The air tightness, as already mentioned in previous diary entries, is crucial for the overall success of the building and for achieving the stringent passive house standards that were set at the outset of the project. In previous diary entries, we have discussed the high risk air leakage components such as wall openings, floor-to-wall junctions and wall-to-roof junctions. Care need to be taken at all these locations to ensure the integrity of the air tight membrane. The testing of SIP frame construction is more straightforward than the testing of ‘typical’ construction as we don’t have to wait for any application of wet plaster (traditional construction relies on wet plaster in order to achieve air tightness). The SIP panels and airtight membrane should in theory by airtight before the application of plasterboard and internal plaster finishes.
The image below shows the airtight membrane being covered over by insulation and plywood that will form the internal stud work walls. Before the contractor can close the walls up completely, they need to ensure that the initial air test has been achieved as it will become increasingly more difficult to rectify any leaks after the walls have been closed.
As mentioned in earlier diary entries, joints in the air tight membrane were taped using airtight foil type adhesive tape to ensure air tightness is maintained. The application of this air tightness tape on site has been a lengthy one as accuracy is vital for its success. The tape is designed to last the lifetime of the building and whilst powerful and durable it moves and responds to the building fabric.
The images above show the air tight membrane behind the frame work that is used to support the curved plasterboard ceiling. The joints between the sheets of membrane need to be carefully sealed using the specified air tight tape.
The air tightness in a building of this specification must be less than 0.6 air changes per hour. This figure is extremely difficult to achieve using conventional building details. The combination of specialist details and taping to all joints on site will allow the air tightness test to be successful.
On site, the contractor is currently completing the first stage of the air tightness testing. We have prioritised air tightness from the very beginning of design detailing and throughout construction of the pavilion. While our calculations allowed us to be quietly confident that the building would achieve its required 0.6 air changes per hour, we were nervous of the unknown and what the initial test results would be.
The performance specification stated that the building was to achieve the required Passivhaus air tightness standard of 0.6 air changes per hour at an air pressure difference of 50 Pascals. This requirement is about 16 times more airtight than the current Building Regulations.
The air tightness of a building is tested by closing all apertures such as windows, doors, mechanical ventilation and heat recovery ducting that runs throughout the building. A large fan is then fitted to an external door opening to lower the air pressure within the building, making the difference between the internal and the external atmospheres 50 Pascals. This differential represents a wind speed of 20mph around the structure and is known as the fan pressurisation technique.
A variable speed fan is attached to one of the external doors, using an adjustable frame and air tight panel. The airflow rate required to maintain a number of specific pressure differences across the envelope of the building is then measured and recorded. The more air leakage that occurs the greater the air flow required to maintain a given pressure differential.
The following photograph illustrates the temporary airtight door screen and fan used to gain the difference of 50 Pascals between internal and external atmospheres. A small amount of leakage throughout the building is to be expected and accounted for when interpreting the results of the test.
After the difference of 50 Pascals is achieved, the contractor is then able to proceed with the ‘smoke’ test, this aims to illustrate any weak points in the airtight barrier were leakage might occur.
This test involves a compact smoke machine directing a small amount of smoke at potentially vulnerable locations in the buildings such as window openings and barrier breaches (i.e. where services and duct work passes through the envelope). If the smoke stays in the room then the junction is effectively airtight. However, if the smoke is sucked into the junction then it illustrates that air is leaking and it has failed, during the smoke test if any weaknesses are discovered, they are highlighted and then the contractor can apply airtight foil adhesive tape to the affected region and the junction can then be re-tested with the smoke machine to ensure that the leak has been fixed.
The image below shows how the contractor has taped around a service that has to pass through the air tight membrane. This taping is required to maintain the integrity of the air tight membrane.
In the next diary entry we will discuss the ventilation system and its installation at the CREST Pavilion at South West College, Enniskillen.