Crest Diary Entry No.7 – Achieving Air Tightness

Since the last Article, the erection of the SIPS panels is nearing completion and both the vapour barrier and air tight membrane is being attached to the panels.

The image above shows the blue vapour barrier fixed to the outside of the building, held in place by battens used to hold the external cedar cladding, the black air tight membrane is shown in the images below.  We will discuss the details of the external envelope and the weather proofing of the building in more detail when the cladding is being attached.

Addressing air leakage is key to reducing heat loss and associated CO2 emissions.  Air leakage is the uncontrolled flow of air through gaps and cracks in the fabric of buildings. This is not to be confused with ventilation, the controlled flow of air into and out of the pavilion using the mechanical heat recovery and ventilation system.  Excess air leakage leads to unnecessary heat loss, discomfort from cold draughts and increased energy costs for the occupants.  As the build progresses and the envelope becomes air tight, an air pressure test is to be carried out and we will discuss this when it happens on site.

To overcome these problems for achieving air tightness, all junctions between the air tight membrane have to be taped using an airtight foil type adhesive tape to ensure the integrity of the air tight barrier.  The accuracy of the application on the tape on site is important as it is vital to successfully achieving the air pressure test results that we need for Passive certification. Both the membrane and the tape is designed to last the lifetime of the building, to do this it has to be powerful and durable but it has to move and respond to the differential movement of the building fabric.  The images below show the black air tight membrane fixed to the internal face of the SIP panels, the sections of membrane are joined together and sealed using the yellow air tight foil tape.

Air tightness is crucial for achieving Passive House Certification.  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.

The Glulam and SIP construction (above image) has been extensively modelled and tested using the PHPP (Passive House Planning Package) software.  Various permutations were modelled at design stage to allow us to address issues with insulation and air tightness.  Considering the floor and foundations, a critical area for air tightness is the junction between the concrete slab and the walls, differential movement can occur here causing ineffective air tightness. The timber framing system facilitates successful air tightness as it can be covered externally with vapour barriers and internally with air tight membrane and air tightness tapes etc.

The SIPs panels arrived on site with all the wall openings already formed for the installation of the window and doors units.  When they are installed and screw fixed into these openings, any gap between the frames and the SIPs panels is sealed with air tight tape to eradicate any other weak points in the openings.

The services for the pavilion have been designed in such a manner that there are minimal penetrations through the air tightness barrier.  To reduce the number of openings for services in the envelope of the building, a service void has been left internally for the duct work and pipes.  Only the MVHR (Mechanical Ventilation and Heat Recovery) system duct work penetrates through the barrier.  Where this occurs additional air tightness tape has been applied to the ducting to prevent air leakage.

We will discuss how to minimise thermal bridging and the installation of the window and door units in our next article