I am in a northern climate with current temps at around freezing to +10°c highs. Winter temps much lower (-20°C or lower). I am just completing a greenhouse style solar kiln with clear polycarbonate roof about 18'x10' (single layer). Unfortunately I was not able to angle the kiln to face due south so I only get sun from about 11am onward in its south-west orientation.
I have a large amount of eastern white pine that is air dried but has old house borer infestation. I also believe I have an infestation of PPB in some air dried maple (about four years) that has been stored flat stacked in another location.
As such I need to run the kiln to sterilize all this stock in the cold months. I tried adding a 14000btu propane "sunflower" type heater and it wouldn't raise the temp more than 10°C. Added a 30,000btu construction heater (propane) and it seems to be able to keep up with heat loss but have not tried to hit 160°F yet but want to get that high to set sap and kill bugs.
Questions: would stapling up a layer of 6mil poly vapor barrier 3" below the polycarbonate roofing substantially decrease my heat loss but still allow solar input? Am I better off to forget about solar in the winter and try to insulate the solar collector roof with say, 1" foil faced foamboard that could be removed in summer? Keep in mind I am not drying from green here although I would like to dry from about 12% down to 6-8% in winter but bug eradication is the priority.
Lastly, the kiln gets up to 30°C or more for about four hours on a sunny day at present with no vents open. Will the solar design will still work with my SW orientation (just slower drying). Should I be considering scrapping the solar, insulating and just using conventional heat?
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
A second layer would be very helpful. Insulating the roof when you raise the heat is even better. Solar is somewhat questionable, but should work.
I have added a 220v 4800w construction heater on a thermostat so with luck I can use propane to jack the heat up to temp and maybe, just maybe, the electric heater might be able to maintain the temp. Could also keep the propane on but throttled back if the electric needed just a little help.
Those 4800w heaters are close to 19,000btu I believe, my propane is 35,000 max. I put polycarbonate roofing on (supposed to be good to 250F) but still a bit concerned about damaging it. I've added 6mil poly to the underside of the rafters and may add 1" foamboard in channels attached to the underside of the rafters that can be removed easily for normal solar operation and added back for HT or setting sap. The extra R7 of the foam board should help a lot when supplemental heat is on. Does this all sound reasonable or is there a better way?