dormer bedroom plasterboard pulled down, re-insulating confirmation/advice (with pics)

[Tirglas_7]

hi

after contemplating overboarding with insulated plasterboard on the inside of the the dormer bedrooms in a house i've just bought, i decided if i was going to do it the only way to do it properly was to start again.

The dormer exteriors were going to be done by our roofing contractors with 100mm kingspan on the front faces, cheeks and flat roofs. but since i started removing the plasterboard inside i think the only way to ensure i have full coverage (with no gaps), is to batten out then fill with 120mm kingspan then plasterboard over it.

i have a rough idea on what needs doing but would like some advice/confirmation if possible please.

i'm planning to batten out to to allow 120mm kingspan with an airgap, should the gap be 50mm? then cut kingspan to size and fill any gaps with expanding foam. do i then seal any gaps with foil tape? then foil backed plasterboard on top?

if i batten out from the underside of the dormer flat roof, is it sufficient to just create the airgap of do i need some kind of airflow through there, with a vent or similar?

will have more questions ahead i'd imagine, hoping to turn this into a bit of a guide/build thread as i go if poss.

heres some pics of how its looking now, any other advice or recommendations welcome, thx in advance

 

[endecotp]

You can fill between the existing woodwork, then fix sheets across the tops without any battens.

Is acoustic performance a concern? Perhaps not as there is currently nothing. If you did want better acoustic insulation, you would fit acoustic mineral wool between the woodwork then kingspan or similar over the top.

I'll let others comment on gaps & tape.

 

[Tirglas_7]

many thanks for the reply

not too bothered about acoustics if i'm honest, i'm looking for the best insulation i can get in there though.

is it better to install insulation between the existing timber, then fix sheets of insulation on top, then plasterboard on top of it all? or install the plasterboard with the insulation already bonded to it? i'm looking fr the most cost effective method if poss.

i watched a few videos on youtube and they've installed kingspan/celotex with the foil backing, then taped between any gaps then installed normal plasterboard on top. is this ok or should i be looking for foil backed plasterboard as well? or installing any other vapour barrier?

ripped part of one of the dormer flat roof internal ceilings down today, the previous insulation had been installed with no airgap at all, as i removed it i was engulfed in powdered osb, looks like the dormer has had a repair beofre and they've just overboarded the old roof. the newer roof is also spongy now pics to follow once i've got it all down.

 

[ibby]

Insulated Plasterer boards are more expensive and only come in 2400x1200.
Only a tad more expensive, but saves time when as you don't need to.comr back to the same place again.

I hate flat roofs

 

[Tirglas_7]

Haha, I'm getting that way!

so you're saying the added cost when using the insulated boards is worth it for speed/convenience right?

Any opinions on taping/filling etc?

if i install 60 mm of the foil faced insulation boards kingspan/celotex etc, between the timbers (retaining the 50mm airgap all round)

do i tape the gap between the timber and the board?

then install 62.5/72.5 insulated board on top? do i need another vapour barrier or anything inbetween or under?

 

[ibby]

I foam injected and taped all the holes all the gaps!

No vapour barriers between the two.

The way I have done it is, have the large boards delivered and the cut to size downstairs and take upstairs.

Trim and fit perfect.