Failing an air tightness test at practical completion can prove very costly – it causes delays to completion, plus the added costs of remedial work and retest fees. Building airtight buildings doesn’t have to cost more, provided you have an air tightness strategy and implement it throughout the design and construction stages.
In this blog you’ll learn:
- The three key factors behind passing an air test – design, build and ventilation
- What to get right at each stage of construction
- How to prepare properly before test day
Three Key Factors in Passing an Air Tightness Test
- Design Tight – develop an air tightness strategy during the early design stages, and communicate it to everyone: the design team, project managers, site managers, labourers and subcontractors.
- Build Tight – one person must be responsible for carrying out regular airtightness checks to ensure all site operatives are fulfilling their responsibilities to the air barrier.
- Ventilate Right – airtight buildings must have an airtight ventilation strategy to avoid poor air quality, condensation and overheating.
Design Tight
It’s essential to develop an air tightness strategy early and communicate it throughout design and construction. At the design stage, consider:
- Firstly, define the air tightness barrier for the entire heated envelope of the building and mark it on all relevant drawings.
- Secondly, specify appropriate material build-ups that will form the air barrier, e.g. VCL membrane, wet plaster, and parged concrete.
- Provide clear construction details for all material junctions i.e. where the floor slab meets the wall (use accredited construction details and enhanced construction details).
- Provide construction details for all penetrations through the air barrier, including structural details for floor joists, beams, etc and for all services penetrations (water, gas, electrics, and ventilation).
- Make sure the air tight design allows for sufficient ventilation in line with Part F of building regulations.
- Ensure the air tightness design is defined in an air tightness strategy document and then disseminated to all relevant parties in the design and construction teams.
- Name one person on the project as responsible for the air barrier – checking it and making sure the strategy is actually implemented throughout design and construction.
Build Tight
All contractors and subcontractors must understand the air tightness strategy and the importance of not breaking the airtight barrier. An air tightness briefing should be part of the site induction, and whoever’s responsible for the air barrier should regularly check contractors’ and subcontractors’ work at each stage.
Foundations & Ground Floor
- The floor damp-proof membrane and the damp-proof course in the walls should form a continuous air barrier. Any tears or punctures in the membrane should then be repaired.
- Beam and block floors should have a seal between the blocks and around the perimeter. This can be done with a grout or cement wash or with a membrane.
- All gaps around floor beams and joists should be sealed if they penetrate the external walls.
- Services penetrations should be cut to the minimum requirement and sealed appropriately.
- All penetrations should be sealed as soon as possible to avoid them being covered up.
Walls
Traditional block masonry
- All mortar joints in brickwork should be fully filled.
- Wet plaster or parging should be used to form an airtight seal to masonry walls. Plaster right to the top and bottom of all internal walls even when skirting and or coving will be fitted.
- No gaps should be left around service penetrations through the wall’s blockwork.
- All window and door cavities should be sealed using cavity closers.
- The external walls should be sealed before internal stud walls are built.
Timber and frame
- A vapour control layer should be used to provide a continuous air tight barrier.
- All laps and joins in the membrane should be taped.
- Dry lining should not be relied upon as an air barrier for timber or masonry walls.
Floor Joists & Eaves
- Floor joists hangers should be used where possible, when joists penetrate the wall they should be either sealed with a flexible sealant or with a proprietary sealing box.
- Sealant should be applied between the ceiling board and the wall to ensure the air barrier is continuous at eaves level where the wall and roof / ceiling meet.
Ceilings & Roofs
- Ensure there is a continuous air barrier between heated rooms and unheated loft space.
- All downlights, switches and light fittings should be sealed.
- All services penetrations into the loft space should be sealed.
- Boiler / riser cupboards should have the same air barrier as all walls, with all penetrations sealed.
- The loft hatch should have an airtight seal, this cannot be taped during testing.
Windows & Doors
- Cavity closers should be used to seal the cavity at the wall reveals and sills.
- All windows and doors must have a tight seal around the edge.
- Door frames must be sealed underneath, especially with double doors.
Services Penetrations
- Check all fire safety regulations and manufacturers’ guidance before sealing all services.
- All service penetrations should be sealed with a permanent solution, don’t forget behind kitchen units, sink units and bath panels.
- There are multiple solutions available including collars, top hats and pipe and cable grommets.
- Small to medium gaps can be sealed with a suitable sealant, larger gaps can be sealed with a mortar or expanding foam.
This is exactly what catching leakage patterns early looks like in practice. On Woodstock Homes’ nine-home Baily Court development, a single diagnostic Blower Door test on one plot ahead of final testing identified leakage risks at typical junctions and service entries – giving the site team time to implement preventative measures across the remaining plots before final testing began.
Ventilate right
Approved Document F of the Building Regulations gives guidance on how to achieve comfortable ventilation – mechanical ventilation, extract fans, trickle vents and other designed openings. Part F assumes a design air permeability of 3-4 m³/h.m², so mechanical ventilation should be provided if building below this.
How to Pass an Air Tightness Test: What to Prepare Beforehand
Run through our air tightness checklist a few days before the test is booked, allowing time to complete any required remediation work.
What’s Next?
Following the steps above and working with qualified professionals significantly increases your chances of passing first time – a successful air tightness test means your building is energy-efficient, comfortable and healthy for occupants.
If you’d like advice on your air tightness strategy before test day, we’re happy to talk it through.
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