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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

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