Dormer insulation

thechump

Hi,
I am currently in the process of replacing the cladding on our dormer windows but have discovered that there is no insulation behind the plywood that the cladding is nailed to.

I have googled but I'm a bit confused as to how I can add insulation from the outside without touching the internally decorated rooms.

My plan so far -

1) Remove old cladding
2) Remove non breathable felt
3) Carefully, remove ply boards to gain access to space between vertical supports
4) Add insulation
5) Recover with boards
6) Cover with breathable membrane
7) Add cladding battens
8) Fix new cladding

My concern is with step 4. Can I Use Celotex rigid board and if so should there be an air space on the internal or external side? Also, a friend mentioned that I could use something called SuperQuilt across the outside of all the verticals and then attache the plywood boards over that.

I would appreciate any advice.

System

You could do this, might be a bit of a faff trying to cut boards to fit up there (unless you get a proper scaff in place)
The only thing I'm thinking is that normally you would tape the joints on the warm side of the boards to create a vapour barrier which you wouldn't be able to do, but it's probably not crucial on a retrofit to something with zero insulation!
The other issue will be that you won't necessarily be able to get every bit insulated on the cheeks because you'll have flashings which you really don't want to disturb...

thechump

Thanks for such a swift reply! I'm fortunate in that the majority of the dormer is situated above a flat roof so working up there isn't a problem. Could I clarify which of the following would be best practise -

1) Completely fill the void between internal plasterboard and external plywood with solid insulation.
2) Push the insulation up against the face of the internal plasterboard and leave an air gap between the insulation and the external ply.
3) Ensure the insulation is thinner than the vertical supports and then keep it flush with the external face so leaving an air gap on the internal side.

I'm concerned about mould growth etc if I don't get this right!

System

You would normally fully fill the timber frame with the insulation (sometimes you might leave a gap on the inner side for services/cables etc) but if it's clear just fill the lot, if you're worried you can get a few point calc done by celotex (phone them, don't think they're available online)

thechump

Thanks for your advice. One final thought - as the void is empty and the plywood in place, could I use a Superquilt type product on top of the plywood, cover that with the breathable membrane and batten over it for the cladding?
Just trying to consider how tricky it may be to remove and replace the ply boards.

FreeBear

thechump wrote: »

One final thought - as the void is empty and the plywood in place, could I use a Superquilt type product on top of the plywood, cover that with the breathable membrane and batten over it for the cladding?

You could fix the Superquilt over the existing plywood (give the manufacturers a call for recommended construction methods). Fitting a breathable membrane would be a pointless expense - The quilt would be impermeable so moisture isn't going to get through and you'd probably need to leave an air gap on between the quilt & external cladding.

If you take the plywood off and use Celotex/Kingspan PIR boards, seal the gaps with expanding foam. It would be cheaper than a Superquilt, but extra work to cut & fit the insulation.

Which ever way you go, I'd recommend using screws to fix plywood/cladding rather than nails - Banging in nails could result in cracks appearing on the internal plasterwork.

System

Tbh i wouldn't do the super quilt on the outside of the sheathing when you don't have a vapoir barrier on the inside of the wall.
Although I don't have great knowledge of the superquilt standard/approved details, I seem to remember it needing a certain amount of space either side to be effective (while ago so maybe changed recently) - however you would then have a construction which is permeable for vapour all the way through until it hits the impermeable superquilt and if condensation forms on the timber at that point it will slowly rot. With all these modern insulations the vapour barrier is fairly critical in timber framed construction, with insulation between studs and a breather membrane externally you are still allowing vapour to pass and not be trapped at any point in the construction.
Speak to the respective companies about how to do this on your wall build up before you buy expensive materials

thechump

Thank you for all of your replies. I have contacted a few companies and I'm just awaiting responses. I think the best way forward would be to remove the ply and fill with solid insulation. This will result in more work but would improve the thermal insulation.

One last question - would anybody recommend a particular product for the rigid insulation boards that I could insert between the vertical uprights?