ASHRAE Journal - October 2013 - 55
Column Building ScienceS
never get overstretched—no “plastic deformation” in the
classic sense. And the springs never wear out—no “fatigue”
in the classic sense. No matter how many expansion and
contraction cycles. Wood is a perfect harmony of coil
springs and rubber bands.
Some woods expand and contract more than other
woods. As the density of wood increases, dimensional
change increases. A good number to remember for the
density of wood we use to build with (SPF) is approximately 25 lb/ft3 (400 kg/m3). Not all woods are the same.
Oak is close to 50 lb/ft3 (800 kg/m3). Hardwoods have
higher densities than softwoods. That is why they don’t
hold paint as well.
The lower the density of wood, the better it holds
paint. Balsa wood is great at holding paint because of its
low density—less than 10 lb/ft3 (160 kg/m3). Because it
does not expand and contract very much, the paint film
on it does not stretch and shrink as much. But balsa is a
terrible construction material except for building model
airplanes.
Paint films and coatings in general like dimensionally stable substrates. The more the substrate moves,
the more likely the paint will debond. One of the magic
characteristics that make a good paint is elasticity. The
more elastic it is, the more likely it will be able to stick to
something that likes to move a lot—like wood.
Paint used to coat wood also needs another really
important characteristic—it needs to be vapor permeable so the wood is able to dry. The easier and
faster wood dries the less likely it will get real wet
and get as big as it can get. Because once it gets real
wet it will also eventually get real dry, and the difference between real wet and real dry is a big change in
dimension.
The trick is how to make the paint both elastic and
vapor permeable. Not an easy thing to do because the
things you do to paint films to make them elastic also
tend to make them less permeable. Even more difficult
is if you get a paint film to be elastic and vapor permeable, how do you get it to stay that way when you put it
outside and expose it to the sun and other nasty things
like oxygen?
We actually have paint coatings that now do both of
these things extremely well. They are called latex paints
or acrylic latex paints, paints that are both elastic and
breathe. The trouble with them is that they do not
always age very well.
Think of a latex paint film as a bunch of long chain
polymers that resemble spaghetti that are very slippery when first applied. The spaghetti sauce—Mama’s
secret recipe that contains ammonia and detergents
and other stuff whose proportions are guarded more
closely than NSA’s surveillance plans—coats these spaghetti strands making them slide by each other with
great ease. Fresh spaghetti and fresh sauce make for a
very elastic film.
However, over time, energy from the sun in the form
of ultraviolet radiation and heat “dry out” the sauce
and the spaghetti does not slide past itself so easily. The
spaghetti polymers begin to stick to each other. This is
called cross linking. Also, some of the ingredients in
Mama’s sauce evaporate or are washed away or react
with stuff in the air. The sauce gets sunburned and
begins to disappear. So over time, paint films tend to get
more brittle.
It gets worse with paint films on the south and west
side of buildings, the sides with the most sun. Wood
claddings move more on the south and west side
because they go through greater temperature differences, which lead to greater relative humidity differences, which lead to greater moisture content differences. The biggest stresses on the paint films also happen on the orientations where they get the most brittle.
Paint tends to fail on south and west orientations more
because of greater solar exposure and greater substrate
movement (Photo 1).
Nothing new here so far. Old-timers learned to put
wood siding on furring to back ventilate the wood, so
the wood would stay drier and not move as much so
the paint would stay on the siding longer. Old-timers
also learned to “back prime” the siding, so it would not
get wet at the overlaps due to capillarity so it would not
move as much so the paint would stay on the siding
longer.
We have been trying to improve wood for more
than 5,000 years. Folks started gluing thin strips of
wood together to improve its properties in Ancient
Mesopotamia. Plywood goes back that far. Modern plywood was invented by Immanuel Nobel, the father of
Alfred Nobel, in the 1800s. Neat connection, eh? The
apple doesn’t fall far from the tree.
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Somerville,
Mass. Visit www.buildingscience.com.
O ct O b e r 2 0 1 3
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ASHRAE JouRnAl
55
ASHRAE Journal - October 2013
Table of Contents for the Digital Edition of ASHRAE Journal - October 2013
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
Sustainable Design of Water Source VRF
Economizer for Data Center
Total Dissolved Solids in Reclaimed Water
Hot, Humid Lab Hood Facility
Engineer's Notebook
Building Sciences
The Performance Gap
ASHRAE Honor Roll
ASHRAE Research Report
Technical vs. Process Commissioning: Basis of Design
Emerging Technologies
Data Centers
HVAC Applications
Energy Modeling
Refrigeration Applications
Products
Classified Advertising
Advertisers Index
ASHRAE Journal - October 2013 - Cover1
ASHRAE Journal - October 2013 - Cover2
ASHRAE Journal - October 2013 - 1
ASHRAE Journal - October 2013 - 2
ASHRAE Journal - October 2013 - Contents
ASHRAE Journal - October 2013 - Commentary
ASHRAE Journal - October 2013 - 5
ASHRAE Journal - October 2013 - Industry News
ASHRAE Journal - October 2013 - 7
ASHRAE Journal - October 2013 - 8
ASHRAE Journal - October 2013 - 9
ASHRAE Journal - October 2013 - Letters
ASHRAE Journal - October 2013 - 11
ASHRAE Journal - October 2013 - Meetings and Shows
ASHRAE Journal - October 2013 - 13
ASHRAE Journal - October 2013 - Sustainable Design of Water Source VRF
ASHRAE Journal - October 2013 - 15
ASHRAE Journal - October 2013 - 16
ASHRAE Journal - October 2013 - 17
ASHRAE Journal - October 2013 - 18
ASHRAE Journal - October 2013 - 19
ASHRAE Journal - October 2013 - Economizer for Data Center
ASHRAE Journal - October 2013 - 21
ASHRAE Journal - October 2013 - 22
ASHRAE Journal - October 2013 - 23
ASHRAE Journal - October 2013 - 24
ASHRAE Journal - October 2013 - 25
ASHRAE Journal - October 2013 - 26
ASHRAE Journal - October 2013 - 27
ASHRAE Journal - October 2013 - Total Dissolved Solids in Reclaimed Water
ASHRAE Journal - October 2013 - 29
ASHRAE Journal - October 2013 - 30
ASHRAE Journal - October 2013 - 31
ASHRAE Journal - October 2013 - 32
ASHRAE Journal - October 2013 - 33
ASHRAE Journal - October 2013 - 34
ASHRAE Journal - October 2013 - 35
ASHRAE Journal - October 2013 - 36
ASHRAE Journal - October 2013 - 37
ASHRAE Journal - October 2013 - 38
ASHRAE Journal - October 2013 - 39
ASHRAE Journal - October 2013 - Hot, Humid Lab Hood Facility
ASHRAE Journal - October 2013 - 41
ASHRAE Journal - October 2013 - 42
ASHRAE Journal - October 2013 - 43
ASHRAE Journal - October 2013 - 44
ASHRAE Journal - October 2013 - 45
ASHRAE Journal - October 2013 - 46
ASHRAE Journal - October 2013 - 47
ASHRAE Journal - October 2013 - Engineer's Notebook
ASHRAE Journal - October 2013 - 49
ASHRAE Journal - October 2013 - 50
ASHRAE Journal - October 2013 - 51
ASHRAE Journal - October 2013 - 52
ASHRAE Journal - October 2013 - 53
ASHRAE Journal - October 2013 - Building Sciences
ASHRAE Journal - October 2013 - 55
ASHRAE Journal - October 2013 - 56
ASHRAE Journal - October 2013 - 57
ASHRAE Journal - October 2013 - 58
ASHRAE Journal - October 2013 - 59
ASHRAE Journal - October 2013 - The Performance Gap
ASHRAE Journal - October 2013 - 61
ASHRAE Journal - October 2013 - 62
ASHRAE Journal - October 2013 - 63
ASHRAE Journal - October 2013 - 64
ASHRAE Journal - October 2013 - ASHRAE Honor Roll
ASHRAE Journal - October 2013 - HR 2
ASHRAE Journal - October 2013 - HR 3
ASHRAE Journal - October 2013 - HR 4
ASHRAE Journal - October 2013 - HR 5
ASHRAE Journal - October 2013 - HR 6
ASHRAE Journal - October 2013 - HR 7
ASHRAE Journal - October 2013 - HR 8
ASHRAE Journal - October 2013 - HR 9
ASHRAE Journal - October 2013 - HR 10
ASHRAE Journal - October 2013 - HR 11
ASHRAE Journal - October 2013 - HR 12
ASHRAE Journal - October 2013 - HR 13
ASHRAE Journal - October 2013 - HR 14
ASHRAE Journal - October 2013 - HR 15
ASHRAE Journal - October 2013 - HR 16
ASHRAE Journal - October 2013 - HR 17
ASHRAE Journal - October 2013 - HR 18
ASHRAE Journal - October 2013 - HR 19
ASHRAE Journal - October 2013 - HR 20
ASHRAE Journal - October 2013 - HR 21
ASHRAE Journal - October 2013 - HR 22
ASHRAE Journal - October 2013 - HR 23
ASHRAE Journal - October 2013 - HR 24
ASHRAE Journal - October 2013 - HR 25
ASHRAE Journal - October 2013 - HR 26
ASHRAE Journal - October 2013 - HR 27
ASHRAE Journal - October 2013 - HR 28
ASHRAE Journal - October 2013 - HR 29
ASHRAE Journal - October 2013 - HR 30
ASHRAE Journal - October 2013 - HR 31
ASHRAE Journal - October 2013 - HR 32
ASHRAE Journal - October 2013 - ASHRAE Research Report
ASHRAE Journal - October 2013 - 66
ASHRAE Journal - October 2013 - 67
ASHRAE Journal - October 2013 - 68
ASHRAE Journal - October 2013 - 69
ASHRAE Journal - October 2013 - 70
ASHRAE Journal - October 2013 - 71
ASHRAE Journal - October 2013 - 72
ASHRAE Journal - October 2013 - 73
ASHRAE Journal - October 2013 - 74
ASHRAE Journal - October 2013 - 75
ASHRAE Journal - October 2013 - Technical vs. Process Commissioning: Basis of Design
ASHRAE Journal - October 2013 - 77
ASHRAE Journal - October 2013 - 78
ASHRAE Journal - October 2013 - 79
ASHRAE Journal - October 2013 - 80
ASHRAE Journal - October 2013 - 81
ASHRAE Journal - October 2013 - Emerging Technologies
ASHRAE Journal - October 2013 - 83
ASHRAE Journal - October 2013 - 84
ASHRAE Journal - October 2013 - 85
ASHRAE Journal - October 2013 - 86
ASHRAE Journal - October 2013 - 87
ASHRAE Journal - October 2013 - Data Centers
ASHRAE Journal - October 2013 - 89
ASHRAE Journal - October 2013 - 90
ASHRAE Journal - October 2013 - 91
ASHRAE Journal - October 2013 - 92
ASHRAE Journal - October 2013 - 93
ASHRAE Journal - October 2013 - HVAC Applications
ASHRAE Journal - October 2013 - 95
ASHRAE Journal - October 2013 - Energy Modeling
ASHRAE Journal - October 2013 - 97
ASHRAE Journal - October 2013 - Refrigeration Applications
ASHRAE Journal - October 2013 - 99
ASHRAE Journal - October 2013 - Products
ASHRAE Journal - October 2013 - 101
ASHRAE Journal - October 2013 - 102
ASHRAE Journal - October 2013 - Classified Advertising
ASHRAE Journal - October 2013 - Advertisers Index
ASHRAE Journal - October 2013 - Cover3
ASHRAE Journal - October 2013 - Cover4
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