Heating and Cooling

Published on November 6th, 2012 | by Shelley Awad


Consider The R-Value

What is the R- Value?

Adding insulation to any greenhouse structure is necessary to provide resistance to heat flow. You have certainly heard the expression “heat rises” which in technical terms means that heat naturally flows toward the cold.

For example, during the winter the warm air in your greenhouse will automatically gravitate toward the areas that are not heated evenly like the upper roof area or cooler side walls that do not have insulation. By the same token, during the summer months the heat struggles to enter your house where the temperatures are cooler. Therefore to restore the heat lost in the winter we need a heating system and to maintain cool temperatures we need a cooling system.

Proper insulation will keep temperatures more balanced and help defer the cost of heating and cooling.

The R-value is the measure of resistance to heat flow which is thermal resistance. The thermal resistance indicates the temperature difference when a unit of heat runs through it.

The better quality materials and insulation for both heating and cooling a greenhouse have a higher R-value. The R-value ratings give you a specific method of evaluating and comparing different materials for your greenhouse. The following are some examples of material comparisons:

  • Single paned glass has an R-value of .95 compared with a double pane which is 2.0. Therefore double pane is the better, more efficient choice for a greenhouse glazed with glass.   Twin wall polycarbonate has an R-value of 1.54 while five wall polycarbonate has an R-value of 3.2, making the five wall polycarbonate the best choice for greenhouse glazing.

The following is a chart showing the R-value of various materials for greenhouse use:

Polycarbonate 6mm twin wall R-value is 1.54

Polycarbonate 8mm twin wall R-value is 1.69

Polycarbonate 8mm triple wall R-value is 2.0

Polycarbonate 16mm triple wall R-value is 2.5

Polycarbonate 16mm five wall R-value is 3.2

Acrylic twinwall R-value is 1.82

Double Glass layer R-value is 2.0

Double Glass layer low E R-value is 2.5

Fiberglass glazing- single layer R-value is .83

Polyethylene Double 5mm film R-value is 1.5

Polyethylene Double 6mm film R-value is 1.7

Polyethylene single film R-value is .87

Just remember that the higher the R-value number, the more heat efficient is the material.


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About the Author

Greenhouse Consultant, Shelley Awad, provides everything you need to know about greenhouses. Whether you are buying a greenhouse or you already own one, you’ll learn tips and tricks on greenhouses and how to grow year-round. Questions always welcome.

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