House extension   UK
 Home extension guide - how to build a house extension and refurbish your home


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Recommended inspection points and checks for ensuring air tightness within a house extension

Foundation/ground floor

• Check that the wall and floor damp-proof course/membrane forms an adequate airtight layer.

• Check that any penetrations through the air barrier (e.g. service pipes) have been dressed. Pre-formed collars, sometimes referred to as ‘top hats’, which seal to the membrane and around the throat of the pipes are effective means of achieving a good airtightness seal (Figure 17).

• With timber frame construction, check that the sole plate is sealed to the foundation/floorslab.

Intermediate floors

• Joist hangers can limit penetrations through the air barrier.

• If joists are to be supported by the wall, check that there is no air leakage into the cavity.

• Ensure timber floorsheets/boards are well fitted and sealed at their edges as well as at junctions with perimeter walls.

• Check that the ceiling-to-wall joint has been sealed.


• Check that the airtightness layer between the wall and ceiling/roof is continuous (Figure 18).

Ceiling below the roof space.

This normally forms the air barrier in domestic construction.

• Check that there is a continuous air barrier over the whole ceiling area.

• Check that service penetrations (ventilation ducts from extract fans and light fittings) have been properly sealed where they penetrate the air barrier.

• Check that loft hatches are airtight and surrounds are sealed where they penetrate the air barrier.

Alternatively, consider using a membrane that follows the pitch of the roof.

• Sealing this membrane at eaves level can be difficult. It will need careful detailing and construction.

Windows and doors

• Specify good quality windows and doors.

• Check that the wall-to-frame junction is properly sealed and continuous with the wall’s airtightness layer, particularly at sills (Figure 19).

• Check that all the windows and doors have an appropriate weatherseal between the opening unit and the frame.

• External doors (and letterboxes) should be fitted with draught excluders.

Service penetration

• Check for seals at service entry points (pipe and cable routes), e.g. around incoming water pipes, gas pipes, electrical cabling, as well as waste water pipes for sinks, baths, washing machines, dishwashers, etc. Seals should be provided internally and externally (see Figure 20 for masonry construction and Figure 21 for timber frame).

• Where several services penetrate at the same point, there should be sufficient space to fully seal round each of them.

Brick/block masonry construction

• Check the quality of construction as the work proceeds. Good mortar joints are required (i.e. no gaps around the blocks or bricks) on both internal and external faces.

• Blockwork is unlikely to provide an adequate air barrier on its own. However, the application of wet plastering, parging or the addition of fully-sealed dry lining will create a good air seal.

• Parging is an effective method of sealing around joists that penetrate the inner leaf of an external wall.

• Check that there is a good seal around all services that penetrate the wall.

Dry lining

• Check the plasterboard is continuous (e.g. there are no large holes behind the kitchen units/bath). Ensure that airtightness measures have been incorporated at all edges, particularly at the floor/ceiling junctions and around openings.

• Check the plasterboard is correctly detailed at joints, corners, reveals and window sills. Plasterboard should be mounted on ribbons of plaster or adhesive around all the edges (rather than dabs) to prevent air leaking through the porous blockwork behind.

• Parging the blockwork prior to applying the plasterboard will improve airtightness.

• In timber frame construction, the edges of the boards can be sealed direct to the timber framing.

Wet plaster

• The application of a wet plaster finish can provide an adequate seal to what would otherwise be a leaky brick or blockwork wall.

• Check that the wet plaster finish covers the whole surface of the wall or ceiling.

Vapour barrier

• Where the vapour barrier is used as an air barrier, check that it is complete, that all joints have been sealed and that the material has not been damaged.

Timber frame construction.

• In general it is easier to make a timber frame dwelling airtight than other forms of construction. This is partly due to pre-fabricated construction and the use of the impermeable vapour barrier as the air barrier, but also because the plasterboard itself can be sealed to form an air barrier.

• Where the vapour barrier is used as the air barrier, care will be needed to avoid it being torn. Any damage to the vapour barrier must be repaired.

Sealant materials

There is a range of different sealant materials available. The main choices are:

• Membranes

– Polyethylene sheeting is widely used when a sheet vapour control layer or airtightness layer is needed in timber frame construction. While 500 gauge polyethylene is sometimes used, 1000 gauge polyethylene is more robust.

– Ensure all penetrations through the membrane (for example, service pipes and cables) are well sealed.

– Joints need to be sealed with appropriate tapes or sealant strips.

• Expanding foam sealants

– While these can be used to fill large voids, not all expanding foam products are airtight. Care is needed to ensure that the whole of the opening being filled is sealed. It may be necessary to cap the foam sealant with an airtight finish.

• Gun-applied sealants

– Check that the material has fully filled the gap and is correctly bonded to the surface.

– Gun-applied sealants must NOT be applied to dusty or wet surfaces as they will not adhere properly.

• Tapes

– Tapes used to seal polyethylene (or similar membranes) being used as air barriers must be fit for purpose. It is advisable to check this with the manufacturers as not all tapes will be suitable.

• Fillers

– Not all the fillers available for filling larger gaps will be airtight: some may crack on drying.

• Fire stopping

– Some products used for fire stopping are not airtight when installed. Intumescent products, for example, only expand to fill gaps when they are exposed to heat. It may be necessary to use additional sealing to provide airtightness in these areas.






























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