It took me a while to figure out that a building is basically just a structural framework with control layers. Yeah, there's also electrical, plumbing, mechanical systems, and finishes, but the heart of a building is the stuff that holds it up and that moderates the flows of heat, air, and moisture between in and out.

One of those control layers is the air barrier. Of course, everyone knows now that a house can never be too tight (right?), so we want the air barrier to stop as much air flow across the building envelope as possible.

Many people believe that by putting house wrap on a building, you've got your air barrier taken care of. Although house wrap can qualify as an air barrier material, it usually doesn't behave as an air barrier when installed. Here's why:

  • All seams must be sealed. Builders usually do a decent job here. After installing the layers of housewrap, they go back and tape the seams.
  • Top and bottom edges must be sealed. This rarely happens, and there's a good reason why you shouldn't seal the bottom edge - You'll trap water inside when it gets behind the house wrap and can't drain out the bottom.
  • All the edges where it's cut at rough openings must be sealed. Doesn't happen. Also, not a good idea in places where water needs to be able to drain out.
  • Cladding contractors cut it with their box cutters to get it to lie flat. When house wrap goes through an inside corner, it often 'cuts the corner,' making it impossible to install siding without slitting it.
  • Innumerable tears, rips, and cuts allow air to move across it. They rarely get sealed before being covered with the cladding.

Because there are so many places that air can move across the house wrap, it doesn't really perform as an air barrier when installed. There is one type of air leakage that it does help reduce, however - infiltration, air moving into the house (as opposed to exfiltration, air moving out of the house).

When the air pressure outside the house is higher than the pressure inside, air wants to move to the inside. This pulls house wrap into the seams, gaps, and penetrations and inhibits the flow of air from outside to inside (infiltration). If the house pressure is higher than the outside pressure, on the other hand, the house wrap pushes away from the holes and air moves out of the house (exfiltration).

So, if house wrap isn't an air barrier, why do we use it?

House wrap's best use is to act as the drainage plane on a building. This control layer keeps liquid water that gets behind the cladding from wetting the building materials. Tar paper (also called building paper or felt) is the traditional material used for this application, but house wrap has is the dominant choice these days. It's kind of like Goretex for the house. It stops liquid water from going through, but it allows water vapor to pass so when the building materials do get wet, they can dry through the house wrap.

The more general term house wrap is weather resistive barrier (WRB), which implies that it's there to keep the weather out. It does that for rain that gets behind the cladding, but it won't resist air leakage due to wind, since wind puts a positive pressure on a house.

Bottom line: Don't think of house wrap as an air barrier. It's there to be the drainage plane.

 

This article has been cross-posted from the Energy Vanguard blog.

Views: 517

Tags: air, barrier, house, resistive, weather, wrap

Comment

You need to be a member of Home Energy Pros to add comments!

Join Home Energy Pros

Home Energy Pros

Home Energy Pros was founded by the developers of Home Energy Saver Pro (sponsored by the U.S. Department of Energy,) and brought to you in partnership with Home Energy magazine.

Latest Activity

Trip Smith joined Hal Skinner's group
Thumbnail

The RCC Classroom (Radiant Control Coatings)

A group where energy professionals can learn about all the different energy saving aspects of…See More
28 minutes ago
Trip Smith commented on Diane Chojnowski's group Facebook Pages
"Just went through the first few pages liking the active Facebook pages. I've found that…"
30 minutes ago
Trip Smith joined Diane Chojnowski's group
Thumbnail

Facebook Pages

Does your company or organization have a Facebook Page?This group is for pros who have facebook…See More
52 minutes ago
Profile IconVictor Couto, Melvin Calahan, Mike Burton and 1 more joined Home Energy Pros
1 hour ago
Steven Lewis replied to Christopher Talarico's discussion Heating with Tankless Water Heater & Hydronic Air Handler vs. Gas Furnace
"It looks better on paper than in real life applications.  Both Amana and Lennox came out with…"
18 hours ago
Dennis Heidner replied to Christopher Talarico's discussion Heating with Tankless Water Heater & Hydronic Air Handler vs. Gas Furnace
"Rinnai has application note on how to use their tankless hot water heater with a Rinnai water to…"
yesterday
Hal Skinner replied to Hal Skinner's discussion How much heat energy is lost through the floor of a house?
"I found the buildingscience article interesting.  Interesting that they noted the wet bottom…"
yesterday
Bud Poll replied to Hal Skinner's discussion How much heat energy is lost through the floor of a house?
"With conduction that ends at the bottom of the joists and convection that is naturally suppressed…"
yesterday

© 2014   Created by Lawrence Berkeley National Laboratory.

Badges  |  Report an Issue  |  Terms of Service