Insulation strategy for room in roof/attic conversion + EWI?

12 Oct 2012
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United Kingdom

Looking a bit of advice on the best way to go about insulating our attic conversion.

It was done 25-30 years ago and had a 50mm layer of fiberglass behind the plasterboard. Weve only owned the house for 18mths but are trying to renovate it to good standard of efficiency and airtightness.

Looking to fit 100mm Xtratherm board between the rafters (leaves 50mm ventilation gap) then over board with 25mm + PB. Had thought of using Rockwool 3-400mm over the celing /floor outside of the room area to save bit on insulation cost or is best to do the whole of the roof pitch with the board or both???

Ill also be fitting EWI later in the year, should i be aiming to tie the top of the EWI into the roof insulation whilst maintaining ventilation from soffits to the ridge?

Ive attached the options as i think they are, which is the best bet?

For info, Bungalow hipped roof, 150mm rafters, 15mm Sarking boards, Felt, flat concrete tiles.

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I have pretty much the same issue as you, except all mine has is foil backed plasterboard with inch thick polystyrene randomly nailed on the back of the studwork. No insulation inside my dormer roof either.

As I am planning on extending the sixe of my conversion (hip to gable) I will also be looking at properly insulating it. My ideal plan is to insulate directly under the roof rafters thereby also insulating storage spaces under the eaves etc. Just insulating directly on the back of dwarf walls can give cold spots I believe.
If I were drawing it I'd say option 1.

I'd say that it'd be a cheaper option plus you only want to be insulating the habitable space.

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