Insulating our attic trusses

[Barryscotland]

Our roof space is attic trusses for future development, gable an velux windows already installed. I would like to purchase the insulation for it before I put in my vat reclaim but as it’s not getting done yet it’s not on the spec sheet so unsure as to what thickness pir or even which face it go’s on. Thinking 100mm on the “walls” and 450mm wool above ceiling and wrapped in breathable membrane? We don’t plan on putting in a staircase or internal walls anytime soon but as soon as it’s signed of I’d like to get the insulation in as it’s currently a book store an the books are starting to curl.

Anybody have a drawing of an attic that they have done in the past? I understand we need to keep the ventilation from the eaves to underside of rafters

[ETC]

1 hour ago, Barryscotland said:

Our roof space is attic trusses for future development, gable an velux windows already installed. I would like to purchase the insulation for it before I put in my vat reclaim but as it’s not getting done yet it’s not on the spec sheet so unsure as to what thickness pir or even which face it go’s on. Thinking 100mm on the “walls” and 450mm wool above ceiling and wrapped in breathable membrane? We don’t plan on putting in a staircase or internal walls anytime soon but as soon as it’s signed of I’d like to get the insulation in as it’s currently a book store an the books are starting to curl.

Anybody have a drawing of an attic that they have done in the past? I understand we need to keep the ventilation from the eaves to underside of rafters

I’d normally expect to see a minimum of 100mm PIR between the rafters an a minimum of 62.5mm insulated plasterboard under the rafters. I’d forget about the “walls” and insulate right down to the eaves and overlap with the wall insulation. Don’t forget you will need 25mm eaves ventilation (Glidevale FV250 or other equal) and 5mm ridge ventilation.

 

If you intend to make the first floor habitable don’t forget about FR and sound resistance to the intermediate floor.

 

You will also need 22mm plywood flooring (which should extend to the eaves) and 10kg/m2 plasterboard to the ground floor ceiling as well as 100mm of sound insulation - 10kgm3 - between the floor joists.

 

450mm mineral wool insulation above the flat ceiling will be fine - I normally see 300mm.

 

Looking at your photograph I’d also expect to see diagonal bracing to the roof and chevron bracing rather than diagonal bracing to the uprights as well as a bit more longitudinal bracing. Perhaps this was taken before the trusses were complete?

 

Are your trusses cut in the middle? Looks strange.

[Barryscotland]

20 hours ago, ETC said:

I’d normally expect to see a minimum of 100mm PIR between the rafters an a minimum of 62.5mm insulated plasterboard under the rafters. I’d forget about the “walls” and insulate right down to the eaves and overlap with the wall insulation. Don’t forget you will need 25mm eaves ventilation (Glidevale FV250 or other equal) and 5mm ridge ventilation.

 

If you intend to make the first floor habitable don’t forget about FR and sound resistance to the intermediate floor.

 

Looking at your photograph I’d also expect to see diagonal bracing to the roof and chevron bracing rather than diagonal bracing to the uprights as well as a bit more longitudinal bracing. Perhaps this was taken before the trusses were complete?

 

Are your trusses cut in the middle? Looks strange.

Have already bought 22mm chipboard flooring, hopefully that’s ok for building regs

 

whats FR?

 

Skiddly bits of wood on the trusses was just bracing to hold in position until the sarking went on. Nothing else has been added to the roof except sarking an there’s joins on all the trusses with the spiky metal joining plates, presumably as you don’t get 9m long bits of wood

[Iceverge]

7 hours ago, Barryscotland said:

whats FR

 

Fire resistance i think. 

 

Do you have any pics of the roof today as it is. Might give some clues as to the best solution. 

[Barryscotland]

Basically as it was in the first pic but with a bit of loft insulation 

[Iceverge]

Ok, that helps. 

 

Looking at those rafters I guess they are ~150mm so something like 100mm insulation between them , ( I would use flexible batt insulation) and then as another 100mm below across the rafters. A/tight membrane then. 22 mm *50mm  battens and plasterboard. 

 

I assume the roof ventilation is happening below the sarking by the way.