Airtightness and Building Energy Ratings
A key parameter for achieving an efficient Building Energy Rating (BER), is that the building envelope in designed and insulated to a high level and the fabric airtightness is to high standard. Furthermore, when considering the Building Energy Rating of a construction, contractors and homeowners can make cost savings by aiming for greater levels of airtightness, which could be used as a trade off to reduce a reliance on more expensive energy reduction measures. The software used to calculate a BER is called the Dwelling Energy Assessment Procedure (DEAP). This software allows the user to either in put a default value, depending on the method of construction, or to insert a superior value, if the building is designed to achieve a higher specification and energy rating. If the latter option is chosen, the improved airtightness value should be confirmed by means of a blower door measurement.
AIR PERMEABILITY TESTING
Air permeability testing measures the rate at which air is able move through a building’s external ‘envelope’ due to random gaps and cracks, at a reference differential pressure of 50Pa (between inside and outside the building). The external envelope can be defined as the area of the air barrier which separates the conditioned “heated” internal air from the unconditioned “cool” outside air in a building. This includes all external walls, floors and roofs or ceilings. The result is expressed in cubic metres of leakage, per square metre of building envelope per hour, at a differential pressure of 50 Pascals, i.e. m3/(m2.hr) @ 50Pa.
The DEAP default value for airtightness is still quite leaky, in fact, depending on the method of construction and building design this default value could easily equate to an air permeability of at least 10 m3/(m2.hr) @ 50Pa. The Airtightness Testing and Measurement Association (ATTMA), recommend for best practice, buildings which are naturally ventilated should achieve an air permeability as low as 3 m3/(m2.hr) @ 50Pa. This gives an indication as to how “leaky” and inefficient a building of 15 m3/(m2.hr) @ 50Pa would be.
Airtightness and Changes to Technical Guidance Document Part (TGD Part L), in the Rep. of Ireland:
For the first time (Since July 1st 2008), building regulations in the republic of Ireland are including a requirement for the maximum allowable air permeability for new dwellings. This follows a standard similar to that of the UK (a Q50 <10 m3/(m2.hr) @ 50Pa). In this way, new dwellings will for the first time, have to confirm, that they are built within this relatively weak standard. This is by no means onerous, and should be viewed as the poorest standard allowed for in our building regulations. For low energy constructions, a much higher level of airtightness should be achieved. It is envisaged that future changes to Part L of the Irish Technical Guidance Documents will reduce the upper limit currently included in our regulations with an emphasis on controlled ventilation.
The new requirements for testing published in the new 2008 Part L Technical Guidance document are listed below.
On each development, an air pressure test should be carried out on at least one unit of each dwelling type.
One dwelling from the first four units of each dwelling type planned for completion should be tested. The basic number of tests for each dwelling type is presented in Table 4 below. The total number tested is related to the number of units of that type in the development and to the results achieved in any earlier tests carried out.
Where a number of apartment blocks are constructed on the same site, each block should be treated as a separate development irrespective of the number of blocks on the site.
Ecological Building Systems provide the pro clima Intelligent airtight system. A key component of the airtight system, INTELLO PLUS, is an intelligent vapour check which prevents vapour from diffusing into structural elements during cooler periods of the year. If moisture penetrates into structural elements (e.g. timber/steel frame walls),due to damp installed building materials, convection or diffusion, a conventional vapour barrier will not permit any drying to occur towards the inside during warmer periods of the year.
INTELLO PLUS intelligent vapour check will allow vapour to dry-out of the structure towards the inside during warmer periods of the year. In this way pro clima INTELLO PLUS provides constructions with exceptional protection against potential condensation risk, as well as acting as a durable continuous airtightness layer.
High levels of air tightness are best achieved by good design and then by execution during the building phase rather than by retrospective works. Intermediate testing of the airtightness layer often lends itself to a positive result upon completeion of the project. Use of proprietary membrane, tapes, adhesives and sealing solutions, that are durable designed to last in the confines of building fabric and not just for the day of the test are a must. By combining pro clima INTELLO PLUS, with the pro clima range of durable tapes, glues and sealing agents an efficient BER can be achieved through high levels of airtightness and minimal risk of interstitial condensation for the life of the building.