Curved ceiling / Vaulted rafters insulation

[InspiredSloth]

Hi all, renovating a home and stuck on one thing (that’s a lie many) but this question does my nut in. 

I need to install 100mm of insulation with a 50mm air gap in a vaulted ceiling in on a curved bedroom ceiling, the wood rafters measures only 70mm. I am looking to install extra timber to extend the rafters but it’s making my room smaller. Seems strange to do this but that’s the recommendation from BC. 

 

how do I ensure there’s a 50mm air gap? I’m worried for future condensation as I have black felt in the roof. 
 

Appreciwte any guidance thank you 

[DevilDamo]

Has the room always been a Bedroom because if so, why does it need insulating at all?

[Gordo]

Why increase the thickness of timbers? If you lowering plasterboard slightly just use insulated plasterboard with say 25mm PUR lining. Then 70mm PUR between existing rafters or whatever to leave 50mm ventilation air gap. U value for these iminimum insulation thickness likely less than 0.18 tho. 

Edited December 24, 2024 by Gordo

[Iceverge]

Any pictures? 

 

How big is the roof area you need to cover? 

 

Ultimately if you want to make a proper job of it you'll need a thicker roof. I would avoid rigid boards between the rafters. 

[Redbeard]

Agree, we need pictures of the curves. If you really need to follow a curve then (a) it depends on the radius and (b) while plasterboard can be persuaded to follow a curve, insulated plasterboard would need slots cutting out of the insulation, making efficacy and continuity a bit iffy. Flexi-something followed by something like Intello membrane and 'steamed' plasterboard.

 

As far as

On 22/12/2024 at 19:40, InspiredSloth said:

how do I ensure there’s a 50mm air gap?

 

If you are concerned re over-enthusiastic 'stuffing', staple some breather membrane between each rafter pair, 50mm from the felt. Stretch as tight as you can and then you can 'stuff' with abandon. How about using 'Larsen Trusses' ('poor persons' I-beams')? Basically a sub-rafter hanging below the existing (parallel or at 90 degrees). The latter would help if you really need to achieve a smooth curve.

[ProDave]

I am still waiting for the pictures to show me in what way this roof is curved.

[Gordo]

Yeah rigid PUR insulation and like isn't ideal for a curve, but if depths limited may be only way to achieve acceptable U values