Air retarding – Tyvek Protec 160
dan7210329
| Posted in General Questions on
My roofer wants to install Tyvek Protec 160 over plywood roof deck, before installing a standing seam metal roof. Dupont says this Tyvek product has a permeability of <.05 perms. Does that tell me anything about the air retarding ability of this product? My goal is to learn to what extent this system can block hot humid summer air from condensing in the insulation installed below roof rafters. Assume all edges are taped to maximize air retardance. The house is in mixed humid, Zone 4, Moisture regime 4, near Valley Forge, PA. Assume a second air barrier for winter months, installed on the inner side of insulation.
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Replies
How is your roof constructed? With a vented roof, the roofing underlayment air sealing ability or permability do not matter.
The perm rating of the product is a measure of how much moisture transfer it allows and has nothing to do with its ability to limit air flow.
For an un-vented roof, generally it is better to have a permeable (greater than 1 perm) underlayment to allow for a bit of drying of the roof deck. It is not a requirement, but helps.
As for air sealing on the un-vented assembly, it is always better to tape the seams of the roof sheathing. I wouldn't rely on the underlayment for air sealing as it can get easily damaged by the roofing installers.