advice on insulation / wall materials for loft please..

24 May 2011
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United Kingdom
Hi, thanks for reading this,

Just boarded out loft floor for a new hobby room, want to insulate roof & sort out the 2 end walls. Got 9.5cm deep roof rafters and was thinking 50mm kingspan polystyrene (or similar) up between all the rafters leaving 45mm for ventilation between kingspan and the roof tiles, then 6mm wall board over the top inside. Cavity walls each end are brick outside / cinder block inside in the loft and have been injection insulated between the gap.

Can I glue wall board straight onto the blocks or should i timber frame it, insulate & then board on top of frame same as roof? will I need to put a moisture barrier between the kingspan and wall board on either roof or walls or between the roof tiles /walls and kingspan ?


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Regards the roof does it have felt or a breathable membrane or none of these ie just tiles? If felt then 50mm air gap, ensure air gap is open to vents at eaves level and you'll need a few vent tiles on each side near the ridge or a continuous vent at the ridge. If breathable membrane or no felt fitted stick the insulation 25 mm below underside of tiles. Use a foil backed plasterboard or a separate vapour barrier instead.

Regards the end walls if you have cavity walls then there should be no condensation issues so no need for any additional vapour barriers etc. So battens with insulation between and then the boards.

45mm polystyrene is a bit crap BTW, a proper loft conversion would require about 120mm Kingspan. You generally get what you pay for.

All a bit of a bodge really but we assume this is 'hobby' space so not to current regs.
thanks very much for your advice on this,

the roof tiles have a breathable membraine circa 1985 so will I still need to put in vent tiles at the ridge if using foil backed kingspan between the rafters?

I definately dont want to bodge this up, do the job once & get it right. Will install more kingspan horizontally across rafters before the wall board to build up insulatation to 120mm, didnt realise how pricey this was going to be in materials !


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Frankly I'm not 100% sure but doubt very much if your membrane on a roof at that age could be a breathable membrane rather than just plain old felt, assuming its normal felt, ridge or high level tile vents on both sides is a must. You don't need to use 120mm insul, I just mentioned that to demonstrate how inadequate your 45mm polystyrene would be, as mentioned we assume you're not doing this to regs and don't expect to be able to call it a habitable room when you come to sell.

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