Insulating between rafters using rockwool

[cdarcy]

My loft has been converted and has two roof voids, whose vertical walls are about four feet high (below the main horizontal roof purlins) which are accessed by small (3ft x 3ft approx) cupboard doors. The tiles in the voids do not have a membrane (the house was built in the 1880s) and there are drafts blowing into the bedroom. The roofing tiles surrounding the internal plastered ceiling are insulated with loose sheet polystyrene.

I plan to use Rockwool-style insulation between the rafters and secure this somehow - I haven't worked out how yet. I was planning on getting 170mm insulation but having read around this forum, there are plenty of posts about leaving various air gaps to prevent condensation.

Should I get a membrane, nail this to the underside of the rafters and then try to attach the insulation to this, leaving a gap of some 70mm between the tiles and the membrane (ie, the depth of the rafters).

Should I stuff the insulation into the 70mm depth of the rafters and use a staple gun to attach a membrane to hold the insulation in place behind the tiles?

Here's a rough plan of what I'm planning to insulate:

Untitled

• cdarcy
• 19 Mar 2012
• 1

Any help gratefully received - Thanks in advance

Christopher

 

[Alastairreid]

That part of the conversion is supposed to be draughty, You should fit draught excluders to the doors and make sure the stud walls are properly insulated.

 

[cdarcy]

Thanks Alastair

I am now thinking of attaching (staple-gunning?) a breathable membrane to the underside of the rafters to reduce drafts and 'catch' bits of mortar from below the tiles - I need to use the roof voids for storage, and want to avoid everything becoming covered in sand / dust. I can leave a 'flap' of the membrane where the roof meets the floor so there's the ability for air to circulate - does that sound like a sensible idea?

The walls up to the purlins are insulated with sheets of polystyrene and seem to be fine (While I could double-insulate, it would be a nightmare to manouvre 8x4 sheets of Kingspan through the cupboard doors anyway!!) so I'll concentrate my efforts on draught-proofing the doors.

Thanks for your help