The vapor diffusion factors for plywood differ ten times.
That of Fraunhofer-IBP is 20(@RH of 1.0) vs that of LTH Lund University is 2.63(@RH of 1.0)
Which one should I use?
Vapor diffusion factor for plywood
Re: Vapor diffusion factor for plywood
Dear drnamba,
'plywood' is a very generic name for a large number of different versions of wood boards. Their properties may be different in different countries, for different manufacturers, or even between different production runs of the same manufacturer. (The same applies to most other building material properties which are not regulated by building codes or strictly controlled for quality reasons.)
If you know which kind of plywood is generally used in your region, you should use the one which is closer to that, or you may even create your own set of material data based on your local kind of plywood.
If you don't know, do a test calculation with each of them and see if it makes a difference in the results. If there is no difference or only a small difference which is negligible for your purposes, then the construction you are investigating is insensitive with respect to this property and it doesn't matter which one you are using.
If it turns out that the results are significantly affected by the choice of the plywood, there is no way around doing the research and determining the precise material data of the plywood that has been used in the construction you are investigating. These data may even be different from all the various plywoods provided in the database.
If you are doing design work for a type of construction that will be built in future and you have determined that the construction is sensitive with respect to the mu-value of the plywood (or some other property), you must make sure that the construction is built only with the right kind of plywood, or you may change the construction design in order to avoid this sensitivity which entails costly material selection and quality control.
An integral part of doing a computational simulation as well as designing a real-life construction is to get a feel for what is important and what is not. WUFI can help you to find out: do test calculations while varying the properties and see how much the variation affects the results.
Kind regards,
Thomas
'plywood' is a very generic name for a large number of different versions of wood boards. Their properties may be different in different countries, for different manufacturers, or even between different production runs of the same manufacturer. (The same applies to most other building material properties which are not regulated by building codes or strictly controlled for quality reasons.)
If you know which kind of plywood is generally used in your region, you should use the one which is closer to that, or you may even create your own set of material data based on your local kind of plywood.
If you don't know, do a test calculation with each of them and see if it makes a difference in the results. If there is no difference or only a small difference which is negligible for your purposes, then the construction you are investigating is insensitive with respect to this property and it doesn't matter which one you are using.
If it turns out that the results are significantly affected by the choice of the plywood, there is no way around doing the research and determining the precise material data of the plywood that has been used in the construction you are investigating. These data may even be different from all the various plywoods provided in the database.
If you are doing design work for a type of construction that will be built in future and you have determined that the construction is sensitive with respect to the mu-value of the plywood (or some other property), you must make sure that the construction is built only with the right kind of plywood, or you may change the construction design in order to avoid this sensitivity which entails costly material selection and quality control.
An integral part of doing a computational simulation as well as designing a real-life construction is to get a feel for what is important and what is not. WUFI can help you to find out: do test calculations while varying the properties and see how much the variation affects the results.
Kind regards,
Thomas
Vapor diffusion factor for plywood
Dear Thomas:
Thanks for the reply. I am studying the precipitation possibility of open cell spray urethan foam (icynene).
If you would spray against the high diffusion factor plywood without vapor barrier room side, it would precipitates during the winter. Whereas if you use the low one, it seems to be OK.
Because it is difficult to obtain the diffusion factor value for the specific plywood, I think it would be safer to use vapor barrier.
Regards
Nam
Thanks for the reply. I am studying the precipitation possibility of open cell spray urethan foam (icynene).
If you would spray against the high diffusion factor plywood without vapor barrier room side, it would precipitates during the winter. Whereas if you use the low one, it seems to be OK.
Because it is difficult to obtain the diffusion factor value for the specific plywood, I think it would be safer to use vapor barrier.
Regards
Nam