(h.m2) [ref]. The following table provides current normal and best practice airtightness criteria for different building types
The Building Regulation Requirements in the new Part L Building Regulations Part L requires reasonable provision to be made to limit heat gains and losses through the building fabric. This includes heat transfer by air leakage. For new dwellings, Building Regulations Approved Document L1A requires pressure tests to be carried out on a representative sample of dwellings. Approved Document L2A relating to new buildings other than dwellings requires an air leakage test to be carried out on all buildings. Extensions to existing building over 100 m2 and increasing the floor area more than 25% will need to be treated as a new building and to comply with L2A. Alternative approaches are given for small non-domestic buildings less than 500m2 floor area but most will probably require air tightness testing in order to meet the required asset rating. Large complex buildings, under exceptional circumstances, may avoid air tightness testing, but these are very much the exception. |
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