Rock heat storage bed.
- The tabs on the frame members used to attached the purlins were, in
some cases, welded so that the purlins would be slightly above the face
of the frame. This would compromise the air sealing of the glazing system.
To compensate, I affixed a 2" wide strip of cured EPDM repair tape
Technology sells a 6" wide roll that is a EPDM and butyl sandwich
with one adhesive side). The tape was thick enough for one layer to
build up the frame so that the gaskets would make good contact with
the glazing. Additionally, the tape will cover the steel frame and slow
- The cavity in the 2"x3" steel frame was filled with extruded
polystyrene foam by cutting strips of the appoximate size and raming
them up into the tube from the bottom with a hand sledge and an appropriately
sized 2x4. I used two brands of foam that I had. One was stiffer, and
consequently was easier to ram 18 feet worth in each frame.
- sills: on top of the foundation wall, I put down a bead of
Tremco acoustical sealant (available from EFI),
a layer of the pink foam sill seal, and another bead of Tremco, before
putting down the sills.
- glazing system: the EPDM gaskets and glazing system made achieving
an air seal easy, with one major exception. We used "self-tapping"
screws to fasten the sandwich (consisting of 1/8" aluminimum bar
stock, 1/8" EPDM gasket, 8 mm polycarbonate glazing, 1/4"
EPDM gasket, EPDM/Butyl repair tape, 3/16" thick wall of steel
frame) together. The steel frame was too tough for the self tapping
screws, so we had to pre-drill the holes. All drill bits used except
cobalt were useless after a couple holes. The cobalt drills lasted through
the hundreds of holes but drilling was challenging, requiring about
20 seconds of pressure, usually in an awkward position. Perhaps predrilling
the holes before the frame is erected will make this job much easier.
Also, be sure there is enough grab in the screw to go through all the
layers. Our screws, which were considered 1-1/2", had just enough
grab when you subtract out the self-tapping end to hold everything together.
Longer screws would be safer.
- glazing: The glazing I used (Thermaglas
from SPS) was perfect for my glazing system. My system squeezed
the panel edges between a 2" wide frame and 2" aluminum bar
stock using 3/8" wide EPDM gaskets. Additionally, the steel greenhouse
frame had rounded corners, further reducing the clamping surface area.
Because the panels expand and contract, allowances for changing panel
width must be accommodated by the glazing system. The integrity of the
panel edges is vital to the success of this clamping system. The thermaglas
panels came sealed at the panel edges with a vertical rib. If the panels
needed to be trimmed to width, it would have been impossible to get
a good seal (unless it was trimmed to another vertical rib). Moreover,
the vertical ribs in the Thermaglas panels were closer together at the
panel edges, adding to the panel strength when clamped. If the panels
did not have a clampable outer edge, a wider clamping system would be
- insulated walls: The steps we took to protect the walls from
moisture, will be more than adequate to handly our air sealing requirements.
From the inside out, our walls had 2 coats of epoxy paint, a sealing
primer, plywood, 6 mil poly, 2x6 cavity with dense-packed cellulose,
plywood, tyvek, white cedar shingles.
- After flip-flopping through many options for interior wall surface,
we settled on plywood walls painted with an industrial quality paint.
While we wanted to avoid wood inside when possible, the other options
seemed too expensive or cumbersome. We used a 2-component water-based
catalyzed epoxy paint (sherwin
williams) over a high quality sealing primer. The literature described
the epoxy in such a way that I was intimidated by the thought of applying
it. It turned out to be very easy to work with. One coat of primer and
two coats of epoxy provided a surface that seems like it will be up
for the task. Time will tell.
Rock Heat Storage Bed
- We decided on 3 inch stone for the rock bed, based on tables for pressure
drop and heat transfer. It was challenging to find consistently sized
3" stone in our area. All the quaries had smaller stone in consistent
sizes, but the larger stone had excessive variability in size. Even
stone called 3" stone might vary from 2" - 9" in size.
We were able to get some stone re-screened to eliminate 2-1/4"
stone and less, but the screening process does not seem precise, and
there is still some variability to our stone. While I don't know how
"perfect" stone would behave, ours seems good enough, in that
the fan we have does manage to blow air through the bed.
- Do to a miscommunication (which means I blew it), we constructed our
rock bed with only one plenum on the fan side of the rock bed. We should
have had another block wall on the down-wind side so that the air would
flow equally through the entire depth of the rock bed. In our system,
the air will tend to flow more toward the top of the bed, compromising
the efficiency of the rock bed. Our air returns on the south side of
the greenhouse where the concrete slab was held back from the wall by
18". We will pull out as many of the stones in that strip as we
can to improve the air flow at lower depths. The data in the perfomance
chart is based on the system as is. Based on the temperature fluctuations
in the bed, it looks like we would have been better off if we constructed
the rock bed as the engineer intended. However, we haven't yet pulled
out the 18" of stone from the return, and the greenhouse was not
fully loaded with plants (and their resulting ventilations requirements
and transpirational cooling), so next winter will be a better indication.
- The fan is thermostatically controlled by a Dramm T42 2-stage thermostat
to blow when the temperature exceeds a threshold (currently ~90 degrees)
or falls below a threshold (currently ~45 degrees). This system seems
to work perfectly.
- The foundation and rock bed were insulated with 2" of extruded
polystyrene. Above grade, the foundation was covered by 24 inch brown
"aluminum" flashing. The flashing was tucked under the sill
and extends quite a ways below grade. On the east, west, and north sides,
the sills extend out past the foundation wall to cover the foam. On
the south side the frames cannot extend beyond the sill, so the foam
was cut to taper to the sill, and the flashing was prebent to follow
the foam's profile. So far, this system was easy to install and seems
to work well.
- The decision to use cellulose in a greenhouse environment was slightly
scary for me. We are big fans of cellulose insulation, however, so we
decided to try it. We worked hard to keep moisture out of the walls,
and to allow any moisture that gets in the walls to get out more easily
than it got in. To get into the wall from the inside of the greenhouse,
moisture would have to get through 2 coats of epoxy paint, a coat of
primer, 1/2 inch plywood with joints sealed with silicone, and a 6 mil
poly vapor barrier. Having constucted the greenhouse, it seems a safer
More notes to follow.