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Air Tightness Testing & Energy Assessments


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How to pass an air tightness test

A guide to building air tight healthy dwellings.

Failing air tightness tests at practical completion can prove very costly. It can cause significant delays in completion not to mention the additional costs of remedial work and re-test fees.

Building airtight buildings doesn’t have to cost more, providing you have an air tightness strategy and implement it throughout the design and construction stages.

The three key factors in building air tight health buildings are:

  1. Design Tight – Develop an air tightness strategy at the early design stages and communicate it to everyone from the design team to the project managers, site managers, labourers and importantly the sub-contractors.

  2. 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.

  3. Ventilate Right – Airtight buildings must have an airtight ventilation strategy to avoid poor air quality, condensation and overheating.

Design Tight

It is essential to develop an air tightness strategy at an early stage and ensure it is communicated throughout the design and construction phases. The following should be considered at the design stage:

  • Define the air tightness barrier for the entire heated envelope of the building and mark it on all relevant drawings.

  • Specify appropriate material build ups that will form the air barrier eg. VCL membrane, wet plaster, 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 disseminated to all relevant parties in the design and construction teams.

  • Appoint an air tightness champion, one person needs to take responsibility for checking the air barrier and ensuring the air tightness strategy is implemented throughout the design and construction phases.

Build Tight

All contractors and sub-contractors must be made aware of the air tightness strategy and the importance of not breaking the air tight barrier. An air tightness briefing should be included in the site induction. The air tightness champion should regularly perform visual air tightness checks of contractors and subcontractors work at each stage of construction. The following should be done and checked in the construction stages:

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 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.


Traditional block masonry

  • All mortar joints in brickwork should be fully filled.

  • Wet plaster or parging should be used to form an air tight 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 walls 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 air tight seal, this cannot be taped during testing.

Windows and 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.

Ventilate right

It is essential to ventilate right when building tight. Too much air leakage leads to draughts and wasted energy however too little air flow causes poor air quality and can cause a condensation risk.

  • Approved Document F of the Building Regulations gives guidance on how to achieve comfortable ventilation. This is done with Mechanical ventilation, extract fans, trickle vents and other designed openings.

  • Part F assumes a design air permeability of 3-4 m3/h.m2 therefore if building to below this mechanical ventilation should be provided.

Air Tightness Testing

Jack Allen