A few questions about vapour permeability etc

[MikeSharp01]

5 hours ago, A_L said:

Hello @MikeSharp01, sorry for the slow reply, went out after replying to Crofter, came back late and was too tired to assemble this reply, unfortunately things are now going to get a little complex and perhaps a little indecisive.

Many thanks @A_L that looks very interesting, will download the software and have a play. Not sure how you could make the VCL fail safe, I suppose its the structure that has to fail safe not just one layer - and a very thin layer at that. The proposed VCL is this one: https://novia.co.uk/uploads/components/variation/Novia VC2 Jan 2016-57bc65ed49d64.pdf which is made by a company local to us in Kent. I am not sure I can see a conversion from the units quoted to the Mn.s/g but perhaps a power of 10 analysis with sort it. I guess we will have to avoid foil covered PU sheets as well because I suspect the vapour resistance of the foil is very high also. Taking your observation aboard means making sure that the VCL is properly installed to ensure it, the layer, has the maximum resistance and is secure and protected, this also ties up with the feelings @JSHarris sets out above about making sure its a gradient with the highest on the inside.

[MikeSharp01]

Sorry @A_LI have had a look and can see that the Novia has a water vapour permeability sd of 3m. The calc sheet says to get from sd to Mn.s/g just divide the sd value by the Vapour permeability of still air (0.2 gm/MNs) and you 15, which seems like a long way from 300 and already well below the 40 you indicate might be a cause for concern. It is late and I have had half a bottle of wine so maybe I am missing something? Even the Intello product only makes it to 37.5 Mn.s/g  - Polythene will be cheaper than the Novia or Intello anyway!

 

Edited September 18, 2017 by MikeSharp01
Typo

[A_L]

If it is possible for you to move the OSB to the warmside of the I-beams (as @ProDave has done) you would not even need a VCL (I would probably still fit one anyway) This corresponds with the approach suggested by @JSHarris

 

[ProDave]

17 minutes ago, A_L said:

If it is possible for you to move the OSB to the warmside of the I-beams (as @ProDave has done) you would not even need a VCL (I would probably still fit one anyway) T@JSHarris

 

Exactly why our OSB racking layer is on the inside.

 

Be warned, a whole host of passers by will tell you that you have put the frame up inside out.

[joe90]

1 hour ago, ProDave said:

Exactly why our OSB racking layer is on the inside.

 

Be warned, a whole host of passers by will tell you that you have put the frame up inside out.

Yes, I also have used osb inside I beams ( foam glued joints) as the vapour " control" layer and racking control.

[Crofter]

9 hours ago, A_L said:

If it is possible for you to move the OSB to the warmside of the I-beams (as @ProDave has done) you would not even need a VCL (I would probably still fit one anyway) This corresponds with the approach suggested by @JSHarris

 

 

So the PUR is vapour permeable enough to go on the outside without causing any problems?

[A_L]

1 hour ago, Crofter said:

 

So the PUR is vapour permeable enough to go on the outside without causing any problems?

 

In this case , yes. However this is only so because the OSB is sufficiently vapour resistant to restrict the rate at which vapour passes through to reach the cold side. If the OSB had only been 9mm thick it would not. In other words it is the build-up as a whole which works, not an individual element.