Insulating vaulted loft space with exposed beams

[JonH]

I’m looking to insulate a vaulted ceiling (which used to be loft space) to create height in a bedroom and ideally expose the roof rafters in the pitch.
The roof is clay tiles with a bitumen felt underlay. The apex roof rafters are approximately 100 thick. I’ll not sure what insulation method to use that maximises thermal properties without loosing the rafters or causing damp / condensation problems. I’m also unsure if I can insulate directly against the bitumen felt or if I need to leave an air gap?

Initially I was thinking of stapeling insulting foil to the rafters 25mm away from the bitumen felt to create an air gap and first layer of insulation then used insulated plaster board approximately 50mm think flush to the foil to create a second insulation layer between the rafters that can then be plastered?

Alternatively I was thinking 50mm wood fibre board insulation cut to fit between the rafters leaving a 25mm air gap and then limeplastering over the fiber board?

Any advised greatly revived!!

 

[jamest]

It's not easy to tell from the photos or your description, but opening up that attic space would seem to imply removing the existing ceiling joists and given the absence of a ridge beam, these are acting as rafter ties. Without them, your roof would be in danger of collapse. If you keep them, you'll not be adding any height to the bedroom. I suppose you could move them a bit higher, but you still need them. You'd need to get a structural engineer to look at it really and explain what options you have.

 

[Cubist]

Fully concur with James and would only add that, without the joists, any snow load on that roof could push the walls out and down.

 

[Kearn]

Agree with the above - walls will bow out at the centre due to roof thrust.

Also, you’ll get very little insulation in the 100mm rafter depth once the air is left. However, those commons aren’t meant to be seen, so pack 50mm wood fibre or sheepswool into them, then use min 50mm wood fibre boards over them plus lime plaster.

But not until you’ve reconciled how to deal with the roof structure!

Doesn’t really feel worth it to me from the limited info.

 

[Stevers]

The modern way seems to be put steels along ridges as an enabler for 'vaulted' ceilings. Although I wouldn't accept anything less than a vaulted stone or brick ceiling - I wish!

 

[Cubist]

Even in the Sistine Chapel regular visitors seldom look up. If you can't rival that don't bother with a vaulted ceiling would be my view.

 

[Kearn]

Agree with the above - walls will bow out at the centre due to roof thrust.

Also, you’ll get very little insulation in the 100mm rafter depth once the air is left. However, those commons aren’t meant to be seen, so pack 50mm wood fibre or sheepswool into them, then use min 50mm wood fibre boards over them plus lime plaster.

But not until you’ve reconciled how to deal with the roof structure!

Doesn’t really feel worth it to me from the limited info.

Oh, and what I’ve suggested obviously adds more weight to the roof structure…

We currently have a similar roof, originally a hayloft over animal shelter, inadequate collars too high up and not tie at all. The span is a touch too long and the walls are bellying from the thrust. We can’t tie without it being 2ft above floor level as we have dwarf walls before the roof, which doesn’t really work, our solution was too bolster the 4 central commons on both flanks to add rigidity without too much incremental weight.

 

[JonH]

It's not easy to tell from the photos or your description, but opening up that attic space would seem to imply removing the existing ceiling joists and given the absence of a ridge beam, these are acting as rafter ties. Without them, your roof would be in danger of collapse. If you keep them, you'll not be adding any height to the bedroom. I suppose you could move them a bit higher, but you still need them. You'd need to get a structural engineer to look at it really and explain what options you have.

So I won’t be removing any joists or rafters. The newly opened vaulted roof will be used to hang some chandeliers. But to keep the room as warm as possible i need to insulate the vaulted roof, with some of the beams exposed.

 

[JonH]

Oh, and what I’ve suggested obviously adds more weight to the roof structure…

We currently have a similar roof, originally a hayloft over animal shelter, inadequate collars too high up and not tie at all. The span is a touch too long and the walls are bellying from the thrust. We can’t tie without it being 2ft above floor level as we have dwarf walls before the roof, which doesn’t really work, our solution was too bolster the 4 central commons on both flanks to add rigidity without too much incremental weight.

So I won’t be removing any joists or rafters. The newly opened vaulted roof will be used to hang some chandeliers. But to keep the room as warm as possible i need to insulate the vaulted roof, with some of the beams exposed.

 

[Cubist]

Thanks for the clarification. Others I'm sure will bet able to help re the insulation question.

 

[Kearn]

So I won’t be removing any joists or rafters. The newly opened vaulted roof will be used to hang some chandeliers. But to keep the room as warm as possible i need to insulate the vaulted roof, with some of the beams exposed.

My suggestion therefore still stands, you won’t get enough bang for your buck with 50mm insulation between rafters. Go over them, they’re not designed to be seen, use them as the frame to hang your internal insulation from otherwise you’ll end up with a a cold space.

 

[jamest]

Absolutely with Kearn on this one. Plus if you need to meet building regs (which technically you probably would, but in practice . . . ?) you'd never be able to get anywhere close to the required U value (which is less than 0.3, but could be as low as 0.15, haven't checked recently) without internal insulation covering the rafters. If you just used 50-60 mm of insulation between the rafters, you'd be getting a U-value for the insulated spaces of around 0.7 at best and worse for the rafters, which would then be acting as cold bridges. You'd also need to ensure airtightness, which can be tricky.