VCL on warm pitched roof

[Will K]

Hi Folks,

 

Looking to better understand the role of the vcl in a warm pitched roof please, and keen to hear what the general consensus is. If there is post that covers it, apologies and pls do link in/ elaborate.

 

About to start our first (and possibly only) self build. Planning an insulated slab, icf walls and a truss roof with osb/ ply over rafters with 2 staggered layers of PIR, then battens, membrane, tile battens and then tiles. 

 

Conscious people typically spec a VCL between the osb and the pir. Question is twofold- is this actually doing anything assuming I have properly taped the outside of the osb / junctions with wall plate (and assuming the osb is airtight (?)and given its on warm side too so dew point issues are mitigated ?).

 

And secondly, if needed, is there a more foolproof way of doing it than laying a vcl given i will be driving long hex screws through pir into osb (ie is there any value in a liquid/ paint applied vcl or perhaps a peal and stick)?

 

And finally, is there anything you'd do differently. Not keen on hybrid roofs with insulation between rafters and trying to make as simple for me as possible to make airtight given sealing from inside with truss connections will likely be a nightmare.  

 

Thoughts appreciated,

 

Thanks

 

Will

[Gus Potter]

Hiya Will.

 

Welcome to BH!

 

Your starting point here is to really understand how a flat warm roof works. Research this and it will give you a good basic understanding of the principles.  

 

It's essential that you have a impermeable VCL between the OSB and the underside of the insulation. Have a look at Alutrix type membranes. 

 

Once you get your head a round how a flat warm roof works that then pitch it up, as you now understand the basic principles. 

 

Key things that will change are that as you pitch the roof up to say 5- 35 degrees is that everything that is above the trusses will want to roll down the roof, like slip off.  The fixings and roof make up need to be designed for this, what we call roughly rolling shear, to stop everything slipping off the roof! The fixings need to be carefully designed as the rolling effect causes them to bend not least. 

 

It can be done but you have to learn a bit more first. Much also depends on your actual roof covering, each type of covering needs a different set up. 

 

Give hybrid roofs a miss until you have learnt more.  

 

You're best bet is to post a sketch of what you think you may like to do.