ASHRAE Journal - February 2013 - 56

building sciences

Figure 1 (left): Water Entry at Window Opening. It is
important to address water running behind building
wraps and housewraps at window heads and the sides of
window openings. Figure 2 (right): Control of Water at
Window Head. The top window flange is taped directly to
the sheathing with “flashing tape.” Note how the building
wrap or housewrap is cut and folded upwards to expose
the sheathing so that the “flashing tape” can be installed.
Once the tape is installed, the flap is folded back down.
This works exceptionally well.

Stuck on You
By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

O

ne of the dirty little secrets that never gets talked about is that
water leaks through building papers, building wraps and house-

wraps and runs down between them and the sheathings that they cover.
The water enters through nail holes and staple holes and sometimes
just through the field of the material if you have a poor product.* The
good news is that the nail holes and staple holes don’t leak too much,
and we know how to handle the incidental leakage.

In the field of the wall we don’t care if
things get a little wet as long as they can
dry. We promote drying to the outside by
leaving an air gap between the cladding
and the building wrap or housewrap or
water resistive barrier (wrb) or whatever

we are calling this “thingy” this month.
I like to call it the water control layer.
Whatever. Anyway, the gap does not have
to be very large: 0.25 in. to 0.375 in. (6
mm to 10 mm) works just fine. This gap
also controls hydrostatic pressure.†

At penetrations such as windows
and doors (Figure 1) we have learned
to scrape the water off the sheathing at
the tops of the windows by taping the
top flange of the window directly to the
sheathing under the building wrap (Figure 2). This is a big deal. If we didn’t do
this, the water running behind the building wrap would wet the top of the window
opening interior finish, and folks would
see it. We don’t care if this water leaks
through a joint in the sheathing in the field
of the wall because the wet spot isn’t seen
by anyone. And if the wall is designed to
dry either to the interior or exterior or
both it does not matter anyway.‡ Recall,
this amount of water is small.
At the sides of windows we use a “bedding joint.” I love this term. A heavy bead
of sealant is installed behind the flange
of a window, and it acts like a “gasket”
smushing the building wrap against the
sheathing, limiting the water that can run
sideways into the opening (Figure 3).
And all of this works. And it works
well. Building wraps and housewraps

*We looked at some of the material properties of these products in “Leaks & Holes,” ASHRAE Journal, December 2012. What bemuses me is that of all of the
myriad of material tests done to qualify these products, such as resistance to hydrostatic head, the entire point is missed in that the weak spot is the nail hole. If
the tests made sense you would test the materials with nail holes and staple holes—a true system test.
†We looked at this in “Hockey Pucks & Hydrostatic Pressure,” ASHRAE Journal, January 2012.
‡People seem to live in this fantasy world where they believe that walls don’t leak. Almost all of them do. But it doesn’t matter if they dry before any damage
occurs. This balance between wetting and drying is a big deal and it is getting more complicated every day as we reduce energy flows across assemblies thereby
reducing drying potentials. Having said that, we still have a lot of capacity and robustness left in most assemblies if we don’t do stupid things like install vinyl
wall coverings and plastic vapor barriers in climates that don’t need them or assemblies that don’t need them.

56

ASHRAE Journal

ashrae.org

February 2013



ASHRAE Journal - February 2013

Table of Contents for the Digital Edition of ASHRAE Journal - February 2013

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
R-22 Hard Act to Follow: Ammonia Low-Pressure Receiver Systems
Long-Term Commercial GSHP Performance: Part 7: Achieving Quality
Thermally Active Floors: Part 2: Design
Future of DCV for Commercial Kitchens
Standing Columns and Special Sections
Building Sciences
Emerging Technologies
ACREX India 2013 Show Guide
Refrigeration Applications
InfoCenter
Data Centers
IAQ Applications
Special Products
Classified Advertising
Advertisers Index
ASHRAE Journal - February 2013 - Intro
ASHRAE Journal - February 2013 - Cover1
ASHRAE Journal - February 2013 - Cover2
ASHRAE Journal - February 2013 - 1
ASHRAE Journal - February 2013 - 2
ASHRAE Journal - February 2013 - Contents
ASHRAE Journal - February 2013 - Commentary
ASHRAE Journal - February 2013 - 5
ASHRAE Journal - February 2013 - Industry News
ASHRAE Journal - February 2013 - 7
ASHRAE Journal - February 2013 - 8
ASHRAE Journal - February 2013 - 9
ASHRAE Journal - February 2013 - 10
ASHRAE Journal - February 2013 - 11
ASHRAE Journal - February 2013 - Letters
ASHRAE Journal - February 2013 - 13
ASHRAE Journal - February 2013 - Meetings and Shows
ASHRAE Journal - February 2013 - 15
ASHRAE Journal - February 2013 - R-22 Hard Act to Follow: Ammonia Low-Pressure Receiver Systems
ASHRAE Journal - February 2013 - 17
ASHRAE Journal - February 2013 - 18
ASHRAE Journal - February 2013 - 19
ASHRAE Journal - February 2013 - 20
ASHRAE Journal - February 2013 - 21
ASHRAE Journal - February 2013 - 22
ASHRAE Journal - February 2013 - 23
ASHRAE Journal - February 2013 - 24
ASHRAE Journal - February 2013 - 25
ASHRAE Journal - February 2013 - Long-Term Commercial GSHP Performance: Part 7: Achieving Quality
ASHRAE Journal - February 2013 - 27
ASHRAE Journal - February 2013 - 28
ASHRAE Journal - February 2013 - 29
ASHRAE Journal - February 2013 - 30
ASHRAE Journal - February 2013 - 31
ASHRAE Journal - February 2013 - 32
ASHRAE Journal - February 2013 - 33
ASHRAE Journal - February 2013 - 34
ASHRAE Journal - February 2013 - 35
ASHRAE Journal - February 2013 - Thermally Active Floors: Part 2: Design
ASHRAE Journal - February 2013 - 37
ASHRAE Journal - February 2013 - 38
ASHRAE Journal - February 2013 - 39
ASHRAE Journal - February 2013 - 40
ASHRAE Journal - February 2013 - 41
ASHRAE Journal - February 2013 - 42
ASHRAE Journal - February 2013 - 43
ASHRAE Journal - February 2013 - 44
ASHRAE Journal - February 2013 - 45
ASHRAE Journal - February 2013 - 46
ASHRAE Journal - February 2013 - 47
ASHRAE Journal - February 2013 - Future of DCV for Commercial Kitchens
ASHRAE Journal - February 2013 - 49
ASHRAE Journal - February 2013 - 50
ASHRAE Journal - February 2013 - 51
ASHRAE Journal - February 2013 - 52
ASHRAE Journal - February 2013 - 53
ASHRAE Journal - February 2013 - 54
ASHRAE Journal - February 2013 - 55
ASHRAE Journal - February 2013 - Building Sciences
ASHRAE Journal - February 2013 - 57
ASHRAE Journal - February 2013 - 58
ASHRAE Journal - February 2013 - 59
ASHRAE Journal - February 2013 - 60
ASHRAE Journal - February 2013 - 61
ASHRAE Journal - February 2013 - 62
ASHRAE Journal - February 2013 - Emerging Technologies
ASHRAE Journal - February 2013 - 64
ASHRAE Journal - February 2013 - ACREX India 2013 Show Guide
ASHRAE Journal - February 2013 - 64b
ASHRAE Journal - February 2013 - S1
ASHRAE Journal - February 2013 - S2
ASHRAE Journal - February 2013 - S3
ASHRAE Journal - February 2013 - S4
ASHRAE Journal - February 2013 - S5
ASHRAE Journal - February 2013 - S6
ASHRAE Journal - February 2013 - S7
ASHRAE Journal - February 2013 - S8
ASHRAE Journal - February 2013 - S9
ASHRAE Journal - February 2013 - S10
ASHRAE Journal - February 2013 - S11
ASHRAE Journal - February 2013 - S12
ASHRAE Journal - February 2013 - S13
ASHRAE Journal - February 2013 - S14
ASHRAE Journal - February 2013 - S15
ASHRAE Journal - February 2013 - S16
ASHRAE Journal - February 2013 - S17
ASHRAE Journal - February 2013 - S18
ASHRAE Journal - February 2013 - S19
ASHRAE Journal - February 2013 - S20
ASHRAE Journal - February 2013 - S21
ASHRAE Journal - February 2013 - S22
ASHRAE Journal - February 2013 - Refrigeration Applications
ASHRAE Journal - February 2013 - InfoCenter
ASHRAE Journal - February 2013 - 67
ASHRAE Journal - February 2013 - 68
ASHRAE Journal - February 2013 - 69
ASHRAE Journal - February 2013 - 70
ASHRAE Journal - February 2013 - 71
ASHRAE Journal - February 2013 - Data Centers
ASHRAE Journal - February 2013 - 73
ASHRAE Journal - February 2013 - 74
ASHRAE Journal - February 2013 - IAQ Applications
ASHRAE Journal - February 2013 - 76
ASHRAE Journal - February 2013 - 77
ASHRAE Journal - February 2013 - Special Products
ASHRAE Journal - February 2013 - Classified Advertising
ASHRAE Journal - February 2013 - Advertisers Index
ASHRAE Journal - February 2013 - Cover3
ASHRAE Journal - February 2013 - Cover4
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