Existing roofs should be insulated to Best Practice standards.
The following two technical factors affect the insulation of existing roofs.
• For an insulated pitched roof (where the insulation is placed between the rafters) the building regulations
specify that a 50mm wide ventilation gap must be maintained above the insulation (and beneath the roofing felt and
tiles); this is to reduce the risk of interstitial condensation. Consequently, the thickness of any insulation
placed between the rafters must be at least 50mm less than the depth of the rafters. Eaves ventilators and ridge or
abutment ventilators must also be installed, in order to let ventilation air in at eaves level and out at the top
of the roof.
• Where new ceiling linings of plasterboard (rather than thermal boards) are fixed beneath the newly insulated
construction, polythene vapour barriers must be fixed beneath the joists or rafters (behind the linings).This
reduces the risk of interstitial condensation. Edges, joints and any breaks in the vapour barriers (e.g. at
electrical switches and sockets) should be sealed with tape.
If the existing rafters are not deep enough to take the depth of insulation needed to achieve the recommended
U-value (plus the 50mm ventilation gap) there are three options:
• supplement the insulation between the rafters by using a thermal board (instead of ordinary plasterboard) for
the internal ceiling lining;
• increase the depth of the rafters by fixing timber battens to them (but this involves a loss of headroom);
• use a form of construction called a 'vapour-balanced' or 'breathing' roof.
These options may be combined.
If an unheated roofspace or residual loft remains beneath a pitched roof, flexible insulation quilt may be
placed immediately above the ceiling, between the timber ties (or ceiling joists).The insulation will be supported
by the ceiling lining (usually plasterboard or thermal board).
For maximum effectiveness:
• the insulation quilt should be in two layers, one between the ceiling joists and the other across them, so as
to prevent thermal bridging;
• the ceiling lining should be thermal board instead of ordinary plasterboard;
• the insulation material should not be compressed when it is tucked into tight corners.
It is important to ventilate this residual roofspace, above the insulation, in order to reduce the risk of
condensation. If the area is to be used for storage, bearer boards should be placed across the existing joists, to
prevent the insulation being compressed.