In another thread the following quote is attributed to vikinghouse

"When you put 50% or more of the insulation outside the air-tightness line then the roof structure becomes too warm for condensation to occur, it becomes like a warm-roof and it doesn't require ventilation."

I just wonder what most people think about this and if anyone can point to any science to back this up. It would be great as a rule of thumb that could be reliably applied to all building elements. Perhaps I'm asking too much?

I had heard that as long as increasingly higher U value elements were placed towards the outsdoors then there was no risk of interstitial condensation.

Googling about turned up this from down under.

http://www.abcb.gov.au/education-events-resources/publications/~/media/Files/Download%20Documents/Education%20and%20Training/Handbooks/2011_CondensationHandbook.ashx

thanks!

RobinB

Timber(and others) - I'd very interested in your views. I am just about to compile information ref the proposed build up of my roof for BC approval without a 50mm ventilation gap!?!

I have a 5 deg mono pitch roof and 25 deg pitched roof which I intend to cover with a steel Tata Colourcoat Urban standing seam.

The roof design has decent overhangs and having looked at a true warm roof, I discounted for two reasons

1. It made the roof depth too deep for a suitable u value
2. With all the insulation above the deck you are left with rafters with no insulation between overhanging the wall plate with a big potential for heat loss and difficult to make airtight.

The use of a cold roof with 50mm ventilation gap was looked. The 5 deg mono pitch roof is approx 12m from eaves to ridge at a shallow pitch which I believe might limit the effectiveness of cross ventilation?

My proposal (for 5 and 25 deg roof) is as follows (outside to in) giving a U value of 0.12 w/m2K

Tata Steel roof
Tyvek Metal / Klober Permo Sec breather membrane
18mm Class 3 WBP plywood deck
306mm I-Beam fully filled with Icynene spray foam insulation
ProClima Intel Plus vapour check barrier
150mm void (in which I intend to run MVHR ducting and provide a void for ceiling lights etc so that they don't penetrate the VCL)
Suspended ceiling
Plasterboard & Skim.

Icynene have produced a condensation risk analysis that says this will work, but as we all know any roof will work with a VCL! What happens though if moisture makes its way in or is trapped in during construction?

The idea is to create a well sealed roof construction with very minimal thermal bridging utilising an I beam filled with Icynene which is claimed to be an air barrier which will hopefully reduce the moist vapour being carried far into the insulation and hopefully not coming into contact with a cold deck?

The use of an intelligent vapour check barrier will aim to keep moist air from getting into the structure during the winter and then hopefully allow any moisture that might have made its way into the structure or got caught up during the construction stage (Wet Wales) or through flanking diffusion from masonry elements etc to escape during the drying season?

Do you think this is a suitable roof construction that a) will work in the long term b) be acceptable to BC?

The alternative is to create a 50mm ventilation cap above the 306mm i-Beam. To continue to use Icynene I would have to put an additional layer here to create a gap. Using Ply/OSB seems a bit silly as they are fairly vapour resistive and not allow any trapped moisture out. The alternative is to place a breather paper between the I beam and 50mm batten and spray up against (flash coat first, then further spray)

I'd really appreciate yours (and all users) views?

Thanks

Well I might start to sound like a broken record, but the plywood deck below the metal deck, as well as the top of the I joists might not be too happy.

I assume the breather membrane is a furry hair-suit type product - this might save the situation. I have heard varying reports on the merits of the pro-clima product and some possible issues, but I have not seen any issues with it first hand, so cannot comment on the validity.

Run some of your own numbers in build desk or similar and see what you get.

Thanks for the comments.

Timber

- Yes its the furry type with a mesh in it to create a drainage channel under the steel. See here http://www.roofingsuperstore.co.uk/browse/pitched-roofing/metal-roof-underlay.html

- Do you have any links to issues/performance of the Pro Clima product. Looks very positive in their literature and surely can't be worse than a poly VCL.

- I would most like use a breather paper rather than a second layer of ply at rafter level.

- Question is, does a 50mm ventilation gap add any 'real' benefit. i.e. will it actually provide extra drying out of timber structure?

- What would you suggest as an alternative?


Tony - Can you please expand on 'pumping problems'? What do you mean by composite roof sheets? SIPS?

Looking at Icynene global website in more detail http://www.icynene.com/fb/document/Detail-ICY-ENG-10-Vented-cathedral-ceiling-US-Icynene-2012-ENG.pdf they seem to recommend a ventilation gap when using a low permeable roofing (which a ply and metal roof will be). I'll drop them an email to ask if there is a better way of creating this void as per the detail.