Thanks for the email Jean, I can not immagine why you felt that you had to send me this and not put forward your point of view on this thread.
Getting back to the subject:
If Tom is going to construct this Pod using a typical timber frame construction system then it is a fair assumption to assume that the Sarking Board used would be either OSB or Ply, both of which in their off the shelf state are impervious to the passage of air. Of course they will not completely eliminate all air movement through them but Tom is building a Summer House and not a CAT 5 Laboratory.
Install both of these materials into an air permeability test rig and measure the air permeability rate through both and then cover them in a plastic sheet and re run the tests (plastic sheet = VCL I am sure that you understand that). Do you think that you will be able to measure much difference in respect to air permeability? The VCL is just a visual safety net for those who have trouble building to a high quality.
We have just finished a warm roof constructed vaulted ceiling with insulation between and on top of the rafters. The insulation in question is a typical hard foam board with a low emissivity coating
(again you should really know what I talking about). Is there any vapour/air movement through this type of insulation?
On the surface of it and consulting the insulation manufacturer’s data sheet then a resounding NO can be heard across the country, but hang on how about the detailing and tricky little interfaces (gaps and holes to the un educated).
It is not the product per say but it is how it is installed that makes it work or not! That was the point that I was making but you seemed to take offense to.
Getting back to the warm roof, which has 120mm of said type insulation as previously described, on top of this I have had to install (according to building regs) a very expensive breather membrane. Perhaps you can enlighten us all Jean with your superior knowledge of construction exactly what this breather membrane does?
We are both guilty of not asking Tom more questions about exactly how he proposed to construct his summer house and then advised him accordingly having first collected this information! In my defence my reply was more aimed towards people who would default to the standard VCL/Breather membrane approach to timber frame construction without actually thinking or considering WHY!
Just because someone has been involved in construction for a number of years does not mean that their advice is correct or should be given, is anyone that good that they can sum up an entire construction methodology in one sentence?
I stand by my reasoning that we would need more information from Tom before any advice were to be given.