Above Rafter Insulation

[thebadger7]

Hi folks
We're getting our roof replaced. We have two attic rooms so looking at getting insulation over rafters. SPAB recommend woodfibre sarking boards laid directly onto rafters, then counter-battens over this for ventilated gap before slates.
Most roofers we speak to want to use TLX Gold, as I suppose this is what they are used to. Woodfibre boards will also raise the roofline which has alot of complications with chimneys etc. This product claims to be vapour open, but instinctively I don't like the sound of it given it's not a natural product and has a lot of marketing around it.
Any views on TLX Gold above rafters vs wood fibre? One of the main considerations is reducing summer heat gain. I understand woodfibre is good for this as has high thermal mass, but then TLX Gold should be reflecting heat gains?

Any help appreciated!
Cheers
Luke

 

[paulc]

A couple of years back, I got asked to do some work in a top floor flat that involved access to the roof space. It had 100mm or so of fibreglass insulation between the ceiling joists, and a multifoil insulation stapled to the rafters. It was like an oven up there, and so hot, I could only stay there for ten minutes at a time.
These multifoil products are good at reflecting radiant heat, but perform poorly with conducted heat - The application notes I've seen all recommend it is used in conjunction with Celotex type boards (presumably to improve performance with conducted heat). I would argue that if you are putting in Celotex type boards, the multifoil is going to add precious little to the overall u-value. I would also take any claims of breathability with a sizable bucket of salt.

 

[thebadger7]

Thanks that's interesting. We're not using celotex boards, we'll be using sheepswool between the rafters and intending to use woodfibre above the rafters

 

[Lacan07]

I'm confused, are you using TLX or wood fibre boards for the insulation, and why are you raising the roofline?

 

[thebadger7]

I want to use wood fibre boards - roofers tend to want to use TLX

Roofline would be raised because you'd have the 32mm woodfibre boards over rafters plus counter battens