Hi This is my first thread so please bear with me. I am a reasonable DIYer....extension, new garage ect (not much good with a computer!!!) I live in a bungalow which had a dormer fitted about 25 years ago and the felt on the roof has gone. No problem I have fitted a rubber membrane to 2 garages. However I want to insulate the roof which is a problem for the following reasons. 1. Height of bedroom ceilings is only just over 2m. 2. Felt on roof is almost up to the ridge tiles and goes straight under the ridge tiles so little scope for insulation above felt. 3. When built 2" of polystyrene was placed against underside of deck but then joists were ventilated at window side making insulation worthless!!! I intend to take off felt and then renew deck with 18mm osb. It has a good fall away from ridge. Can I put 100mm of Kingspan/celotex between joists, block ventilation and make it a warm roof. Will this cause damp problems...there is no bathroom upstairs only a toilet and wash hand basin. I have had a quote from a roofing firm which seems to be doing the above but for silly money. Thanks for your help!!!!
Technically that's a cold roof & with no ventilation you would need a vapour barrier between the celotex & the plasterboard. Better to have the Celotex & to have a soffit for ventilation to prevent the build up of moisture vapour in the roof void. A warm roof the insulation is fixed beneath the roof covering to the decking. As for roof covering have a look at glassfibre roofing, far superior, it will outlast a felt roof.
I hate it when two supposedly same diagrams show different information. ie No vapour barrier shown in 'Conventional cold roof', but shown in 'Cold flat roof'. Mr. HandyAndy - Really
Thanks for your reply KIAB. Not sure if I have explained the problem well. Can't put any insulation below joists as low headroom in bedrooms. Can't put insulation above joists as almost up to ridge tiles. No vapour barrier between plasterboard and joists but foil backed plasterboard used? Is only option to fill between joists leaving a 50mm gap at top for ventilation? (could be no more than 50mm of insulation) Have seen one diagram on net showing a warm roof where gaps between joists was totally filled with insulation up to decking with comment that with no air space moisture can't be a problem. Has anybody seen or done this? Thanks.
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