Celotex between Rafters

[Doggit]

Although you're using a breathable membrane, a 25mm air gap is still a good idea, so you'd need an inch batten under the joists. By the time you've added the weight of the celotex and plasterboard, the existing 3x2s won't be strong enough, so I suspect this is why he's doubled them, but I'd say he's done it the wrong way round, but adding the 3" to the top of the existing joist won't give the same structural integrity as a new 6" joist. Chat to your local BCO to see what they'd specify without an independant BC.

And using 75mm insulated PB means you're going to be using very longs screws, so I'd glue and screw 50mm battens to the existing joists, tack a 25x25 batten by the membrane to give an air gap, then use 100mm celotex, and then 50+pb over the top.

 

[FRAN1870]

Aye .... Kind of why I'm here tbh. I'm not happy with the service at all.
Live and learn but If I can resolve these issues without employing another SE or going back n forth between BC then I'd appreciate the help.

I'll email the BC and SE again !!

 

[FRAN1870]

Although you're using a breathable membrane, a 25mm air gap is still a good idea, so you'd need an inch batten under the joists. By the time you've added the weight of the celotex and plasterboard, the existing 3x2s won't be strong enough, so I suspect this is why he's doubled them, but I'd say he's done it the wrong way round, but adding the 3" to the top of the existing joist won't give the same structural integrity as a new 6" joist. Chat to your local BCO to see what they'd specify without an independant BC.

And using 75mm insulated PB means you're going to be using very longs screws, so I'd glue and screw 50mm battens to the existing joists, tack a 25x25 batten by the membrane to give an air gap, then use 100mm celotex, and then 50+pb over the top.

You mean rafters right ?

 

[FRAN1870]

Although you're using a breathable membrane, a 25mm air gap is still a good idea, so you'd need an inch batten under the joists. By the time you've added the weight of the celotex and plasterboard, the existing 3x2s won't be strong enough, so I suspect this is why he's doubled them, but I'd say he's done it the wrong way round, but adding the 3" to the top of the existing joist won't give the same structural integrity as a new 6" joist. Chat to your local BCO to see what they'd specify without an independant BC.

And using 75mm insulated PB means you're going to be using very longs screws, so I'd glue and screw 50mm battens to the existing joists, tack a 25x25 batten by the membrane to give an air gap, then use 100mm celotex, and then 50+pb over the top.

You mean rafters right ?

Also he's not spec'd to put 3x2's under the rafters, he's said to double them up so I'd end up with 4x3 rafters. But only a 350mm wide cavity for the celotex which seems daft

 

[FRAN1870]

Although you're using a breathable membrane, a 25mm air gap is still a good idea, so you'd need an inch batten under the joists. By the time you've added the weight of the celotex and plasterboard, the existing 3x2s won't be strong enough, so I suspect this is why he's doubled them, but I'd say he's done it the wrong way round, but adding the 3" to the top of the existing joist won't give the same structural integrity as a new 6" joist. Chat to your local BCO to see what they'd specify without an independant BC.

And using 75mm insulated PB means you're going to be using very longs screws, so I'd glue and screw 50mm battens to the existing joists, tack a 25x25 batten by the membrane to give an air gap, then use 100mm celotex, and then 50+pb over the top.

You mean rafters right ?

Also he's not spec'd to put 3x2's under the rafters, he's said to double them up so I'd end up with 4x3 rafters. But only a 350mm wide cavity for the celotex which seems daft

 

[^woody^]

You should only need to double up rafters to each side of a roof window

 

[FRAN1870]

It might have something to do with the tiles ? The original tiles were slate but it's had a new roof using Marley modern which are of course quite chunky ?

 

[Doggit]

Sorry Fran, yes rafters. And if you've changed the slates to tiles, then you've got extra weight on the roof, so need more support, and then some more for the insulated plasterboard, but I'd still glue and screw to the top/underside of the rafters rather than double up. Having a gap of 350mm for the celotex isn't really an issue, as you're then being told to put celotex across the top of them. Your original post suggests using a 2x2 batten under the rafters, and then 50mm insulated PB on top, and I've suggested a 3x2 batten, and 125 between the rafters so you get and air gap, and then 25mm IPB on top, so we're pretty close in our ideas.