Calculating Sd values

[Keith Rankine]

Could someone help me with a problem I have with WUFI. It's not the software itself but trying to model a vapour barrier (Thermakraft Vaporshield 101). It has been some years since I studied any sort of maths.
The information I have for the product to be used is a resistance to water vapour transmission of 308MN.s/g and a thickness of 103 micrometres.
I have calculated the Sd value using the following

(308 x 0.2) / 0.000103 = 598,058

then

598,058 x 0.000103 = Sd 61.59

The 0.2 is the typical value for the vapour permeability of still air in the UK. NZ may be different. Does anyone know?

Can you see any flaws in my calculations? I propose to use the Sd of 61 within my model.

Thanks in advance for any help you can give.

[Thomas]

Hi Keith,

you probably took that procedure from the PDF document
http://builddesk.co.uk/wp-content/uploa ... tances.pdf
and it looks okay.

Please note that you first divide and then multiply by 0.000103; these two steps cancel and it is sufficient if you simply compute

308 MN.s/g x 0.2 g.m/MN.s = 61.6 m

The 0.2 is the typical value for the vapour permeability of still air in the UK. NZ may be different.

The vapour permeability of still air slightly depends on the air temperature and pressure. For example:

at 0°C and 1013.25 hPa we have 1.9e-10 kg/m.s.Pa = 0.19 g.m/MN.s
at 20°C and 1013.25 hPa we have 2.0e-10 kg/m.s.Pa = 0.20 g.m/MN.s

So for a precise conversion you'd need to know the precise temperature and barometric pressure. But since the vapour barrier will be exposed to a variety of temperatures and pressures, all you can do is to use a sufficiently typical value. It is thus not so much a question of UK air versus NZ air, it is more a question of what the typical conditions for the vapour barrier will be in the building structure.

On page 4 the PDF document states:

"This value of 0.2gm/MNs is used by convention in the UK. Outside the
UK, local values for atmospheric temperature and pressure can be used
to calculate the vapour permeability of still air from basic principles."

But since 0.2 is a nice round number, and you probably have no specific reason to use something different, you should simply join the convention and stick to this number.

Kind regards,
Thomas

[Keith Rankine]

Thanks Thomas.