ASHRAE Journal - April 2023 - 16
BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
Shakespeare
Does Roofs
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
To vent, or not to vent-that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous moisture,
Or to take arms against them
And, by opposing, end them. To rot, to rust-
To rust, perchance to scream. Ay, there's the rub.
With apologies to the Bard of Avon, after several
decades of dealing with roofs, I have come to the answer
to the roof vent question: do not vent-except. Great
answer, eh?
Vented roofs blow off more than unvented roofs.
Vented roofs burn more than unvented roofs. Vented
roofs are energy inefficient compared to unvented
roofs. Vented roofs cause moisture problems south of
the Mason-Dixon Line and east of Interstate 35 in Texas.
Venting a roof in a hot-humid and mixed humid climate
is a very, very bad idea.
The only time you should vent a roof is to control
ice-damming. And when I do a vented roof to control
ice-damming, I install the vented roof over the top of an
unvented under-roof.
The original reasons to vent roofs were pretty reasonable.
In cold climates roofs were vented to control
moisture accumulation and, yes, to control ice-damming.
In hot climates roofs were vented to increase
thermal comfort.
In cold climates, roof venting controlled attic moisture
accumulation only in poorly insulated attics. Let me
repeat that to have it sink in. Roofs needed to be poorly
insulated. In the winter the air outside is cold. Yup. Cold
air is not capable of holding much moisture. So bringing
cold air into your attic does not remove moisture unless
there is heat loss from the building. The heat loss warms
up the incoming cold air, giving it the ability to pick
up moisture and carry it to the outside. All of the attic
ventilation ratios-1 to 300, 1 to 150-were based on little
or no insulation. The code now calls for R-60 in attics.
There is no effective heat loss from the building into the
attic. It gets worse: the relative humidity of the outside
air is very high-vapor pressure is low-hygroscopic
materials like wood, plywood and OSB " see " relative
humidity not vapor pressure. Venting highly insulated
roofs in cold climates actually makes them wet.
You want to have some fun, ask a Canadian about how
well the CHIP program worked in the 1980s. Oops. It
has to be an old Canadian. CHIP was the short form for
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford, Mass. Visit www.buildingscience.com.
16
ASHRAE JOURNAL ashrae.o rg
A P R I L 2023
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ASHRAE Journal - April 2023
Table of Contents for the Digital Edition of ASHRAE Journal - April 2023
Contents
ASHRAE Journal - April 2023 - Cover1
ASHRAE Journal - April 2023 - Cover2
ASHRAE Journal - April 2023 - 1
ASHRAE Journal - April 2023 - Contents
ASHRAE Journal - April 2023 - 3
ASHRAE Journal - April 2023 - 4
ASHRAE Journal - April 2023 - 5
ASHRAE Journal - April 2023 - 6
ASHRAE Journal - April 2023 - 7
ASHRAE Journal - April 2023 - 8
ASHRAE Journal - April 2023 - 9
ASHRAE Journal - April 2023 - 10
ASHRAE Journal - April 2023 - 11
ASHRAE Journal - April 2023 - 12
ASHRAE Journal - April 2023 - 13
ASHRAE Journal - April 2023 - 14
ASHRAE Journal - April 2023 - 15
ASHRAE Journal - April 2023 - 16
ASHRAE Journal - April 2023 - 17
ASHRAE Journal - April 2023 - 18
ASHRAE Journal - April 2023 - 19
ASHRAE Journal - April 2023 - 20
ASHRAE Journal - April 2023 - 21
ASHRAE Journal - April 2023 - 22
ASHRAE Journal - April 2023 - 23
ASHRAE Journal - April 2023 - 24
ASHRAE Journal - April 2023 - 25
ASHRAE Journal - April 2023 - 26
ASHRAE Journal - April 2023 - 27
ASHRAE Journal - April 2023 - 28
ASHRAE Journal - April 2023 - 29
ASHRAE Journal - April 2023 - 30
ASHRAE Journal - April 2023 - 31
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ASHRAE Journal - April 2023 - 33
ASHRAE Journal - April 2023 - 34
ASHRAE Journal - April 2023 - 35
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ASHRAE Journal - April 2023 - 54
ASHRAE Journal - April 2023 - 55
ASHRAE Journal - April 2023 - 56
ASHRAE Journal - April 2023 - Cover3
ASHRAE Journal - April 2023 - Cover4
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