GREENHOUSE 'R' VALUE

Stashco:

I believe that two radio shows ago you gave a formula for calculating the size requirement for gas heaters for greenhouses. Unfortunately I didn't get the opportunity to record it. Can you repeat the calculation?

My green house is octagonal. The floor area is 230 square feet. The height is 8 feet. Four of the walls are single pane glass measuring four feet by seven and a half feet. The ceiling is insulated plywood. Three of the remaining walls are insulated plywood 2 x 6 construction.

The last wall contains a steel insulated door.

Thank you. Bob
Edmonton

Bob.... Thanks for your interest in the show, and I hasten to relay with the formula for the BTU's necessary to heat your structure.

Thankfully, the formula will be easier to calculate than your physical structure to be heated.

Measure the square feet of uninsulated wall area that is exposed to the outside weather as a transparent or opaque material, even if the glass or poly carbonate is double glazed .

There is no need to include the flooring square feet.

Decide what interior temps. are necessary in your greenhouse through the winter months.
Decide when the greenhouse will first be in operation, ie. late winter to late fall. If not all year around, and including winter, the outside night temps will be more temperate, and less heat will be necessary to maintain plant life.

If the greenhouse. is to be in operation during the severe weather, you will use the figure of the lowest expected temp in degrees F. Now settle on the obligatory degree of temp that is required to continue your interior plant production.

The difference between the two, say minus 30 to plus 60 is a sum of 90, which is called the 'differential 'and important to your equation.

Multiply the differential with the square footage. If the glass area is 150 sq. ft. and the differential is 90, your working number would be 13,500.

This figure is the number of BTUs necessary for a heat source to produce. If but for glass or poly, there would be an R value[resistance] that would be introduced to the final figure. As there is no real insulative value to glass or poly, when part of a greenhouse situation, the figure of 13,000 could be boosted to a 25 or even 50 thousand BTU out-put heat source.

This extra potential would make up and exceed the heat loss all greenhouses suffer, and be capable or overcoming a bout of extreme wind and low temps.

For interest, quarter inch plywood is rated at an R value of 2. Any greenhouse material used to transmit light is rated at 1. If the poly roof is insulated by a forced air fan to inflate the double layer, the R value will increase to a whopping 2 and a half R value. If the R value is used in the equation, it would be divided into the product of the differential times the square foot of exposed wall and ceiling. Not a lot of difference.

Hope this is helpful.

(alias: Stash)