Infiltration and Heat Loss

Infiltration heat loss is caused by unplanned air entering and leaving a greenhouse (Figure 4). Air will infiltrate the greenhouse through gaps around doors and joints, through holes in the glazing, between the ground and the sill plate, and out open flue pipes and other openings in the greenhouse covering. Maintenance of the glazing and the accessories that protrude through it (fans, louvers, heater exhaust) helps reduce heat loss due to infiltration in all types of greenhouses. Infiltration heat loss in glass greenhouses can be particularly high due to the large number of joints compared to plastic films or polycarbonate/acrylic sheets. It can be reduced by replacing deteriorating joint materials (supports, caulking) or by placing one or two plastic films over the top of the glass to reduce infiltration and increase thermal resistance.

Table 1 provides infiltration ranges for greenhouses with different glazing types. The greenhouse type, condition, and wind velocity will affect the infiltration.

 

Table 1: Typical Infiltration ranges for greenhouses with different glazing types. Source: Scott Sanford. Ref: Energy Conservation for Commercial Greenhouses, NRAES-3, Natural Resource, Agriculture, and Engineering Service, 2001.

Table 1: Typical Infiltration ranges for greenhouses with different glazing types. Source: Scott Sanford. Ref: Energy Conservation for Commercial Greenhouses, NRAES-3, Natural Resource, Agriculture, and Engineering Service, 2001.

 

The infiltration rates are difficult to judge without some measuring equipment. A blower door test unit can be used to measure the infiltration in a greenhouse. These units are typically used for testing the infiltration rates of homes but can be adapted for greenhouses. If the greenhouse is large or leaky, multiple test units may be needed. Turning on a small exhaust fan and using smoke sticks or a bee smoker is a low-cost first step to finding and reducing infiltration leaks.

 

Figure 5: Conifer and deciduous tree windbreak.  Source: USDA-NRCS

Figure 5: Conifer and deciduous tree windbreak. Source: USDA-NRCS

Windbreaks

The wind speed over a greenhouse can cause negative pressure within the structure and increase the infiltration rate. Windbreaks built to buffer the prevailing winter wind can help reduce infiltration rates. One quick option is to build some type of wall or high snow fence, although this is often not economical or practical. A more permanent option is to plant a windbreak of trees and bushes (Figure 5). A mix of evergreen and deciduous trees of different species is best, so if there is a disease problem the whole windbreak won’t be lost. To speed up your windbreak, plant fast-growing hybrid poplar, willow and white pine along with a mixture of slower growing trees. The windbreak should be planted to break the prevailing winds, which are generally from the west and north during the colder months. The windbreak should be located upwind of the greenhouses at a distance of four to six times the estimated mature height of the trees. If the windbreaks are too close to the greenhouse, it may block sunlight during certain times of the day. This could result in slower plant growth and large amounts of snow being deposited on the greenhouse roof.