How to keep foil-faced PIR airtight when insulating from the inside

Only certain insulation boards have an aluminium laminate on both faces. Assess the heat and moisture behaviour of a wall using these boards, particularly where outward drying is limited.

Remove fittings that conceal the floor and ceiling junctions. Check substrate adhesion, flatness and evidence of leaks. Remedy moisture sources and verify the condition of the wall before covering it. Foil cannot dry masonry and may reduce its capacity to dry towards the room.

Match the fixing method to the system and substrate. A bonding pattern suitable for a different material may leave a cavity that connects leaks at floor level to the top of the wall. The design needs to control this airflow as well as holding the boards in place.

Trace the airtight layer on a section through the room, including reveals and adjoining walls, floors and ceilings. Use system-compatible connections to plaster, concrete and window frames. Tape needs a firm, clean bonding surface. Dust or flaking paint can weaken even a neatly applied strip.

An overlap or rebated edge does not in itself provide a seal. Follow the specified joint details for plasterboard-faced composite boards too. The foil may be inaccessible from the face, so resolve the joint design before ordering.

Sockets, cables and brackets create numerous small penetrations. A service void on the room side can reduce their number. Include its depth in the overall build-up to retain clearance for skirting boards and window reveals.

Seal unavoidable penetrations with components compatible with the system. Anchor heavy cupboards and radiators in a suitable substrate. Plan fixing positions early, as drilling through the finished wall can damage the concealed air barrier.

Before covering the layer, mark future fixings for curtain rails, shelves and screens. Planning these positions reduces the risk of puncturing the facing when the room is furnished. Show any necessary reinforcement on the drawings.

Detail how the insulation terminates at corners. Assess surface temperatures on the adjoining wall and ceiling. A poorly insulated narrow strip beside a window can remain cold even with a favourable result in the centre of the wall. Select the thickness and extent of return insulation for each junction.

At floor level, check moisture and allow for skirting fixings. Form a lasting seal independently of the decorative skirting board. In an older building, a small area of plaster may need removing to expose a sound surface for bonding.

Inspect while all joints are visible. Check tapes, overlaps, terminations and repaired damage. Photograph corners and service penetrations. If specified, an airtightness test can locate air leaks before painting and fitting out the room.

After the retrofit, maintain ventilation and monitor humidity during the heating season. A warmer wall surface improves comfort, but cooking and bathing still introduce moisture. Heating savings also depend on the heating system, occupants and the rest of the envelope.