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some treated wood is carcinogenic
in cold areas, heat in the colder seasons would be very expensive. so an insulated unit would be needed if you start bedding plants in febuary. I saw clear bubble wrap at lowes that was two feet wide, which might make a cheap insulating material. also if you can put the unit three feet in the ground and then concentrate on a slanted roof like older heat boxes that people used. any type of heavy plastc with a small fan blowing air could insulate if it was sealed right.