Loft Conversions

Converting a cold roof space into living accommodation is a popular way of increasing the internal size of a house, creating an extra bedroom or living area…

Since the loft becomes part of the heated part of the building, it is necessary to insulate all the way round the envelope of the loft conversion. This can involve insulating up to 5 different elements.

In England, Wales and Northern Ireland each element must be insulated to achieve a level of heat loss, or U-value, as defined in Approved Document Part L2B 2010.

    Pitched roof, sloping ceiling: U-value 0.18

If the roof has an old non-breathable roofing felt, type 1F, then the rafters will need battening out to accommodate a 50mm ventilated air space beneath the felt and PIR between rafters. TLX Silver is fitted below rafters, battened and plasterboard added.

If there is a breathable underlay, PIR between rafters can go almost up to the top of the rafter space. TLX Silver is fitted below rafters, battened and plasterboard added.

Pitched roof solutions 

    Dormer cheek: U-value 0.28

Dormer cheeks are generally constructed of timber studs with sheathing and breather membrane on the outside and tile, slate or weatherboard cladding. Insulation fits between the studs with TLX Silver across the face of the studs. A batten provides a service cavity before the plasterboard.

Dormer cheek solutions

    Flat roof dormer: U-value 0.18

A flat dormer roof usually employs a cold deck structure to reduce height. Ply deck and felt are above the joists, insulation between and TLX Silver below joists. This needs good ventilation below the deck

Dormer solutions


Dwarf wall U-value 0.28

A dwarf wall separates the room from an unheated part of the roof. It is not load bearing, and usually consists of timber studs with insulation between, TLX Silver across the front of the studs, batten and plasterboard.

Dwarf wall solutions

    Flat ceiling area: p. U-value 0.16

A flat ceiling separates the room from an unheated part of the roof. The simplest and most cost effective solution is to install 270mm of glass fibre. If insulating in this way it is important to maintain a continuous vapour barrier around the room envelope at junctions.

    Good practice

BS5250 Control of Condensation recommends that insulation and vapour control layers should be fitted all the way down the pitch of the roof, from ridge to eaves, rather than down dwarf walls and across the ceiling. This minimises the number of junctions, and makes it much easier to maintain continuity of insulation and vapour barrier,and to reduce air leakage.

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