Internal finishes

The building should be allowed to dry out for a reasonable period after roofing before any internal finishings are applied to the walls.

Plaster undercoats
The following mixes are recommended for floating coats: 

    a) 1 : 1 : 6 cement: lime: sand by volume 
    (a 1 : 2 : 9 cement: lime: sand mix may be used, except under cold winter 
    conditions). 
    b) 1 : 6 cement: sand by volume, with the addition of an approved plasticiser used 
    in accordance with the manufacturer’s instructions. 
    c) Pre-mixed gypsum bound lightweight aggregate undercoat plaster. 
    d) Pre-mixed cement-lime bound lightweight aggregate undercoat plaster.

Note: The sand used in both external and internal render finishes should comply with BS EN 13139.

Plaster finishes
Undercoats a) and b) may be finished with a neat gypsum Class B finish coat. Undercoats c) and d) may be finished with a lightweight pre-mixed gypsum, or gypsum lime bound finish coat, respectively. Machine-projected plasters may be suitable for use on Thermalite walls.
 
Dry linings
The following systems are suitable for use on Thermalite walls:

a) Plaster dab method
Bonding compound dabs should be used to fix the boards to the wall. The dabs are applied to the wall and should be the length of the plasterer's trowel and 50mm to 70mm in width. Dabs should be 50mm to 70mm apart vertically and approximately 300mm apart horizontally. The boards are then pressed tightly into position and the face plumbed and lined. They should be wedged in position until the plaster dabs have set. Dabs should only be applied for one board at a time.

It may be advantageous to apply a bonding agent to the walls prior to the dry lining operations. Multi-purpose plaster should be used to fix insulating plasterboard linings.

For sound insulation purposes, it is recommended that a continuous horizontal ribbon of adhesive is applied at floor and ceiling levels.

b) Timber batten method
Timber battens 50mm wide x 20mm thick should be fixed at centres in accordance with the lining board manufacturer's recommendations. The battens should be correctly aligned and fixed with cut nails of sufficient length to penetrate the Thermalite® by at least 50mm (it may be necessary to increase the depth of penetration into Turbo). The board should then be fixed to the battens using flat-headed galvanised nails.

c) Metal framing systems
British Gypsum Limited manufacture a metal framing system that may be fixed to Thermalite® walls and will provide a satisfactory internal finish. The application should be carried out in accordance with the manufacturer's instructions.

Taper edge boards should be jointed with a proprietary joint filler, taped and finished.

Sealing edges
It is a requirement of the Building Regulations that gaps be sealed between dry lining and masonry walls at the edges of openings, such as windows and doors, and at the junctions of walls, floors and ceilings. One method is to seal with continuous bands of fixing plaster.
 
Painting direct
Paint Grade Smooth blocks built fair-faced can be painted internally with any alkali-resistant paint, plastic emulsion paint being particularly suitable. Any small holes that occur on the faces of the blocks should be filled before painting.

Storage
As with all concrete products, it is desirable that Thermalite® blocks be kept dry by stacking them close together, with blocks laid flat on top of the stack. Blockwork should be allowed to dry out thoroughly before commencement of rendering or plastering (BS 5628: Part 3 and BRE Digest 342).