Insulate to save heat

In performing energy audits on several hundred greenhouses, it has become apparent that there is considerable heat savings and a good payback to insulating the wall areas of a greenhouse that are not important for sunlight. Payback is usually less than two years.

The savings that can be obtained will vary with the length of the heating season, the growing temperature and the interior layout of the greenhouse. In a typical 30’ x 100’ freestanding greenhouse, the amount of wall area to 3 feet height is about 15% of the total surface area. Insulating reduces the amount of light reaching the plants very little. It can reduce fuel use significantly and lower the heating bill. Insulating exterior walls behind fin or bare iron heat pipes is very important and will reduce the wall temperature at least 50 F.

What type of insulation can be used?

Insulation is available in many forms and materials. The most common materials utilize the principle that air is a good insulator. Air, trapped in foam bubbles or the tiny spaces creates a barrier to heat movement.

Heat loss is measured by “R-value” (°F- square-foot-hour/Btu), the resistance to heat flow. See Table 1 for selected R-values. “U-value” is the inverse of R-value. Adding insulation provides a significant reduction in heat loss.

Fiberglass/polystyrene beadboard

These are not a good choice in the greenhouse as moisture is absorbed, reducing its insulation value.

Reflective foil bubble insulation — A low-cost choice that has worked well for growers is to install a double-bubble insulation with foil faces. This provides insulation due to the air space in the bubbles and the reflective aluminum. Aluminum has low emissivity (high reflectivity) that reflects heat. Foil facing on the insulation achieves four purposes:

• Reduces moisture migration — greenhouses have a high humidity
• Provides some mechanical protection to the insulation
• Increases fire rating
• Reflects most of the radiant heat back into the greenhouse

There are two types of flexible materials available — single and double bubble with foil on both sides and a 3/16-inch thick closed cell polyethylene foam with foil on both sides. The foam material has a slightly higher R-value as it has more bubbles that trap the heat. These materials are available in rolls of 500 or 1000 square feet in several widths from hardware stores and home centers. The life is about five years.

Double-bubble with aluminum foil on both sides is easy to install on hoophouse walls by placing it between the inner layer of film and the hoops. In greenhouses with roll-up sidewalls, it can be removed and rolled up when the sides need to be opened in the spring for ventilation. Wall insulation allows plants to be grown right up to the sidewall without cold damage.

Insulation board

To get a greater R-value, rigid board insulation is commonly used for glass or double polycarbonate walls. The material should be a closed cell material such as polystyrene, polyurethane or polyisocyanurate. These materials are available in several thicknesses from ½ inch to 2 inches with R-values from 5-8 inches. The addition of a foil face on one or both sides reflects the heat back into the greenhouse. The insulation board can be attached to the walls with large head nails, tek screws or clips. This material is readily available at home centers and lumber yards.

A more permanent material commonly used for kneewalls in greenhouses with rigid glazing is Thermax Heavy Duty Insulation by Dow Chemical Company. It consists of a glass fiber reinforced isocyanurate foam core with 4 mil aluminum on one side and 1.25 aluminum on the other. Available in 4’ x 8’ sheets, it can be purchased with a joint closure system for easy installation. R-value is 7.2 for 1-inch thickness. This material will provide good service for many years. Manufactured insulated kneewall panels are available from some greenhouse suppliers.

Final thoughts

Growers should check with the building inspector or fire marshal to see if the material is allowed by the building code. In some cases, the officials may want the insulation covered with sheet rock or other material.