Notes from Windward: #70
A Metal Roof for PropHouse
Over the past few years, we've gradually been putting together a
small 6'x10' research greenhouse known as PropHouse, which is short for the
Propagation Greenhouse. That name refers to our long-term project of
working out what it takes to be able to develop a year-round working
supply of Black Soldier Fly larva.
Propagation Greenhouse in winter
People who live in temperate, coastal areas can count on the wild
population of BSFs to come in and lay eggs in their waste containments.
Unlike houseflies, adult Black Soldier Flies don't have a mouth, so they have no reason to come
around bothering people in a search for food or water. Being extremely shy creatures,
they only come near human habitation in order to lay their eggs.
Many areas of the US are either is too elevated, too cold or too dry
for them to survive in the wild; Windward is all three, so if we can
"domesticate" them here, others will also be able to put a powerful
biotechnical tool to work for them.
cutting a shingle from a chest freezer lid
Since the mature flies lack a mouth, they can't avoid dehydration
by drinking dew. As a result, ideal breeding conditions would involve a
relative humidity of around 85% and temperatures in the mid 80's. We see
those conditions at various times of the year, but never both at the
same time. When it's damp enough for them here, it's too cold for them
to fly, and when it's warm enough, it's too dry for them to survive long
enough to breed.
It's only within a contained environment that we can satisfy those
two requirements simultaneously. But that brings up the next problem,
which is that BSFs mate in flight and use the sun to orient themselves
during mating. Multiple light sources disorient them, so bringing them
inside a controlled environment will only work if it's adequately lit
solely with sunlight.
preparing a notched shingle
PropHouse is designed to meet these needs. As part of that design, the
northern wall and the northern half of the roof are insulated and
covered with plywood so that the structure retains heat during the
winter. But that ability creates problems in the summer in that the
thermal efficiency is such that PropHouse can rapidly overheat on a
To prevent that from happening and cooking everything inside, we have a thermostatically-controlled grid-powered
fan that kicks in when the interior temps exceed 90°F, something
which I've seen happen on a bright sunny day in February when the
outside air temp was in the mid-20's.
installing a notched shingle
In anticipation of taking PropHouse off-grid, we've installed
three vents on the north side of the roof with the intent of driving
them with the 12 VDC fans that blow air through the radiator on some of
the smaller Japanese cars such as Honda Civics.
Because we live in a forest, we only install non-flammable roof
materials, usually a form of corrugated sheet metal. But the three vents
made that difficult since the corrugations below the vents are difficult
to seal. As an alternative, we decided to give PropHouse a metal roof by
cutting sections of enameled steel from some "dead" appliances we had
stored up in the Depot where we keep things that are too potentially
useful to haul to the dump.
the bucks want to help
We used a metal abrasive wheel mounted on our heavy-duty handsaw
to cut the steel shingles. Using a tub of roofing tar and a caulking
gun, the metal shingles were bedded in tar and screwed into place.
Sections had to be cut out of some of the shingles to provide space for the
vents, but otherwise the task went forward smoothly. The biggest problem
was that the two rams in the pen north of PropHouse kept rubbing
themselves on the ladder, which made working on the roof a bit more
exciting that I usually care for.
With all the shingles in place, it was time to give the roof a coat
of reflective paint to make it even more insulative since reflective
surfaces don't absorb heat as much as dark surfaces‒thereby keeping
them cooler in summer‒and don't transmit as much heat, thereby
keeping them warmer in winter.
the new roof finished with a coat of reflective paint
Now that the northern roof is installed, the next task will be to mount a photovoltaic panel.
Notes From Windward - Index - Vol. 70