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Perimeter Enclosure Part I
By George Porter
Well, we aren't supposed to call it skirting anymore are we?
In some ways it would be a mistake to think of it as just a sort
of "cover-up" for the frame, pipes and wheels under
the home. There are reasons why we need it on our homes and extremely
good reasons for it to be done correctly. Improperly applied skirting
can damage the home. Yes, I said skirting! The way I see it a
perimeter enclosure can be several things. It can be vinyl sheets
you screw to the sides of the home (we will get back to this),
or it can be a block wall around the edge of the home required
by most banks called a "permanent foundation" (even
if it doesn t touch the home at all), or it can be a real foundation
(all types of which should be permanent by the way), or it could
even be a basement wall. Let s look at each in some detail:
Some basic rules for perimeter enclosures:
- 1. The ventilation required is directly proportional to the
moisture that can get under the home. The more it leaks, the
more air it needs.
- 2. It must comply with the factory installation manual. This
is how we maintain the warranty.
- 3. If the ground and/or home move in relation to each other,
then provisions must be made to accommodate this difference as
- 4. Enclosures should be made of something at least as durable
as the exterior of the home.
If you look in a manufactured home installation manual, you
will see that there are requirements for proper venting in the
skirting. 99.9 % say that ratio is 1:150. This means that for
every 150 square feet under the home you must have at least one
square foot of free vented area. Free vented area means the size
of the actual hole in the skirting. You must also have a vent
located within 3 feet of every outside corner of the home. When
you apply the skirting you can t mess up the home and if the ground
is subject to frost heave then the enclosure can t be allowed
to affect the side wall of the home.
Why is it a good thing to have skirting? Properly applied
skirting does many jobs. It makes the home look better, no doubt
about that, but it performs some other very important functions
as well. Eliminating wind chill, believe it or not, is one and
controlling rain water is another. There is also the ability to
create a geothermally heated area under the home. Creating this
barrier changes the environment below the floor, for the better
we hope. For instance, in the State of Kentucky, if you have skirting
on a home, the frost depth required for the footings more than
24 inches from the perimeter of the home is cut in half. If the
home has no skirting, the footings must be full depth. This policy
has been in effect for about 10 years, covering many thousands
of homes, and has caused no problems. There has been enough concrete
saved to probably build a highway across the entire State. Frost
heave needs water to make the ice and the skirting keeps it out.
It is a barrier to the wind and eliminates wind chill. Relatively
still air will not carry heat away from the structure and ground
like rapidly moving air.
Notice I said "relatively" still air. You do have
to vent the area under the home or you will have a moisture build
up from the ground and damage the structure. The ground by the
way is warm. Allowed to seek its natural temperature without influences
such as sunshine and cold air it will maintain a temperature of
about 55 degrees. All deep caves are this temperature plus or
minus one degree. Shelter the area under the home and you will
trap some of this heat. If you could seal off the entire area
under the home then you would need no ventilation, but you can
t, because skirting is not air or water tight. As a little bit
of moisture (humidity) gets in the below floor area, you must
have a little bit of air to take it out. A failure to recognize
this simple fact has caused lots of rot.
The skirting installer has several problems to solve here.
1. Most people can figure out how much ventilation they need
by simple math. Divide the area of the floor by 150 and that s
how much venting you need, but how much ventilation is in a skirting
panel? The manufacturer of the panel is the only one who knows,
read the literature. You will probably find that you will need
two out of three panels around the home to be of the vented variety.
Some skirting manufacturers use three or four rows of little vents
running vertical on their panels, some use just two rows, and
others come with none, you have to cut them in and screw vents
on the skirting. Whatever it is, you have to meet the ratio of
1:150 found in the home manual. It is very possible to never be
able to get enough ventilation under some homes! For instance,
suppose you had a triple section home made with 16 wide sections
and it was sited fairly close to the ground, about 24 inches to
the sidewall. The first and third boxes have three sides to get
ventilation from, but the middle has only the ends. You will need
some very heavily vented skirting to make this work. You will
probably need either bigger vents or raise the home to solve this
2. Where will you attach the skirting? Interestingly, the manufacturer
of the skirting may also make the vinyl siding on the home. The
directions for the siding say that you can t "face nail"
a panel. It will cause it to not be able to expand as it should
and will wrinkle the panel. Vinyl siding is of course applied
by the slots at the top so it can move with changing temperature,
if it gets lots of screws or nails through it making it tight
to the side of the home, it gets ugly. So . . . how does one put
vinyl skirting on vinyl siding without messing up the siding?
According to most skirting directions you should drill a 2 inch
hole through the siding before putting the skirting screw in the
middle of that hole. The skirting must be slightly loose so the
siding behind it can move and not wrinkle up!
There has to be a better idea out there than this and happily
there is. Many installers simply attach a treated 2x4, on edge,
under the perimeter of the home on the bottom of the floor joists
with long galvanized deck screws. If this board is recessed about
one inch, then the top of the skirting cap is under the home and
this will help keep the water from getting behind it.
3. If the home is located in an area of the nation where the
ground freezes then either the skirting must have a non-movable
frost free foundation or be able to adjust itself as the need
arises. Solid skirting with stiff bracing will severely effect
the sidewalls of the home if the ground heaves. I have personally
seen a 12 x 50 home lifted off some of the pier blocks by galvanized
skirting on a 2 x 2 wooden frame against the ground. Most commercially
made vinyl skirting has provisions for frost expansion in the
top track; most "home made" skirting does not. Without
room for expansion something has to give, either the skirting
buckles or the home edge moves.
4. When skirting gets tall enough it will need help or it will
blow out. Somewhere around four feet in height is when vinyl starts
to lose its ability to resist wind forces. The panels will fold
and cave in so for this reason most skirting manufacturers have
devices for stiffening the sheets. It is usually a steel rod that
clips behind each panel but there is a limit to the height at
which it is effective and it will tell you in the directions.
If it is necessary to exceed this height you are on your own!
It might be best to call an engineer or architect so you are covered
but it you "engineer" it yourself, it is yours alone
5. There is some relief from all these venting calculations.
Just use all vented panels, you can t over vent. As time goes
on the venting ratio of 1:150 may change. Right now only about
2% of the manufacturers will let you do this. Their manuals say
if you put a vapor barrier down on the ground and seal the moisture
in, they will allow a new venting ratio of 1:1500. This is one
tenth of the original formula and it works if all conditions are
right. The lot must be properly graded with no barriers next to
the home like landscape timbers or concrete sidewalks and the
vapor barrier must be at least 6 mil poly with all seams overlapped
and sealed tightly.
Next time we will discuss "permanent" ground enclosures.