Loft Conversion U values and Insulation Regulations
Part of the Building Regulations requirements is to ensure that the new habitable roof
formed within the roof void is thermally efficient and compliant under current regulations. This means achieving a
certain maximum U Value for the uprated structure of the roof, walls, ceilings and sometimes floors (over garages
The Current Version of the approved documents within the Building Regulations will have the actual figures that
the loft conversion design must achieve for the insulation or thermal performance of the structure. Please remember
that these are minimum standards for the loft conversion insulation so do not be afraid to improve them.
Typical insulation materials for a loft conversion could be made up of the following:-
- For dormer walls and cheeks:- 100mm Crown Dritherm cavity wall insulation bats cut between the studs with a
40mm Celotex (Ref. GA3040Z) internal lining covered with 12.5mm plasterboard and set.
- Cold Deck - For insulation at joist level:- 200mm Crown Wool insulation quilt laid between the ceiling
joists plus 100mm Crown Wool insulation laid over the ceiling joists.
- Cold deck - For insulation at rafter level (sloping soffit):- Two layer system comprising of 40mm Celotex
(Ref. GA3040Z) laid under the rafters with 100mm Celotex (Ref. GA3100Z) fixed between the rafters. Joints to be
sealed with Idenden sarking tape.
- Warm deck - For insulation at rafter level:- Two layer system comprising of 60mm Celotex (Ref. GA3060Z)
laid over the rafters secured with 25 x 50mm treated s.w counter battens. 100mm Celotex (Ref. GA3100Z) fixed
between the rafters and abutting top insulation held in place with fixing pins. Top side of joints to be sealed
with Idenden sarking tape.
- Warm deck flat roof:- Celotex 116mm o/a thickness composite insulated decking comprising of 110mm
Celotex pre-boned to 6mm plywood decking laid over the joists prior to laying of additional 12.5mm OSB
- Cold deck flat roof:- Celotex totalling 140mm. 100mm celotex placed between the joists with 40mm Celotex
underlining. Tape all joints with idenden sarking tape. This roof system requires cross venting and a 50mm min.
air gap above the insulation.
- For new scaling ceilings using breathable felts :- 100mm Celotex cut to fit between the rafters with a 40mm
Celotex underlining with the joints sealed with Idenden Sarking tape prior to 12.5mm plasterboard and set.
For loft conversions utilising existing non breathable felts :- Add 50 x 50mm battens to the underside of
the existing rafters so that 100mm Celotex insulation board can be installed between the rafters and still
achieve a 50mm air gap over. Install a further layer of 40mm Celotex insulation board as an underlining
with the joints sealed with Idenden Sarking tape prior to 12.5mm plasterboard and set. This roof system
needs cross venting.