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. 

 

 

[Stu789]

We are doing the same thing, specified by our architectural technician - as @Gus Potter says above special fixings through the PIR and VB+underlay (Proctor products).

Below is an excerpt of one of the drawings that may be useful.

 

[JpManning]

It's worth being clear what the VCL is actually doing in this build up, because it answers both of your questions.

With a breathable membrane above the PIR, any moist air or vapour that gets past the deck and into the insulation joints can reach the cold side and condense on the underside of the membrane or on the battens. The foil facings on the PIR are very vapour tight in themselves, but the board joints, the screw holes and the edges aren't. So the VCL isn't really about the dew point in the OSB, it's about stopping vapour getting into the insulation layer at all.

Taped OSB will give you a decent airtight layer, but I wouldn't rely on it as your vapour control. Its vapour resistance varies between boards and with moisture content, so it's not something you can design to with confidence. A separate membrane laid over the taped deck is cheap insurance.

The screws aren't the weak point you might expect. They go through under compression from the counter battens, and a self adhesive membrane tends to seal round the shank reasonably well. The places that need real care are the laps, the junction between the roof and your wall airtightness line at the wall plate, and anywhere services or roof windows go through. Get those detailed on the drawings before you start, as they're much harder to put right once the PIR is down.

A liquid applied membrane over the whole deck would work, but it's slow and costly on a roof that size. Where liquid or paste products earn their keep is round awkward penetrations and at the wall to roof junction.

As mentioned above, get the fixing design for your exact build up from the screw manufacturer, as long fixings through two staggered layers of PIR on a pitch need to be calculated, not guessed.

[ADLIan]

+1 to above from JPM.

 

Generally a VCL is not required with this form of construction, perhaps with the exception of high humidity buildings such as swimming pools. Check MIs and relevant BBA cert for your chosen insulation and breather type roofing membrane. Adding a VCL will make the roof safer from condensation risk.