Diagram B11 Hollow-block wall, internal

insulation lining

(Par. B.6.4)

The insulation is installed on the inner face of the masonry walls. It may be installed between preservative-treated timber studs fixed to the wall, or in the form of composite boards of plaster backed with insulation, or as a combination of these.

Installation guidelines and precautions

Exposure

Table 11 of S.R.325:2013 (+A1:2014)

provides guidance on exposure categories for single leaf masonry walls for the assessment of resistance to rain penetration.

Air movement

Air gaps in the insulation layer should be kept to a minimum. If using insulation between timber studs, there should be no gaps between insulation and studs, between insulation and the VCL, between butt joints in the insulation, around service penetrations, etc. If using composite boards, they should be tightly butted at edges, and should provide complete and continuous coverage of the external wall.

When mounting composite boards on plaster dabs or timber battens, there is a danger that air will be able to circulate behind the insulation, reducing its effectiveness. To minimise such air movement, the air gap behind the boards should be sealed along top and bottom, at corners and around window and door openings, e.g. with continuous ribbon of plaster or timber studs. This also serves as a fire barrier.

Condensation

A suitable VCL should be installed on the warm side of the insulation to minimise the risk of interstitial condensation on the cold masonry behind the insulation. Care should be taken to avoid gaps in the VCL at all joints, edges and service penetrations. The location of service runs in the air gap on the cold side of the insulation should be avoided. Refer to paragraph B.4 for further guidance.

Thermal bridging

Care should be taken to minimise the impact of thermal bridging. Critical locations where care should be taken to limit thermal bridging include lintels, jambs, sills, roof/wall junctions and wall/floor junctions (see Acceptable Construction Details, section 6 details).

Other areas where there is a risk of significant thermal bridging include:

Junctions with solid party walls and partitions Internal partition or party walls of solid dense concrete blockwork can create significant thermal bridge effects at junctions with single leaf masonry external walls (see Acceptable Construction Details, details 6.05 and 6.06).

Junctions with intermediate floors

The external walls in the floor space of intermediate floors should be insulated and protected against vapour movement. Along the wall running parallel to the joists, insulation can be placed between the last joist and the wall. Where the joists are perpendicular to the wall, the insulation and VCL should be continuous through the intermediate floor space and should be carefully cut to fit around the joist ends (see Acceptable Construction Details, detail 6.04).

Stairs, cupboards and other fittings supported on or abutting the external wall

Insulation should be carried through behind such fittings.

Ducts, e.g. soil and vent pipe ducts, against external walls

Insulation should be continuous at all such ducts, i.e. the insulation should be carried through on either the external or internal side of such ducts. Where the insulation is on the external side, particular care should be taken to prevent ingress of cold external air where ducts etc. penetrate the insulation.

B.6.4 Construction W4: Hollow concrete block wall, rendered externally, internal insulation lining with plasterboard finish

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