This design is relatively inexpensive. It can be adaptable to any size.
A kiln constructed to the specifications given in the diagram above will dry 450
board feet of 1" thick hardwood lumber in 6 weeks or less of good weather,
or 600 board feet of 2" hardwood lumber in 15 weeks. Soft woods will take about
half the time of hard woods. Drying times vary with sun intensity, moisture
content of the lumber and the relative humidity.
When deciding on kiln size, consider the maximum capacity of the kiln in board
feet to be 10 times the roof area in square feet. For maximum year round performance,
the roof angle of the kiln should be equal to its latitude in degrees north of
the equator.
Frame the kiln floor with 2x6 inch joists; cover with 5/8 inch plywood top and
bottom. For durability, use pressure treated lumber and "exterior" plywood.
Space floor joists 16 to 24 inches apart and mount with joist hangers. Insulate
the floor with blanket-type or solid foam insulation. Avoid insulations with a
foil vapour barrier, as they may trap moisture inside the floor. Do not use poured
in or blown in insulation. Apply two coats of aluminum or oil based paint to the
top sheet of plywood (the kiln floor) to prevent moisture from seeping into the
floor. Cover with flat black paint for maximum solar absorption.
Construct the walls of 2x4 inch studs and 3/4 inch plywood. Be sure the studs
on the side walls frame openings for doors at least as large as the end dimensions
of the wood pile. The back wall studs should frame four 1 foot square vent openings,
two on top and two on the bottom. Paint the interior walls in the same manner as
the floor. Cover vents with screen, to keep birds and rodents out, and simple doors.
Fasten a 3/4 inch plywood fan baffle to the side walls. The fan baffle ensures
that air flows through the wood pile rather an over the top of it. (See above.)
Cut holes for mounting two electric fans as close to the roof as possible to eliminate
dead hot air pockets in the upper corner. The baffle should extend from the roof
to within 6 inches of the lumber pile. A sheet of plastic or canvas can be used
to close the gap between the baffle and the lumber pile.
Mount electric fans to the baffle. Temperature can exceed 150 degrees F inside
the kiln, so avoid fans with plastic parts that could melt; typically, multi speed,
metal window fans are used. Fans should blow towards the front of the kiln
(away from top vents).
Raise the kiln off the ground with cinder blocks or railroad ties, or construct
a cement foundation. The kiln floor structure will deteriorate rapidly if resting
directly on the ground. Align the kiln so the angle roof faces south. Stain
the exterior of the kiln with a dark colored stain. Do not use any covering that
is impervious to water and may trap moisture in the insulation, such as oil
based paints.
Cover the kiln roof with one or two layers of translucent fiberglass, plastic
film or glass. Two layers will decrease drying times. Fiberglass is inexpensive,
resistant to breaking and the easiest to work with. Apply a non-hardening,
silicone caulk to the outer surface of the frame to provide a seal between the
plastic and the wood. Secure the fiberglass to the frame with 1x4 inch treated
wood strips.
The following chart may be helpful in determining the safe rate of drying for
various wood species.
SPECIES
MAX RATE OF MC LOSS
PER DAY
Beech
4.5%
Birch
6.1%
Hard Maple
6.5%
Soft Maple
13.8%
Red Oak
3.8%
White Oak
2.5%
Walnut
8.2%
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