New Construction: Upgrading House Wrap in the Rainy Southeast?

2 years ago

last modified: 2 years ago

Hi everyone,

I’m wondering if there would be a significant advantage in asking our builder to upgrade the standard house wrap that he uses and completely cover the increased cost ourselves. From some basic research online, it seems like his standard choice is lower-quality and cheaper than other house wraps on the market.

Here are the details for what he uses by default:

Manufacturer: Alpha Pro Tech
Product: REX™ Wrap Housewrap
Distributor: Builders FirstSource
ESR Report: ESR-1602 (link: https://icc-es.org/report-listing/esr-1602/)
Composition: Perforated Microporous Woven Polyolefin Fabric with a Polyolefin Coating
Moisture vapor transmission rate [MVTR]: 8 perms
Thickness: 0.0055”
Breaking strength ASTM D882 (lbs/in): 45/51

Our house will be a 3,200 sq. ft, 3-storey “drive under” in coastal Charleston, SC. The siding will be HardiePlank. This is a very humid environment especially in the summer that gets a lot of rain and occasional hurricanes. Winters are dry and mild.

I understand that sheathing wraps have at least two purposes:

Provide an effective barrier to water penetration in the first place (i.e., a “raincoat” for the house under the cladding to shed water)
Allow the sheathing to dry as quickly as possible should it end up getting wet anyway

The wrap’s permeance to water vapor is an important point to consider, although I’m having a hard time understanding if “only” 8 perms is favorable or unfavorable in this climate. It seems low compared to Tyvek house wrap which can range up to 56 perms. Also, building codes require a minimum allowable level of 5 perms, so my builder’s choice is pretty close to that. I know a higher perm rating essentially speeds the escape of trapped moisture, but does the climate in this area affect the calculus?

Other things beside permeance that I want to understand are the impact of woven vs. nonwoven as well as perforated vs. nonperforated material. Based on my research, it seems that a nonwoven, non-perforated, vapor-permeable house wrap is what you’d want in the rainy Southeast. My builder’s house wrap is literally the opposite of these things, so I think it would be a reasonable upgrade for my future house in this climate.

Looking for any opinions, validation, or pushback here. Thanks for reading my novel and weighing in with your expertise!

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New Construction: Upgrading House Wrap in the Rainy Southeast?

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