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Benefit of Air Space
Behind Siding
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 Cedar
siding, like most wood
claddings, absorbs rain
water. Painting or staining
siding reduces its water
uptake, in the liquid form.
Claddings in general and
sidings in particular, are
not water tight.
Rain water penetrates the
cladding systems at joints,
nail holes, penetrations,
overlaps, etc. and runs down
their back surfaces.
Additionally, water can be
pulled upwards at overlaps
due to capillary action. For
these reasons it is
important to protect all of
the surfaces of wood
claddings from liquid water
uptake.
This is independent of
sheathing or house wrap
choice. However, it becomes
critical with respect to
foam sheathings as we shall
see. Traditionally,
protecting the surfaces of
wood cladding from liquid
water uptake has been
accomplished by
"back-priming" or
"back-coating" (along with
"front-coating").
The key factor to be
addressed is the liquid
water uptake, not the vapor
uptake. Back-priming or
back-coating with a
"penetrating water repellant
preservative" will be as
effective as back-priming
with an oil based primer in
terms of the liquid water
uptake.
The advantage of the water
repellant preservative is
that it is also vapor
permeable. The permeability
will allow wet siding to dry
more rapidly, but only if an
airspace has been provided
for it to dry into. The
disadvantage of the water
repellant preservative is
that it is not as readily
available as oil based
primer and does not work as
well when there isn't an air
space.
Now in some assemblies a
vapor permeable back-coating
is not desirable, such as an
assembly that is releasing
water vapor from the
interior where airspace
between siding and sheathing
does not exist.
Of course this issue can be
easily addressed by
providing a vented airspace
between the wood cladding
and the sheathing.
Experience shows that the
trim typically has the most
critical exposure since trim
often has no airspace
associated with it.
Back-priming or back-coating
trim that is not over
airspace with an oil based
primer is more effective
than back-coating with a
water repellant
preservative. However,
adding airspace behind the
trim, gives the nod to the
water repellant
preservative.
In the absence of the easy
availability of a water
repellant preservative, I
recommend an oil based
primer coupled with
airspace. Why oil based
primer rather than latex
based primer? In woods, such
as cedar or redwood, with
high quantities of
water-soluble extractives
(tannins, wood sugars) a
water-based primer will
leach extractives.
Water-soluble extractives
tend not to be soluble in
oil (by definition); hence
the advantage of the oil
based primers. Newer water
based primers are being
developed which may be able
to address this issue. If
the extractive issue can be
handled, the latex-based
primers will work better
than oil based primers due
to their higher vapor
permeability.
To answer another question
that people keep asking me
about. Does the Forest
Products Lab argue against
back-priming? Actually, the
FPL argues in favor of
back-coating with a water
repellant preservative and
providing an air space
rather than back-priming
with an oil based primer. On
this we agree.
Now ask the next question.
In the absence of an air
space and the availability
of a water repellant
preservative what do I do?
FPL's answer, and mine, is
to back-prime with an oil
based primer. And the next
question. If I have an air
space but I can't get water
repellant preservative what
do I do? FPL's answer, and
mine, is to back prime with
an oil based primer.
And finally the most
difficult question. If I can
get a water repellant
preservative, but I do not
have an air space what do I
do? I argue that you should
use an oil based primer and
not use the water repellant
preservative. At FPL the
answer depends on who you
ask.
Now, no matter what we do,
moisture will get into the
cedar siding or any wood
siding. It probably starts
out wet. The sun beats down
on the siding and drives the
moisture inward. Temperature
gradients and moisture
gradients get created that
want to redistribute the
moisture.
In the old days, before foam
sheathings and OSB, moisture
was redistributed into the
exterior sheathing (board
sheathing, plywood, and
fiberboard) relieving the
moisture stress on wood
siding that happens when the
front is really dry and the
back is really wet.
This hydric redistribution
does not happen with wood
sidings over foam
sheathings, especially ones
with a foil facing. Airspace
is necessary to help
redistribute the moisture.
Not much of an air space,
1/4 inch to 3/8 inch, will
do.
A tar paper has some
moisture storage capacity,
whereas a house wrap has
none. Installing a tar paper
between cedar siding and a
foam sheathing will provide
better performance than
installing a house wrap
since the tar paper will
facilitate some modest
hydric redistribution.
However, an air space works
far better than a tar paper.
In most applications that I
have seen, tar paper by
itself (without an air
space), does not prevent the
cupping, splitting and paint
peeling associated with
cedar siding over foil faced
foam sheathings. Also, the
tar paper may not if it
stays wet. If the tar paper
rots, watch out, the siding
will rot as well.
Now, this discussion has
been centered on the issue
of what is good for the
cedar or wood siding. What
about the rest of the wall?
Foam sheathings also need to
control rain water that
penetrates the siding. A tar
paper or house wrap
installed behind foam
sheathing may be required as
a drainage plane in high
rain exposures.
Alternatively, ship lapped
foam sheathing at vertical
joints and flashing at
horizontal joints (a strip
of poly) also works well.
The above has been a long
winded way of recommending
coating wood siding (cedar
included) on all six
surfaces and installing it
over a small air space
regardless of sheathing
type. It becomes critical
over foam sheathings, and
essential over foil faced
sheathings in order to
reduce water uptake (the
coating on all surfaces) and
to provide a receptor (the
airspace) for the moisture
in the siding.
If you want to install tar
paper or a house wrap with
foam sheathing, install them
under the foam sheathing,
but keep the airspace
between the siding and foam,
and don't forget to coat all
six sides of the siding.
Now on to the rain screen
question; the original
concept of the rain screen
was to provide air pressure
equalization between the air
space behind a cladding and
the exterior face of the
cladding in order to reduce
rain entry. Pressure
equalization rarely happens,
and to get it to happen
requires considerable design
and supervision (i.e. "brain
damage").
The main benefit of a rain
screen is not to reduce rain
entry (by this alleged
pressure equalization that I
contend rarely happens) but
rather to increase moisture
removal by creating a
ventilated cladding.
The ventilated cladding
allows both the cladding to
dry (out its back surface)
and the wall assembly to dry
(through the sheathing and
building paper) into the
airspace behind the
cladding. Some claddings are
inherently self ventilating
such as vinyl and aluminum
siding and brick veneers.
Other claddings need help,
such as wood siding.
The help occurs by
installing the wood siding
over a spacer or by using
wedges, or clips or oval
headed ("bumpy") nails to
separate the laps of the
wood siding in order to vent
the siding. What is the real
benefit? Well do you want
the siding to not rot and
the paint to not peel?
Before the days of plywood,
OSB, foam sheathings, lots
of cavity insulation and
that stupid plastic interior
vapor barrier, hydric
redistribution of wetted
claddings could happen
towards the interior.
This hydric redistribution
does not happen with today's
low drying potentials (due
to high levels of cavity
insulation and interior
vapor barriers) and
impermeable and semi
permeable sheathings.
Finally, we have the
problems associated with
loss of water repellency of
plastic house wraps and felt
building papers (yes, its
true, it can happen to felt)
due to tannins and other
extractives, not to mention
soaps, detergents, bleaches,
dirt, dust, and paints. None
of this is a problem with
back-primed wood or
back-coated wood over an air
space.
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