Hi,

Hoping someone here can advise. I'm about to have the SIPs manufacturer finalise manufacturing drawings, but I would like to ensure that there are no issues with the opening sizes in the panels when the time comes to install the rooflights. The given size Velux roof windows are 550x980 and looking at typical velux installation instructions, they appear to indicate a tolerance of 40-60mm for the width (EDZ):

http://www.professional.velux.co.uk/en-GB/Documents/Installation%20instructions%20pdf/GGL_GGU452953-2013-03.pdf

So I presume the manufacturing drawings should be 600ish for the width? And I guess with this 40-60mm gap, it's intended to have their insulation kit fitted around the frame (BDX kit)?

Obviously I don't want my installer thinking he can simply hack away at the SIP panels on the day, if the fit is a bit tight. Nor do I want a massive gap that needs to be filled with sealant etc.

Thanks Tony. I will allow for some more width then to get some sheet insulation in there.

Should have said, I'm fixing Tata Colorcoat Urban Steel roofing directly over the SIPS membrane, hence no Battens, etc.

Hi,

Tata recommend fitting directly through the membrane onto the OSB face of the Kingspan SIP.

Thanks

Posted By: DarylPthe condensation that forms under the steel?

Where wd that condensation come from? Assuming a SIP is non-breatheable (with VCL) so water vapour's nor coming to the steel from the interior (until the VCL goes faulty in due course). Until then, that leaves just atmospheric condensation, occuring on any skyward-facing material, prob both faces, on a clear night. How much of a threat is that? Is that why Tata say battens unnecessary? Anyway, battens wouldn't help unless there's deliberate, copious through-ventilation from eave to ridge outlet, of the batten space.

I ask because it makes a massive difference, whether the layers beneath are breatheable, or rely on a (hopefully) perfect long-term VCL. I wonder whether Tata's advice (to fix direct to the OSB, no battens, with or without through-ventilation) is strictly in case of non breathable, VCL'd layers beneath. If no such stipulation, then I'd strongly question that advice.

Don't need to - I can't imagine using metal sheeting that way - but I might alert someone else.

How do you mean? OSB is fair-to-good vapour-breatheable (but pretty airtight), along with everything else in my favourite build-up (as pictured) and when it goes seamlessly from wall to roof (minus the render) the batten/crossbatten-space above it, beneath the corrugated iron, is copiously cross-ventilated.

We've got along fine on that basis - division of labour - for many years so far. I've learned a lot and am extreemely grateful for GBF.