Insulating Existing Mansard Dormer

[RenegadeMaster]

Hi,

We’ve been in our 3 bed Victorian cottsge for 8 years and when we moved in there had already been a mansard dormer extension built. Due to space we are going to go further into the roof space and install some pull out cupboards.

Problem with the dormer is that it’s been poorly insulated and I’ve already installed some rockwool / chicken wire down one side. I don’t know what’s above the ceiling but will have to cross that bridge when we come to it. We will need to replace the roof at some point so trying to pre-empt any future proofing as much as possible.

I’m not sure what to do in terms of insulating the rafters - do I insulate in between and go with a warm roof or leave the gap and fix the insulation board across the rafters.

Any help appreciated.

 

[blup]

Leave a 50mm gap for ventilation and space at the eaves to ensure free passage of air.

 

[catlad]

I would put 40 mm kingspan between rafters if they are 3x2" and then put insulation across the rafters. I would also put in lap vents before I covered anything up.

 

[RenegadeMaster]

Thanks - I’ve followed all the advice. Would you recommend I put any membrane on internal face of the insulation board? If so, what is best way of fixing? Double sided gorilla tape or staple?

 

[robinbanks]

I personally would fit an aluminised/reflective VCL, held in place by battens screwed through the insulation and into the rafters, joints taped (alu tape or something like tescon) and glued (vapour sealants).
The battens also give you the air gap necessary for the aluminising to work as intended (reflect radiant heat) and give a void in which to run cables and pipes etc, and also something to screw your ceiling plasterboards to

If you've made a really neat job of fitting your boards and their aluminised surface is in good condition you may just opt for careful application of alu tape to the joints