Insulation queries - loft conversion

[chris_o]

After some insulation advice.

Currently ripping apart an old loft conversion at my mum's house. The conversion was done well, but with a severe lack of insulation (none in places) and gets extremely cold in the winter and very hot in the summer.

As you can see we are taking down all old boards with a view to insulate properly and install a small en suite.

A few questions though. On all external wall areas I plan to use 75mm celotex in between timbers, a vapour barrier on top, then standard plasterboard. Internal walls I plan to use mineral wool to keep costings down.

There is a big dormer coming off the back with a flat roof as you can see in the picture, then the other section of the room has the pitch above it. The pitch space is vented so celotex will be fine in that portion of the ceiling. My question is what to do about the flat roof ceiling space. It currently isn't vented at the soffitts, so would I be able to celotex the rafters leaving a 30mm gap above? Or do I need to vent the soffits? For 50 years there had been a very thin layer of mineral wool in the flat roof section above the plasterboard and everything timber wise seems spot on, no sign of damp/ rot in the slightest.

 

[chappers]

you would get much better insulation on your walls if you put 50mm in between the studs with25mm over the studs, would help if you can vent at the soffit, but if I was you I would insulate as you propose with a vapour barrier below the plasterboard. try to keep the number of penetrations to a minimum Again if you can under draw the ceiling joists with insulation you will vastly improve it's performance, even if it is just 12mm

 

[chris_o]

Thanks for your reply.

Bringing it 25mm in front of studs might disrupt quite a lot of fittings, especially round the window etc.

Would something like this work stapled over the joists and celotex, acting as a vapour barrier too?

http://www.wickes.co.uk/Thermawrap-...gclid=CLmNsLbAqdQCFdQ_Gwodl1ML7w&gclsrc=aw.ds

 

[chappers]

that's always the issue with refurbing there are always fixtures and fittings to work round.
That foil would have some effect but whether it's cost effective is another matter altogether. just use 1000 gauge polythene for your vapour barrier

 

[Doggit]

At less than £3 per sqm, you can reckon the Therma Wraps going to close to useless; but I use it behind rads. Current regs require 135mm of celotex, normally 100mm between the rafters, and 35mm over it - or variations thereof if you want. Seal any gaps in the sub layer with expanding foam, and then seal any joints in the top layer with aluminum tape. The add the vapour bariier, and fianlly the plasterboard. You can get upvc sills and trims to extend the windows etc, and if there are any sockets that need extending, then you can crimp on extra cabling.

 

[endecotp]

50mm between studs plus 25mm in front is equivalent to about 95mm between the studs with nothing in front.

The silver stuff won't do much.