What Is A Weather Resistant Barrier (Ohhhh…That’s What Tyvek is)?

by Erik North on October 12, 2011

What Is A Weather Resistant Barrier


There’s a great deal of functional overlap between materials that act as an air barrier, vapor barrier and as a weather resistant barrier.

We’ve covered air barriers and vapor barriers in previous posts, but what is a weather resistant barrier? And what’s with the rest of the layers and barriers of your building enclosure … why aren’t they ‘resisting weather’? Are they just lazy?

What Is A Weather Resistant Barrier?

Weather resistant barriers or water resistant barriers (WRB) come in several forms. The most common being #15 building felt, adhering or sprayed membranes or polyolefin building wrap like Tyvek. The purpose of these materials is preventing driving rain from entering the building shell/actual building. Rain is the main source of water most buildings deal with.

In Maine, a rainy storm whipping off the Atlantic can drive rain water into your building sheathing. Water gets behind the siding, finds every crack and seam, possibly entering and accumulating in the wall cavity (yay). This can damage framing elements and soak the insulation. Fiberglass and cellulose insulation which has been saturated with water loses most of its R-value. Water attempts to enter the exterior, driven by storm winds, pulled by gravity or impelled by capillary action and pressure differences. If only there were barriers, weather resistant barrier to help.

And Why Is A Weather Resistant Barrier Important?

This is where WRBs come in. WRBs are materials engineered to stop water penetration, preventing rain from entering sheathing and behind the cladding, with a space allowance for drainage. When properly installed with detailed flashing, a weather resistant barrier prevents any rain water from entering the building enclosure. If the building is using an exterior insulation system (with XPS or polyiso sheathing), the WRB is most often installed between the sheathing and the exterior insulation.

A Weather Resistant Barriers Prevent What Problems?

What if a bit of rain water gets into the walls? Or a lot? Or maybe you host a few extra family members for the summer, meaning 5 or 6 showers a day and your house is dealing with a bunch more internal humidity? Weather resistant barriers allow the wall assemblies to naturally dry out.

Important Note

Important Note!

Some folks hear “keep water out” and their thoughts leap to plastic. A very impermeable plastic layer. A sheet of polyethylene plastic would absolutely keep moisture out but it would also keep moisture in. Trapping moisture in the building enclosure can lead to rot and mold issues. WRBs stop storm rain water but are vapor permeable, allowing the building to dry.

A Weather Resistant Barrier Prevents What Problems?

One of the most difficult problems builders have had to address over time has been how to keep water out of the building but allow it to dry out during incidental moisture events. This past month, Hurricane Irene pounded New England. Maine got off relatively lightly (New Hampshire and Vermont decidedly less so) but we still had 4 or 5 inches of driving rain, at times sheeting horizontally against our house. Under unusual conditions, water can and will get into your house. The high vapor permeability of weather resistant barriers allows the building to dry out after these out of the ordinary happenings.

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