About 92% of cannabis growers in the United States choose to grow indoors while about 6% grow outdoors and just 1% are growing in greenhouses (Oregon Liquor Control Commission, 2015). For many cultivators, the decision to grow indoors is clear: the plants are not at the mercy of outdoor weather conditions, so production can happen year-round. To maintain the ideal indoor environments, facilities use energy intensive processes like heating and cooling systems, dehumidification, venting, and artificial lighting (Hesa et al., 2017). Despite this intensive energy use, outdoor weather conditions can still impact indoor cultivation. This is because many buildings are not designed to withstand extreme weather conditions (Jentsch et al., 2008). This problem will be worsened by global climate warming which is increasing the intensity and occurrence of extreme weather events. This blog post will address three main questions:

How can we measure the impact of weather on our cultivation facility?

Braingrid makes versatile and easy-to-install sensor devices (Sentrollers) that track environmental data such as temperature and humidity. In addition to collecting indoor environmental data, Braingrid taps into outdoor weather stations near the facilities we service to collect information about the outdoor environment. While analyzing the relationship between outdoor and indoor conditions, we were surprised to see that at every facility tested, the outdoor conditions impact the indoor cultivation environment! Braingrid quantified this effect through the Outdoor Weather Impact Score (OWIS). The OWIS indicates the effect of extreme weather on indoor conditions. For example, a room that is unaffected by outdoor conditions will have a score of 0 while a room that is completely affected can reach a score as high as 100. Examples of OWIS scores resulting from hot and cold weather are seen in Figure 1.

Figure 1: Example of OWIS score for a cannabis facility. The OWIS score ranges from 0 to 100 and the lower the score, the more insulated the facility is from outdoor conditions. Each room in the facility (indicated by various coloured diamonds) is assigned a heat and cold score representing the effect of hot and cold outdoor weather on indoor conditions.

What factors make a cultivation facility sensitive to weather conditions?

While it’s helpful to have a measure like OWIS to evaluate and track facility performance, many growers want to know the cause of poor scores. Two main factors impact the OWIS score: (1) the climate, and (2) the facility’s construction. Facilities located in severe climates tend to get higher OWIS measures. In particular, facilities in cold climates (like here in Canada) are most likely to experience changes to the indoor environment, even when the building is highly efficient (Hong, Chang, & Lin, 2014). Hotter climates also impact indoor conditions, but to a lesser extent than a cold climate. Regions with mild or mixed climates have the least impact on indoor conditions (Hong et al., 2014). Extreme cold or hot weather impacts HVAC energy use and can overwhelm HVAC systems. This is because the cooling and heating loads of buildings are dependent on weather. Other outdoor environmental factors, such as humidity, wind speed, and solar radiation can also impact indoor conditions and elevate HVAC use (Hong et al., 2014).

Facility-specific features can also increase the OWIS measure. The facility’s orientation can cause some rooms to receive more solar radiation (and thus be hotter) than others. Large- and medium-sized buildings are more susceptible to weather conditions than smaller buildings (Hong et al., 2014). This is because large facilities have more exposed surface area and window area compared to smaller buildings. Other facility-specific features, such as building materials, choice of HVAC system (and other equipment), and building layout can also affect the OWIS, however these factors need further investigation. Figure 2 shows the OWIS score for three Canadian cannabis facilities. The facilities differ in location, size, and construction, and we can clearly see the impact that this has on the OWIS score. For example, facility A is the north-most facility and it has the highest OWIS measure. Facility B is located south of facility A, but it is the largest facility, and so it is more subject to weather fluctuations than the other facilities.

Figure 2: The OWIS Cold Score varies from facility-to-facility as a result of various factors, such as location, weather, and materials used in building the facility. In this example, Facility A is the north-most facility while Facility B is the largest facility.

Is a High Weather Impact Score a Bad Thing?

While a high OWIS may be concerning to growers, it is important to put this information in context. Many species of plants – including cannabis – are accepting of a range of temperature and humidity conditions. For example, many cannabis strains have a range of temperatures where growth is not significantly affected (Chandra et al., 2008; Werf et al., 1995). Each cannabis strain can have a unique temperature range, and this can be used to manage a facility that is impacted by weather. For example, strains that tolerate hotter temperatures can be positioned in rooms that are affected by hot outdoor weather in the summer. Along the same lines, strains that tolerate cooler temperatures can be grown in rooms that tend to become cool, dry, and drafty in the winter. Research shows that the acceptance of hotter indoor temperatures in summer and cooler indoor conditions in winter leads to less use of air conditioning and heating (Podorson, 2015). The above strategy will result in a higher Outdoor Weather Impact Score but has the potential to save electrical energy and money.

On the other hand, there are plenty of cannabis strains that have low tolerance to extreme environmental conditions. Extreme temperature and humidity can also increase the risk of plant diseases, pests, and other pathogens — and no budtender wants to sweat so much that they irrigate the plants! Fortunately, there are some short- and long-term changes that can be made to the facility to reduce the impact of weather on indoor conditions.

How can a grower compensate for outdoor weather conditions while reducing energy costs?

Short-Term Changes

  • Install Sentrollers in challenging areas

The Outdoor Weather Impact Score is calculated for each room in a facility. If one of your grow rooms is consistently scoring poorly on this measure, you may consider adding additional sensors in these rooms. In particular, you may target drafty areas like windows, doors, and outside facing walls.

  1. Keep windows and doors closed

Just like your own house, keeping the doors and windows closed will prevent outdoor weather from coming indoors. This is especially true if doors and windows are located on opposite sides of the room, as cross-breezes can easily happen.

  1. Look for leaks and use draft blockers

Every few years, look for leaks in your facility that allow air to flow in and out. There are different methods for finding these leaks, and some utility companies will do free building energy audits. Seal up leaks and use draft blockers to decrease air flow in unwanted places.

  1. Change the facility’s schedule

Scheduling the light-on and lights-off periods in different rooms at opposite times can help prevent extreme heat during summer days and cold during winter nights. Utility demand charges are often calculated based on time-of-use, and this scheduling could cut overall energy costs. This method may prevent growers from using natural sunlight as a source of light and could interfere with plant circadian rhythms.

Figure 3: Skylights are one option for introducing free lighting that generates less heat. They benefit plants by providing wide-spectrum lighting and reduce building electricity costs.

Long-Term Changes

  1. Grow deciduous trees outdoors

As many growers know, plants are excellent at absorbing heat in the summer and providing insulation against wind in the winter. Planting trees that stay green year-round will help shade the facility in the summer and will insulate the facility from wind in the winter.

  1. Consider lighting that generates less heat

There is a lot of hype surrounding the use of LED lights in the cannabis industry. Growers report mixed results with LED lights because there is a lot of variation in performance among different LED products. Two things that is clear about LED light is that they produce less heat and are more energy efficient than traditional HPS lamps. There is evidence that some LED lights can match the light output of a traditional HPS lamp with less heat generation and electrical power – so it’s worth doing your homework to find a good LED fixture (Bantis et al., 2018).

Skylights are another option for introducing free lighting that generates less heat. Skylights can be used in combination with traditional or LED lighting in an indoor facility. They benefit plants by providing wide-spectrum lighting and reduce building electricity costs. A variety of glass options exist which allow for specific wavelengths to be filtered out of sunlight that give the grower greater control of their cultivation facility.

  1. Create positive pressure within the facility

Designing HVAC systems to maintain positive air pressure in grow rooms will help prevent hot or cold air from blowing into the building. Positive pressure has the added benefit of pushing spores and insects away from entrances (Mears, 2002). When internal pressure is high enough, air passes through open doors and other building openings at high enough speeds that outdoor air, pests, spores, and pollen are expelled. To remove excess heat, vents must be positioned strategically (i.e., near the roof) so that warm, rising air is pushed out (Mears, 2002). A drawback of creating positive pressure within a facility is that odours can also get pushed out of cracks and crevices before they have a chance to be treated properly.

  1. Add desiccant dehumidification

Like temperatures, outdoor humidity can also impact indoor conditions. One way to combat high humidity in an energy-efficient way is to use desiccant dehumidification in addition to your current dehumidification system. Desiccants are materials that readily attract water without mechanical cooling. Research indicates that desiccants can reduce HVAC electricity use by 30 – 60% (Podorson, 2015). Combined conventional and desiccant dehumidifiers can be purchased and some even have economizers built-in, which will draw in outside air (when it is dry enough) to get free dehumidification. (Podorson, 2015).

  1. Use an economizer

Economizers are devices that can be integrated into air conditioning units that will draw in outside air when it is the cool enough. In the summer, an economizer will have minimal impact, but in cooler months, heat gains of the grow light can be mitigated with cold, free outside air. On dry winter days, this also has the benefit of providing free dehumidification (Podorson, 2015).