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Do I Have a High Water Table Problem?
 If you are contemplating construction of
a new greenhouse, site selection is very important.
Not only should your new greenhouse be constructed in an
area without shading from other structures or trees, but
you should also be careful not to site your facility in an
area with a high seasonal or year ‘round water table
level. Normal construction practices would typically have
you create a compacted site with adequate drainage, even
in a low-lying area. But, if you must build a greenhouse
on a “wet” site you should make special provisions.
This is especially true if you plan to install a warm-floor
heating system. If you install a warm-floor system and
the water table “impinges” or touches the warm
concrete, you could lose a tremendous amount of efficiency
as the ground water will “wick” away precious
heating BTU’s.
How do you determine if you have a high water table on your site? If you haven’t
already been put on notice that you’ve got “wetlands” on
your hands, (in which case you may not be doing much building anyway), there
are some very simple procedures to check. The first and most obvious is to
dig a test hole the depth of your excavation for the site and observe if it
fills with water. Keep in mind that the local water table probably fluctuates
with the season. If the test hole indicates that your water table may be close
to the level of the bottom of your greenhouse floor, you have a “high” water
table. If you have even a slight doubt, follow the procedures for setting up
a warm-floor with a complete layer of insulation below.
Most warm-floor heating systems are created by encasing
thermal tubing in the concrete pour. Boilers make hot water
that pumps circulate through the tubing. The heated water
gives up it’s heat through the tubing and warms the
mass of the floor. A thermostat at or near the floor level
monitors and controls the whole thing.
The goal of a warm-floor is to provide very even and
efficient heat transfer to the entire greenhouse.
To help maximize the efficiency of this type of system, attention should be
paid to reducing “parasitic” losses that can reduce the amount
of heat making it from the boiler, through the floor, and into the greenhouse.
All warm-floor installations should have perimeter insulation installed previous
to pouring the concrete. This is typically done with 2” thick extruded
styrene panels placed vertically on the inside of the forms, at a minimum 2’ depth
into the ground. The best practice would be to insulate with 2” panels
down below the depth of the frost line.
In high water table areas, it is necessary to insulate
the entire site with extruded styrene insulation, otherwise
your system installation could be a big disappointment.
We know first hand of growers that
skipped this step and found that their fuel use was double
or triple what it should have been. We recommend a minimum
of 1” thick material, which gives an “R” rating
of 5. Place this insulation horizontally under the entire
floor. If you plan to use heavy equipment on the slab,
consult with a structural engineer to determine the compressive
strength necessary for this material to handle the loading.
For most greenhouse applications, standard compressive
strength panels (25 PSI) will be fine.
In either area, perimeter or below slab, it is very important
to use true extruded insulation, and not “bead-board” as
the latter tends to absorb water and lose it’s insulative
ability over time.
In these times of high fuel costs, properly insulating your warm-floor application
is more than just a good idea, it’s paramount to helping make a profit
from your new greenhouse.
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