I'm looking to test the performance of different oriented strand boards (OSB) within a wall assembly, using WUFI Pro.
Liquid transport coefficients for these boards have not been measured: the only available information is dry-cup and wet-cup measurements for the water vapour diffusion resistance factor (μ). As far as I understand, accounting for surface diffusion by using a moisture-dependent μ (in lieu of liquid transport coefficients) is an acceptable simplification, as long as there is no significant temperature gradient in the material.
How can the dry cup & wet cup μ measurements be translated to a WUFI material? Should I simply use the dry cup μ for the basic value (0% RH) and the wet cup μ for 100% RH? Or should I add more intermediate points? (I'm aware that the increase in μ is unlikely to be linear)
The existing OSB materials in the WUFI database (which I intend to use as a base for editing) have a constant μ. Is this because the surface diffusion effect is already included in the liquid transport coefficients? If so, should I delete the liquid transport coefficients as soon as I add a moisture-dependent μ?
Many thanks for your help.
Applying dry cup and wet cup measurements to a WUFI material
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Daniel
- WUFI SupportTeam IBP
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- Joined: Tue Jul 19, 2005 2:50 am -1100
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Re: Applying dry cup and wet cup measurements to a WUFI mate
Dear Benat,
first point is correct: for thinner boards it is quite acceptable to use variable µ-value instead of liquid transport as far as you don't use a material which absorbs a lot and often i in contact with water.
Best solution for the curve is to measure different resistances at different RH values. Especially in those cases where wet cup and dry cup differ significantly.
As simplification you can also use average values in the test specimen at test conditions (normally 3 % RH at the dry side and 50 % RH in the climate chamber - means 26,5 % RH in average in the specimen for dry cup / and wet cup: 50 % - 97 %, average 73,5). Here you would use a curve with a constant value from 0 to 26,5 %, than a linear decrease to 73,5 % and then constant again to 100 % (to be on the safe side with the values above and below the measurement range)
But this remains a simplification. As bigger the differences between the two points are, as more it becomes important to do both: additional µ values at furchter RH steps and also refine the curve considering the not linear moisture distribution within the material (especially important for membranes with high spread e.g. sd 0,5 m in wet status and 25 m in dry status).
Concerning the adaption of the existing OSB. The liquid transport coefficients contain both: transport at sorption moisture conditions and in contact with water. It's not so easy to separate the two points. So best in your case may be deleting the liquid transport and use variable µ - but please be aware that you get wron results if you have the OSB in contact with rain water for example.
best regards
Daniel
first point is correct: for thinner boards it is quite acceptable to use variable µ-value instead of liquid transport as far as you don't use a material which absorbs a lot and often i in contact with water.
Best solution for the curve is to measure different resistances at different RH values. Especially in those cases where wet cup and dry cup differ significantly.
As simplification you can also use average values in the test specimen at test conditions (normally 3 % RH at the dry side and 50 % RH in the climate chamber - means 26,5 % RH in average in the specimen for dry cup / and wet cup: 50 % - 97 %, average 73,5). Here you would use a curve with a constant value from 0 to 26,5 %, than a linear decrease to 73,5 % and then constant again to 100 % (to be on the safe side with the values above and below the measurement range)
But this remains a simplification. As bigger the differences between the two points are, as more it becomes important to do both: additional µ values at furchter RH steps and also refine the curve considering the not linear moisture distribution within the material (especially important for membranes with high spread e.g. sd 0,5 m in wet status and 25 m in dry status).
Concerning the adaption of the existing OSB. The liquid transport coefficients contain both: transport at sorption moisture conditions and in contact with water. It's not so easy to separate the two points. So best in your case may be deleting the liquid transport and use variable µ - but please be aware that you get wron results if you have the OSB in contact with rain water for example.
best regards
Daniel
Dr.-Ing. Daniel Zirkelbach, Deputy Head of Department Hygrothermics, IBP Holzkirchen
Re: Applying dry cup and wet cup measurements to a WUFI mate
Many thanks Daniel.