ASK THE EXPERT
Q Our Victorian terraced house has suspended timber floors downstairs laid directly over the ground. How might we insulate them?
Suspended timber floors that are inaccessible from below or can’t be practically insulated on top can be thermally upgraded by lifting the boards temporarily to insert insulation in the form of batts, boards or loose fill. This is cradled using netting or a breather membrane stapled over and between the joists. Permeable insulation like sheep’s wool, hemp fibre, cellulose or woodfibre often works well with old buildings. A space of at least 150mm must be maintained below insulation installed within a sub-floor void that has air bricks in the external walls for ventilation. If the void is unventilated, cross ventilation may be needed before insulating. You should lag subfloor cold or hot water pipes against frost damage and heat loss, and route any electrical cables outside the insulation to reduce the risk of fire. Gaps between boards or under skirtings can be draughtproofed using slips of matching timber, papier mâché or beeswax, or with a breather membrane in the void below.
Q How can I repair cracked and loose lime plaster in my Edwardian house?
Fill cracks using new lime plaster or, if narrow, an interior filler gauged with whiting (i.e. pulverised chalk). They may require widening first to create a good key and, where movement is expected, bridging using a non-metallic mesh or brick mesh. The plaster should also be repaired beforehand if loose, but not if it’s merely springy.
Douglas Kent, technical and research director at the Society for the Protection of Ancient Buildings, answers your renovation queries