Roof vents should close snugly with no cracks or openings.
Greenhouse growers in Pennsylvania and elsewhere are under pressure these days to lower operating costs in order to stay competitive in today's dynamic market as they work to produce high quality, low cost crops.
part from labor, energy is probably the top operating cost for most growers. Therefore, one of the best ways to help reduce costs is to improve the energy efficiency of their operation. Energy efficient greenhouses cost less to operate, and in some cases perform better than less-efficient facilities. When you think about the fluctuating and (usually) rising costs of energy, it only makes sense to ensure that your greenhouse is as energy efficient as possible. The following list gives some of the most common successful measures that greenhouse growers can use to reduce energy use without compromising the performance of their greenhouse.
"Tighten up" the house
Many greenhouses are extremely "leaky", meaning that they have cracks, holes, and openings in the walls or roof that allow cold air to leak into the house and warm air to escape. This "infiltration" can account for a significant portion of a greenhouse's winter heating bill, and can often be eliminated with the help of a sharp eye, a can of spray foam, and a tube of caulk. This measure can often reduce your heating bill by 5 to 10%. Tightening up the house has the added benefit of improving control over airflow patterns in the house, which can make temperatures and humidity levels more uniform.
etal posts and frames that are embedded in a greenhouse's walls or roof are another spot where heat can leak out. Even though there is not a physical opening to allow hot air to escape, the metal, with its high thermal conductivity, provides an easy "pathway" for heat to move from indoors to out. Cover these structural elements with insulation whenever practical.
Add a thermal screen
Greenhouse coverings are clear in order to allow sunlight into the house. Unfortunately, clear panels are also poor insulators. You can minimize your nighttime heat loss in the winter by using a movable thermal screen, that can be drawn across the roof and walls of the greenhouse. Often these thermal screens can serve double duty - providing shade from excessive sunlight in midsummer, and providing thermal insulation during winter nights. The reduction in heating costs will vary depending on your situation, but can be as high as 30 or 40%.
Seal the fans
When ventilation fans are turned off, the fan's louver will automatically close the fan opening. At least that's how it's supposed to work. Unfortunately, bent or malfunctioning louvers are all too common in greenhouses, as well as drilled holes or gaps around the fan housing. This leads to air leakage during the winter, which translates into higher heating bills. Malfunctioning louvers need to be repaired, and any holes or cracks should be covered over. Growers can also cover the fan inlet with a sheet of foam insulation board during the coldest months when the fan is not needed.
Insulate the perimeter of the greenhouse
One of the spots where heat is lost in the winter is along the perimeter of the greenhouse - through the ground and through the bottom part of the sidewall. You can reduce energy losses by installing an insulated board that extends from the height of the greenhouse's benches down into the soil along the greenhouse's perimeter. Typical savings will vary, but are on the order of 5% for Pennsylvania conditions.
Cross section drawing showing installation of perimeter insulation.
Insulate the north wall
The north wall of a greenhouse lets in surprisingly little light - especially in the winter when the sun is low in the southern sky. You may find that it's cost effective to cover the north wall of the greenhouse with insulating board to reduce heat losses. If the insulation is painted white, it can even enhance light levels inside the greenhouse by "reflecting back" winter sunlight that would have otherwise passed out through the north wall.
Fans that are clean and well maintained will use less energy.
Replace ventilation fans with high efficiency models
Ventilation fans vary in their performance. If you purchased the cheapest ones available, chances are they're also very energy inefficient. Now might be the time to upgrade to a higher efficiency model - ask the fan manufacturer to give you their product's "ventilation efficiency ratio", or look it up online. Larger fans are often more efficient than smaller ones, but the variation in efficiency from one fan to another can be quite large. (see graph)
High efficiency fans are often worth the extra cost.
Fan efficiency vs size. Note that the efficiency for a given fan size varies greatly from model to model. Also, larger fans tend to be more efficient than smaller ones.
*The Bioenvironmental and Structural Systems Lab maintains an online database of fan data information.
Upgrade the lighting
Not all greenhouses use electrical lighting, but those that do should look into the possibility of upgrading their system to a more efficient variety. The relatively high cost of lighting in a greenhouse makes this an important measure, especially if the lighting system is kept on for long periods of time.
Reflectors can be added to have bulbs direct more light down to the plants and reduce the number of fixtures needed.
Clean the fans
Greenhouse growers usually care a lot more about their plants than about their fans - that's one of the things that makes them good growers. However, it pays to look over the fans from time to time to see if they are in good operating order or if they need a cleaning. Accumulated dust on a fan's blades and safety screen can increase ventilation energy use by as much as 20%! All that is needed to correct this problem is a rag and some elbow grease. Be sure to de-activate the electrical circuit for the fan before starting, just to be safe.
Replace motors with properly sized, energy efficient models
Electrical motors operate most efficiently if they are running at full capacity - a motor that is twice as big as it needs to be will use extra electricity to keep itself energized, leading to additional energy costs. Electrical measurements by a competent electrician are needed to determine if a motor is oversized or not, but the savings from replacing oversized equipment can be significant. You can also look into switching to "energy efficient" motors that cost a bit more to purchase, but use less electricity than standard motors.
"Tune up" the control system
Automobiles need a regular tune up if they are to run properly, and the same is true of your greenhouse's control system. Whether you use mechanical thermostats or a fancy computer system, it is still worthwhile to check its operation to make sure everything is running properly. A thorough inspection of the control system requires a bit of technical knowledge about the controls equipment. However, it's possible for just about anyone to catch simple problems by simple observation. For example, if the ventilation fans are on while the heat is on, there's probably something wrong. If the lights are on in a greenhouse when there are no plants in the house, it's worth it to figure out why and correct the problem.
Other Ways to Save Money on Energy
The largest energy cost in most Pennsylvania greenhouses is the cost of heating in the winter. If you are using an expensive heating fuel, it may be worthwhile to switch to a more economical option. This does not reduce the amount of energy that you are using, but it can reduce your operating costs significantly.
Typical breakdown of annual energy use for a Pennsylvania greenhouse.
One of the more interesting options for fuel is to burn wood or other biomass - often this is very cost effective as well as being renewable and locally produced. It's not always easy to tell which fuel is the cheapest to use - some fuels are sold by the gallon, some by the cubic foot, others by the ton, and so on. Penn State Extension's "Energy Selector" tool can help you cut through this confusion and figure out which fuels are the most economical to use.
Energy efficiency for your greenhouse is a great way to reduce your operating costs and improve the profitability of your operation. Keep in mind that not every one of the measures described here will be appropriate for every house - sometimes an energy conservation measure is simply too expensive to install, relative to the expected savings. A careful energy assessment is the best way to determine which measures will be worthwhile. However, it's a good chance that some of the suggestions above will help make your greenhouse more energy efficient and cost effective in the coming years. Contact your Extension office if you have questions, or take a look at Extension's website.
Prepared by Daniel Ciolkosz, Penn State Department of Agricultural and Biological Engineering. Reviewed by Robert Berghage and Jay Holcomb, Penn State Department of Horticulture.