Insulating intersection of flat roof and vaulted ceiling.

[If in doubt...]

Am just about to start insulating a part vaulted ceiling of the existing bungalow where the warm flat roof of a new extension joins.

Below is fairly close to the existing flat roof and proposed set up of the vaulted ceiling (blue rectangle) and it's where the air gap above the vaulted ceiling insulation meets the end face of the flat roof insulation I'd like to get right.

I'm thinking the Flat roof Alutrix vapour barrier needs to be taken up vertically to seal off the end of that insulation from the potential cold spot of the upper deck and then maybe pack that area somehow with insulation but maintain the airgap in the upper half of the sloping rafters, maybe vent along the bottom of the tiles somehow - or as it is only a small section of vaulted roof (2.5metres wide by half the slope of the bungalow pitched roof) then will that be ventilated enough already?

 

[noseall]

Am just about to start insulating a part vaulted ceiling of the existing bungalow where the warm flat roof of a new extension joins.

Below is fairly close to the existing flat roof and proposed set up of the vaulted ceiling (blue rectangle) and it's where the air gap above the vaulted ceiling insulation meets the end face of the flat roof insulation I'd like to get right.

I'm thinking the Flat roof Alutrix vapour barrier needs to be taken up vertically to seal off the end of that insulation from the potential cold spot of the upper deck and then maybe pack that area somehow with insulation but maintain the airgap in the upper half of the sloping rafters, maybe vent along the bottom of the tiles somehow - or as it is only a small section of vaulted roof (2.5metres wide by half the slope of the bungalow pitched roof) then will that be ventilated enough already?

View attachment 282762

Like this roof?

Inso1

• noseall
• 3 Oct 2018

Inso2

• noseall
• 3 Oct 2018

Inso3

• noseall
• 3 Oct 2018

 

[^woody^]

Hail the EcoTherm Mitre Meister.

 

[If in doubt...]

Like this roof?

Looks like it could be !

So your mitre / angled deck looks like it will take the vapour barrier up to meet the top deck and keep the warm roof insulation self contained?, which I will have do .... either angled or vertically.... dunno if I have the willpower to mitre it all . Then is your existing pitched roof going to be vaulted or just left as loft space? Mine's vaulted so I'm just trying to suss out the best way to part insulate / part ventilate that area - currently the old roof joists in the vaulted area are still in - not shown in my sketch - so it's currently a wooden spaghetti junction right now

 

[George_engineer]

The vaulted roof is a cold roof, so why wouldn't you do the flat roof as a cold roof too?

 

[If in doubt...]

The vaulted roof is a cold roof, so why wouldn't you do the flat roof as a cold roof too?

Vaulted roof wasn't in the original plan, was just going to do only the new extension and with warm roof for all the various reasons / benefits.... then when things were underway had the idea of vaulting the roof.

 

[If in doubt...]

So if someone could just check I'm on the right track - would love to get this finished for Xmas....summer was the original plan

1 - Seal up the end of that warm roof insulation with the vapour barrier - or is there some reasoning in leaving it exposed to that cold area giving it some ventilation?
2 - Install all the vaulted roof / rear stud wall insulation so it's a continuous layer meeting up with the "external now internal wall" - but with the air gap under the pitched roof.
3 - Finally plasterboard choice - vapourcheck for the vaulted ceiling ? - and also the rear stud wall (which will be insulated as it has the cold loft space behind?

Thanks.

 

[If in doubt...]

Any thoughts chaps, hoping to order the plasterboard in a couple of days.

Ta.

 

[George_engineer]

Could you post an updated diagram?

I think you could do with some ventilation tiles at the bottom of the tiled roof so you've got a way for air into the ventilated void.

 

[robinbanks]

Not sure what putting a vapour barrier against a vapour impermeable PIR board is going to offer..? (Between "150mm PIR" and "flat joist" in that diagram)

 

[If in doubt...]

Could you post an updated diagram?

I think you could do with some ventilation tiles at the bottom of the tiled roof so you've got a way for air into the ventilated void.

Here's where we are at - except I've yet to install the vaulted roof insulation.
The red cross is where I'm asking if I need to seal it up so the Alutrix Vapour barrier completely envelopes the warm roof insulation, which is my understanding it should be and keeps it separate from the cold roof void. No prob to do that if need be.

I was anticipating some kind of vent tile but where I have shown blue arrows "Side Ventilation" I'm wondering if those side voids (photo below - one each side of vaulted section) would be adequate - each triangular void is approx 270 cm2 area. That vaulted roof section is only 2.7 metres wide and the insulated pitched section is only 2.4 metres long with a further 2.4 metres of open loft existing pitched roof above it. New rafters are 200mm so will be 100mm airgap above. There will be 2 small velux (98cm high x 55cm wide) so there will be a slight restriction. Note thats the old existing rafter in photo as I'm taking photo from inside the vaulted section so the new rafters are just behind the camera.

Thanks for your help.

 

[If in doubt...]

Not sure what putting a vapour barrier against a vapour impermeable PIR board is going to offer..? (Between "150mm PIR" and "flat joist" in that diagram)

Hi - the existing Alutrix barrier on top of the deck is the specified warm roof makeup - so as not to rely on the PIR alone, apparently it's the norm? but as per the red X in sketch above I've yet to seal that end off - other 3 ends are all sealed up as part of the roof build which was done by builders, but as this end joins up with inside I'm handling that and ok yeah I'm a lot more pedantic and happy to go overboard, but without causing additional consequences.

 

[George_engineer]

It'll be a pig to do, but I think you'd want to end up with a proper overlap of insulation, lapping the vapour barrier onto the vaulted roof insulation.

 

[If in doubt...]

It'll be a pig to do, but I think you'd want to end up with a proper overlap of insulation, lapping the vapour barrier onto the vaulted roof insulation.

Thanks.

I was thinking more like below though, which would be very easy to do as it's all easy access.
Black squiggle is sealing off the end of the warm roof insulation - my understanding is it needs to be sealed in but hoping to get confirmed.
Orange squiggle is linking up insulation to continuous band.

Then it's just sussing the ventilation - not sure what they are called but something like under eaves vents along the bottom of the tiles ?

 

[George_engineer]

If the gap is big enough.,

I was thinking tile vents like this (loads of different options out there):

Clay in-line tile vent | Marley

Product images and information for Clay in-line tile vent

www.marley.co.uk