The gerneric North American Data Base has the perm in. for SBP at .392. However, Dupont, the maker of Tyvec or SBP, has a rating of about 25 perms. What coversion factor is used to go from 25 to .392. Also, what am I missing.
Thanks,
C. Furtaw
Perm rating of SBP
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C. Furtaw
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Perm rating of SBP
Charles E. Furtaw, P.E.
Re: Perm rating of SBP
Dear Mr. Furtaw,
the perm inches measure the permeability of the material, the perms measure the permeance of a sheet made from that material, having a given thickness.
If you have a sheet of thickness d (in inches), made of a material whose perm inches you know, divide the perm inches by the inches to get the perms of that sheet.
WUFI's material database says that the spun-bonded polyolefine material has a permeability of 0.392 perm inches; Dupont say that their sheets made from that material have a permeance of 25 perms. This implies that their sheets are 0.016 inches thick, because 0.392 perm inches / 0.016 inches = 25 perms.
Another kind of sheets made from the same material, but with half the thickness, would still have a permeability of 0.392 perm inches (because it's the same material) but a permeance of 50 perms (letting pass more vapor because it's thinner).
Regards,
Thomas
the perm inches measure the permeability of the material, the perms measure the permeance of a sheet made from that material, having a given thickness.
If you have a sheet of thickness d (in inches), made of a material whose perm inches you know, divide the perm inches by the inches to get the perms of that sheet.
WUFI's material database says that the spun-bonded polyolefine material has a permeability of 0.392 perm inches; Dupont say that their sheets made from that material have a permeance of 25 perms. This implies that their sheets are 0.016 inches thick, because 0.392 perm inches / 0.016 inches = 25 perms.
Another kind of sheets made from the same material, but with half the thickness, would still have a permeability of 0.392 perm inches (because it's the same material) but a permeance of 50 perms (letting pass more vapor because it's thinner).
Regards,
Thomas
Dear Mr. Furtaw,C. Furtaw wrote:Is there any way that the pressure can be handled or do I have to wait for the 2D version. When will this be available?
the new 2D version which is currently under development will not be able to simulate air flows within the wall. There is a special 2D version for internal experimental use that simulates air flows, but it is not compatible with the other 2D versions and it requires that air speeds be specified as boundary conditions, which are usually not explicitly known. One day, this air flow feature may become part of the regular WUFI 2D, but in any case this will be far in the future.
Regards,
Thomas