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Air Out Your Problems
By George Porter
Do you know what the formula is for ventilation under a home?
First of all it is what ever the factory manual says it is. If
you don't have the factory manual as in the case of a used home
or some other reason, then you can be safe with the recommendations
of the ANSI Code A. 225.1. This code says that there has to be
one square foot of free vented area for every one hundred fifty
square feet enclosed beneath the home.
This means that a 1500 sq. ft. home needs 10 sq. ft. of open
vent area. There should also be a vent within three feet of each
corner of the home.
There are a few questions that need to be answered here. The
first would be, how much free vented area is there in a vent?
If you have a piece of vinyl skirting with lots of little perforations
or louvers in it, how do you add up the space in all those tiny
holes? Well, you don=t. What you do is just look in the manufacturers
literature or maybe even the box the material comes in and it
will tell you. It probably says something like 5 sq. ft. per 12
ft. section but that is not necessarily what the stuff you use
might say so you'll have to look and see yourself. For now let's
just suppose that we have the 5 in 12 kind and we want to enclose
a 1500 sq. ft. home, how do we do it?
Using the formula of 1 sq. ft. of vent per 150 sq. ft. of
enclosed area we can easily see that we need 10 sq. ft. of vent
or 2 complete 12 ft. panels. If the height of the perimeter of
the home is 28 inches then we would cut the two 12 ft. panels
into 10 pieces 28 inches tall. If we have any left over that=s
too short to fill the space then we have to get a piece out of
another panel to be sure we have used enough vent panel. This
means we may have a total of 11 vented pieces of skirting on the
home. Remember we are talking about the minimum here, it is OK
to go beyond that minimum standard but don't use less than it
calls for. As a practical matter you will probably always have
to use an extra piece because to come out exactly even with no
scrap left over is nearly impossible.
Using the same rule for a 14 x 70 would mean 924 sq. ft. divided
by 150 or 6.16 sq. ft. of free vented area. This is approximately
14.6 feet of panel ( 5 divided by 12 equals .42 sq. ft. of free
vent per foot. 6.16 divided by .42 equals 14.6 feet or 175 inches
of panel). If you have a height of 28 inches for the skirting
then you need 6.25 pieces. You can't install .25 of a panel so
you need an extra full piece. In the case of this home you need
7 vented panels.
This venting is required year round and is especially important
in the winter months when the ground is wetter around the home.
High humidity can destroy a home and can lead to the manufacturer
voiding the warranties on the hardboard siding or even the whole
home. Proper ventilation is not hard, not expensive, and is the
right thing to do. You can solve a lot of problems if you just
air them out.