ASHRAE Journal - August 2019 - 63

COLUMN BUILDING SCIENCES

Felt
ASK
High Perm Plastic WRB
Low Perm Plastic WRB
Permeance "Sweet Spot"
Wet Cup

Penetrating Water

100
90
80
70
60
50
40
30
20
10
0

Dry Cup

Sheathing

Figure 4 "Sweet Spot" for Vapor Permeance. With a very small air gap (less than
3/16 inch), the "sweet spot" for the permeance of "the layer" is between 10 and
20 perms. Too high and the moisture driven out of the back side of the reservoir
cladding into the air space will blow through the layer and through the gypsum
sheathing and into the wall cavity where it can cause problems. Too low, and
we reduce the outward drying of the wall cavity through the gypsum board and
through "the layer" (WRB) into the air space during the heating season.
Water Vapor Permeance (U.S. Perms)

Figure 3 Hygric Redistribution Adjacent the Surface of a Material. Penetrating
rainwater hits the sheathing, and is not easily absorbed and is not able to be redistributed within the material or passed inward or outward easily. Redistribution can
occur within an air film adjacent the surface of the material when a small air gap
is provided. How small a gap? Ah, that's the rub, eh? Pretty small, so small that
"crinkled house wrap has a significant effect, as do small "bumps" or even ribbons
of adhesive (think commercial water-managed EIFS).

0
10
20
30
40
50
60
70
80
90 100
your water control layer has a high vapor permeance
Mean
Relative
Humidity
(%)
and to allow sheathings that get wet and that are limited
in drying inwards (such as oriented strand board [OSB])
inch gap is very conservative, we have seen pretty
to dry outwards (Figure 3).
impressive lateral redistribution in air spaces half
There are two reasonable ways to deal with inward
as wide# under controlled environmental exposure
vapor drive - the ﬁrst is to have a "vapor throttle" and
the second is to have an air gap. With a very small air gap (Photo 4). Note the previous discussion on high perm
water control layers.
(less than 3/16 inch) there is a "sweet spot" (Figure 4) for
Recall earlier I noted that we now have lots and lots of
the vapor transmission control characteristics of water
options because with good drainage pretty much everycontrol layers...somewhere between 10 and 20 perms-
thing works. So what do these options look like?
the "vapor throttle." Again, yes, we were here before
Most common are "bumpy" house wraps (Photo 5). The
("Inward Drive-Outward Drying," ASHRAE Journal, June
"bumps" create a drainage gap that provide a drainage
2017). Now comes the neat part, the larger the air gap
efﬁciency of greater than 90%, as measured in accorthe bigger the "sweet spot;" more than 3/16 inch the
sweet spot grows to 10 to 30 perms or higher. If you want dance with ASTM E2273 or Annex A2 of ASTM E2925.
a higher vapor perm
Photo 4 (Left) Hygric Redistribution. Test hut experiment measuring hygric redistribution under various air spaces, cladding types,
water control layer,
sheathing types and thermal resistance levels. In this image the air gap is 3/8 inch coupled with fibercement siding, a high perm
you need a bigger gap
house wrap and OSB sheathing. Photo 5 (Right) "Bumpy" house wraps. The "bumps" create a drainage gap that provide a drainif you are dealing with
age efficiency of greater than 90%, as measured in accordance with ASTM E2273 or Annex A2 of ASTM E2925. The "bumps" also
provide for lateral and vertical hygric redistribution.
reservoir claddings and
inward vapor drive.
With bigger gaps pretty
much everything works
vapor transmission
wise.
In terms of hygric
redistribution a 3/16
#The

"Vancouver test hut"
decade long experiment.
"Parthenon, Eh!", ASHRAE
Journal, March 2012.
A U G U S T 2 0 19

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ASHRAE Journal - August 2019

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