Page 22 - GB Building Science Ebook 2019

03 WEATHER BARRIERS, OLD AND NEW

Heavy duty. Spray on water resistant
barriers such as the DuPont Tyvek Fluid
Applied WB go on quickly, but care must be
taken to prep the envelope.

rapidly, either by spray or roller.
With this technology, however, there’s little room for error. First,
the structure of the building must be carefully detailed, avoiding Among building wrap’s
large cracks or holes. Any such holes must be caulked and filled prior
to application of the liquid weather barrier. Another option is to use advantages: it’s significantly
a sealing system such as AeroBarrier, to guarantee that no hidden
gaps will be left unsealed. lower in cost than premium
In addition, you can’t just slap on a coat of sealant. It has to be
applied at a specific thickness—typically about 20-25 mils. It’s a job systems, goes on even when
best left to experienced crews.
Under the right conditions, however, with an expert crew, liquid
sealants offer a streamlined process. They dry in about 24 hours, weather conditions are not
require no staples or tape, and act as both a bulk water barrier and,
in the case of DuPont Tyvek Fluid Applied Weather Barrier, allows perfect, and comes in
moisture vapor (but not bulk water) to pass through, so that walls
can dry to the outside. enormous rolls.

HOUSE WRAPS: FOOL RESISTANT
It’s not by accident that building wraps have held an important role in
the construction process for decades. When we say “fool resistant,” we WHAT’S THE BEST CHOICE?
mean that it’s a product that forgives minor variations in installation, At the risking of dodging the question, the answer for builders
yet still performs. A natural evolution of a backup system that began is dependent on the type of project, the timeframe required for
with paper and tar impregnated wraps, modern materials such as completion, and the experience of available labor.
DuPont Tyvek’s HomeWrap fuse synthetic technology with a familiar You notice that what’s not included in that list is the important
installation process. question of whether the home will achieve zero net energy
Among building wrap’s advantages: it’s significantly lower in cost performance.
that premium systems, goes on even when weather conditions are That’s because ANY of the weather barrier and insulating systems
not perfect, and comes in enormous rolls. Building science suggests we’ve discussed can be applied to achieve the necessary 2 ACH for
that taping seams is important to optimize reduction of unwanted net zero. The key is to combine systems in a way that makes sense,
air inside walls, but because the wrap is overlapped, it will naturally and doesn’t assume perfection.
repel and carry bulk water to the ground, even if the seams are not That way, you’ll always cover your tracks with backup methodology.
sealed. For example, if spray foaming walls with open cell foam, you may
Products such as HomeWrap are designed to allow water vapor to be tempted to leave building wrap seams untaped. But spray foam
evaporate out from walls, but keep out rain. As noted above, however, typically won’t fill every gap, such as between sill plates, so you may
Tyvek now offers what is essentially a third line of defense for extreme want that extra air sealing factor.
weather conditions. They recommend installing WeatherSmart Approach the question from the perspective of net zero
drainage over Tyvek in storm-threatened areas. performance and the various options will become clear. GB

22 BUILDING SCIENCE SPECIAL REPORT: SIX WAYS CONSTRUCTION IS CHANGING