Thanks Dave, I did read a post referring to using an air pump to keep the layers separated which also allows air exchange. I don't want to reinvent something that may already have good cheap solutions.
My GH gathers moisture which in turn reduces the amount of light which is already at a low ebb this time of the year. .Venting helps but at the expense of air temps. I can imagine using a heat exchange scheme to get around the problem but it will require an investment in time and money.. so i am interested in how others have dealt with the many design problems for growing vegetables in the winter.
I have been looking at Grow lights on The Internet. Most of what I see refers to inside growing. usually Pot. So Mercury Vapor lights ar thought to be ineffective and produce too much heat. However as an adjunct to natural light and where additional heat is desirable can they be effective?
My beds are not on tables per se. but are raised planter boxes. most of my GH is constructed of recycled or surplus materials. Even the industrial heat tape was acquired at scrap price. The GH poly Film came off the internet at a much lower price than the local garden store is asking. The Steel Costco frame was free. My Solar exposure is less than ideal as I live in the woods and have only a few hours of direct sun each day + a climate which has many grey days.
Just talking with you helps me to formulate solutions and identify problems. So thanks for the conversation to date. It is helpful.