ASHRAE Journal - November 2013 - 56

Column Building ScienceS
Joseph W. Lstiburek

'Double, Double Toil and Trouble'

*

Macbeth Does Vapor Barriers
By JoSeph W. lStiBurek, ph.d., p.eng., FelloW AShrAe

Life was simple when I grew up in Canada. Winters were long and cold, we
had no air conditioning, walls dried to both the outside and the inside and
the Toronto Maple Leafs would win Stanley Cups. That all changed when we
started putting plastic on the inside of walls and began to put impermeable
insulating sheathing on the outside. We started to have double vapor barriers.
Shakespeare was right in "that Scottish play."† Double vapor barriers meant
"double toil and trouble." With double vapor barriers if a wall got wet it would
not be able to dry to either the outside or the inside.
In the old days assemblies worked
just fine with no vapor barriers
on either side. That approach still
works today, but for reasons that are
difficult for me to understand, it is
not used as often as it should be or
could be. We looked at this in "Joni
Mitchell, Water and Walls," ASHRAE
Journal, August 2013, and we will look
at it again here.
For the record, it makes no sense to
have a vapor barrier on both sides of
an assembly. If the assembly gets wet
or starts out wet we are pretty much

doomed...except if we are perfect
and we start out dry.‡
Also, for the record, it is OK to have
a vapor barrier on the inside of an
assembly and design and construct
the assembly to dry to the outside.
But there are requirements on how
and limits on where.
Finally, for the record, it is OK to
have a vapor barrier on the outside
of an assembly and design and construct the assembly to dry to the
inside. But there are requirements
as to how based on where.

So, back in the day, Figure 1a ruled
Canada and the United States.
It could dry in both directions.
It could tolerate leaky windows
and missing flashing. It could be
rained on and snowed on during
construction. It was awesome. The
only thing wrong with it was that
it did not have a lot of insulation.
In fact, that is what helped it be
awesome. It was poorly insulated
by the standards of today, and
the resulting energy flow helped
it dry when it got wet or when it

*F.B. rowley is credited with first promulgating vapor barriers to prevent condensation in insulated buildings in 1939, not Macbeth. professor rowley got it wrong and things have ended badly for architects
and contractors ever since. Macbeth also got it wrong and things ended badly for him as well.
†Many believe that the play Macbeth is cursed and refuse to mention its title aloud and refer to it as "the Scottish play." Superstition abounds. According to theatrical legend, speaking the name inside a
theatre will result in disaster, requiring a "cleansing ritual." one of the most common is requiring the offender to go outside, turn three times and recite a line from another of Shakespeare's plays. Sometimes the door is locked, and the offender is refused re-entry. i wonder if that might work with some AShrAe committees as they do become pretty theatrical and sometimes folks feel cursed.
‡it has been common for decades to construct compact flat roof assemblies with double vapor barriers with great success. the outer roof membrane is, of course, a vapor barrier. in most cold climate and
many mixed climate regions, we often install an interior air barrier that is also a vapor barrier. one of the most common and successful roof assemblies used is a fluted metal deck with paperless gypsum
sheathing supporting a fully adhered membrane under rigid insulation. the fully adhered membrane under the rigid insulation is typically vapor impermeable. this works because both membranes are for
all intents and purposes "perfect." if we built walls like we build roofs, we could probably get away with double vapor barriers on them as well. But we don't. We have windows and we have architects; both
result in rainwater leakage and air leakage.
56

ASHRAE JouRnAl

ashrae.org

N ovem ber 2013



ASHRAE Journal - November 2013

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

Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Sequencing & Control of Compressors
Fire Station Renovation
Sewage Plant Heats Village
Standing Columns
Engineer’s Notebook
Building Sciences
Special Supplement
BACnet Today & the Smart Grid
The Performance Gap
Refrigeration Applications
InfoCenter
Energy Modeling
Emerging Technologies
Data Centers
People
Products
Special Products
Geothermal for Big Box Retail
Classified Advertising
Advertisers Index
ASHRAE Journal - November 2013 - Intro
ASHRAE Journal - November 2013 - Cover1
ASHRAE Journal - November 2013 - Cover2
ASHRAE Journal - November 2013 - 1
ASHRAE Journal - November 2013 - 2
ASHRAE Journal - November 2013 - Contents
ASHRAE Journal - November 2013 - Commentary
ASHRAE Journal - November 2013 - 5
ASHRAE Journal - November 2013 - Industry News
ASHRAE Journal - November 2013 - 7
ASHRAE Journal - November 2013 - 8
ASHRAE Journal - November 2013 - 9
ASHRAE Journal - November 2013 - Letters
ASHRAE Journal - November 2013 - 11
ASHRAE Journal - November 2013 - Meetings and Shows
ASHRAE Journal - November 2013 - 13
ASHRAE Journal - November 2013 - Sequencing & Control of Compressors
ASHRAE Journal - November 2013 - 15
ASHRAE Journal - November 2013 - 16
ASHRAE Journal - November 2013 - 17
ASHRAE Journal - November 2013 - 18
ASHRAE Journal - November 2013 - 19
ASHRAE Journal - November 2013 - 20
ASHRAE Journal - November 2013 - 21
ASHRAE Journal - November 2013 - 22
ASHRAE Journal - November 2013 - 23
ASHRAE Journal - November 2013 - 24
ASHRAE Journal - November 2013 - 25
ASHRAE Journal - November 2013 - 26
ASHRAE Journal - November 2013 - 27
ASHRAE Journal - November 2013 - 28
ASHRAE Journal - November 2013 - 29
ASHRAE Journal - November 2013 - Fire Station Renovation
ASHRAE Journal - November 2013 - 31
ASHRAE Journal - November 2013 - 32
ASHRAE Journal - November 2013 - 33
ASHRAE Journal - November 2013 - 34
ASHRAE Journal - November 2013 - 35
ASHRAE Journal - November 2013 - 36
ASHRAE Journal - November 2013 - 37
ASHRAE Journal - November 2013 - 38
ASHRAE Journal - November 2013 - 39
ASHRAE Journal - November 2013 - Sewage Plant Heats Village
ASHRAE Journal - November 2013 - 41
ASHRAE Journal - November 2013 - 42
ASHRAE Journal - November 2013 - 43
ASHRAE Journal - November 2013 - 44
ASHRAE Journal - November 2013 - 45
ASHRAE Journal - November 2013 - 46
ASHRAE Journal - November 2013 - 47
ASHRAE Journal - November 2013 - Engineer’s Notebook
ASHRAE Journal - November 2013 - 49
ASHRAE Journal - November 2013 - 50
ASHRAE Journal - November 2013 - 51
ASHRAE Journal - November 2013 - 52
ASHRAE Journal - November 2013 - 53
ASHRAE Journal - November 2013 - 54
ASHRAE Journal - November 2013 - 55
ASHRAE Journal - November 2013 - Building Sciences
ASHRAE Journal - November 2013 - 57
ASHRAE Journal - November 2013 - 58
ASHRAE Journal - November 2013 - 59
ASHRAE Journal - November 2013 - 60
ASHRAE Journal - November 2013 - 61
ASHRAE Journal - November 2013 - 62
ASHRAE Journal - November 2013 - 63
ASHRAE Journal - November 2013 - 64
ASHRAE Journal - November 2013 - BACnet Today & the Smart Grid
ASHRAE Journal - November 2013 - BCover2
ASHRAE Journal - November 2013 - B1
ASHRAE Journal - November 2013 - B2
ASHRAE Journal - November 2013 - B3
ASHRAE Journal - November 2013 - B4
ASHRAE Journal - November 2013 - B5
ASHRAE Journal - November 2013 - B6
ASHRAE Journal - November 2013 - B7
ASHRAE Journal - November 2013 - B8
ASHRAE Journal - November 2013 - B9
ASHRAE Journal - November 2013 - B10
ASHRAE Journal - November 2013 - B11
ASHRAE Journal - November 2013 - B12
ASHRAE Journal - November 2013 - B13
ASHRAE Journal - November 2013 - B14
ASHRAE Journal - November 2013 - B15
ASHRAE Journal - November 2013 - B16
ASHRAE Journal - November 2013 - B17
ASHRAE Journal - November 2013 - B18
ASHRAE Journal - November 2013 - B19
ASHRAE Journal - November 2013 - B20
ASHRAE Journal - November 2013 - B21
ASHRAE Journal - November 2013 - B22
ASHRAE Journal - November 2013 - B23
ASHRAE Journal - November 2013 - B24
ASHRAE Journal - November 2013 - B25
ASHRAE Journal - November 2013 - B26
ASHRAE Journal - November 2013 - B27
ASHRAE Journal - November 2013 - B28
ASHRAE Journal - November 2013 - B29
ASHRAE Journal - November 2013 - B30
ASHRAE Journal - November 2013 - The Performance Gap
ASHRAE Journal - November 2013 - 66
ASHRAE Journal - November 2013 - 67
ASHRAE Journal - November 2013 - 68
ASHRAE Journal - November 2013 - 69
ASHRAE Journal - November 2013 - 70
ASHRAE Journal - November 2013 - 71
ASHRAE Journal - November 2013 - 72
ASHRAE Journal - November 2013 - Refrigeration Applications
ASHRAE Journal - November 2013 - InfoCenter
ASHRAE Journal - November 2013 - 75
ASHRAE Journal - November 2013 - 76
ASHRAE Journal - November 2013 - 77
ASHRAE Journal - November 2013 - 78
ASHRAE Journal - November 2013 - Energy Modeling
ASHRAE Journal - November 2013 - Emerging Technologies
ASHRAE Journal - November 2013 - 81
ASHRAE Journal - November 2013 - 82
ASHRAE Journal - November 2013 - 83
ASHRAE Journal - November 2013 - Data Centers
ASHRAE Journal - November 2013 - 85
ASHRAE Journal - November 2013 - 86
ASHRAE Journal - November 2013 - People
ASHRAE Journal - November 2013 - Products
ASHRAE Journal - November 2013 - Special Products
ASHRAE Journal - November 2013 - Geothermal for Big Box Retail
ASHRAE Journal - November 2013 - 91
ASHRAE Journal - November 2013 - 92
ASHRAE Journal - November 2013 - 93
ASHRAE Journal - November 2013 - 94
ASHRAE Journal - November 2013 - Classified Advertising
ASHRAE Journal - November 2013 - Advertisers Index
ASHRAE Journal - November 2013 - Cover3
ASHRAE Journal - November 2013 - Cover4
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