ASHRAE Journal - October 2013 - 54

Column Building ScienceS
Joseph W. Lstiburek

Photo 1 Painted Wood Siding. 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. Notice that the bottom edges
lose the paint first. Old guys noticed this as well, hence the practice of back priming
at the overlaps. This is old siding with old paint, the non-elastic, non-vapor permeable
stuff—alkyd paint. Modern paints don’t do this. Having said that, it is still a good idea
to back prime wood siding and back ventilate it by installing it over furring even when
using the new stuff.

Improving and Replacing Painted Wood

Coil Springs, Rubber Bands,
Spaghetti and Tinkertoys*
By JoSeph W. lStiBurek, ph.d., p.eng., FelloW AShrAe

When Mother Nature came up with wood it gave it magical properties that, try
as we might, we mere humans find difficult to improve upon or to transfer to
other materials we come up with. One of these magical properties is its ability to regain its original dimensions after being repeatedly wetted and dried.
Wood gets wet and it expands. It dries and it shrinks. No magic or mystery
there. The magic is that no matter how many times wood gets wet and
expands it always comes back to its original dimension when it dries. Always.
Think about how remarkable that is?
Think of wood as a material made
up of a combination of coil springs
that are compressed by rubber
bands. When wood picks up water
it expands. The coil springs exert an
outward force and expand in length
but are restrained as to how far they

expand by the rubber bands. They
can never expand beyond the limit
established by the fiber saturation
point, the elastic limit set by the
rubber bands. Wood can store water
in its pore structure beyond the fiber
saturation point but it does not get

any bigger. In the dominant species
of wood used in North American
construction, spruce/pine/fir (SPF),
fiber saturation is around 28% moisture content by weight.
Now get this, with wood the rubber
bands never break.† The rubber bands

* this column was inspired by charlie carll, who passed away recently. he was at the Forest products laboratory in Madison, Wis., for as long as i remember. he had an encyclopedic knowledge of wood.

he always had time for me when i asked dumb questions. the answer to every question i asked him usually started with: “now Joe, i am pretty sure that is not how things work…let me help you out.”
he was a kind and patient man who will be missed. charlie liked the rubber bands, coil springs and spaghetti analogy in teaching youngsters, even though it was not exactly how things worked. he said it
was close enough for engineers and then he would grin. † never does not mean never ever, but means never unless really weird stuff happens. Stuff that does not happen in buildings unless really, really weird stuff
happens that should not be happening in buildings, like fire.

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ASHRAE Journal - October 2013

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