Year round greenhouses, or even those used for season extension, face the challenge of extreme temperature swings. The structure collects excessive amounts of heat during the day when the sun is out, causing them to easily overheat. However, they quickly lose this heat at night due to a lack of insulation, resulting in over-cooling or possibly freezing.
Many growers turn to heating and cooling to maintain a stable temperature – ventilating during the day and heating the greenhouse at night. While reliable, these strategies can be costly and unsustainable. Thermal mass materials present a natural alternative to evening out temperature swings maintaining a suitable growing environment without expensive heating and cooling.
Thermal mass materials are dense materials which store heat. They absorb thermal energy during the day, either from direct light or the heat of the greenhouse, and re-radiate this heat back into the greenhouse when the temperatures drop at night.
Water is the most commonly used thermal mass in greenhouses for two reasons: it has the highest heat capacity per volume of any of readily available material, and it is cheap. The only needed component is a storage container – an abundant commodity in our plastic-laden society.
By stacking several large drums of water in a greenhouse, a grower can create a ‘water wall’ — a large and low-cost thermal battery for the greenhouse. The disadvantage with this low-cost climate control strategy is primarily that water takes up considerable space in the greenhouse which could otherwise be used for growing. Incorporating water walls often requires building a bigger greenhouse or taking up some of your existing growing room. Thus, water walls are most commonly used in large backyard greenhouses or small-scale commercial greenhouses – structures large enough to accommodate the extra space easily.
Other disadvantage is a lack of precise control. Unlike a ventilation fan or heater, which you can set to operate at a specific temperature, thermal mass can have a varying and unpredictable effect. To absorb heat during the day, water walls rely on passive solar energy. Thus, they can have are dependent on the outdoor climate, and will have limited effect during periods of cold and cloudy weather. They are particularly well suited for structures which do not rely on electricity or supplemental heating, such as off-grid or passive solar greenhouses.
In the winter months, the goal of a water wall is to absorb as much light and heat during the day, so that it can store this thermal energy for use as heating at night. To this end, water walls should be exposed to light in the winter to absorb as much heat as possible during the day.
In the warmer months, the opposite effect is desired: in most climates with warm temperatures, the goal is to keep the greenhouse sufficiently cool. Water walls can help by passively absorbing heat from the surrounding air inside. During this time, they should be shaded.
Accomplishing these two tasks –exposed to light in the winter and shaded in the summer — is easily done by strategically orienting the greenhouse. Following passive solar greenhouse design, the majority of the glazing should face the south if located in the northern hemisphere. With this orientation, water barrels can be located on the north side of the greenhouse. A section of insulation or covering in the roof will shade the water barrels in the summer (when the sun is higher in the sky), and leave them fully exposed to direct light during the winter.
The length of the insulation or cover in the roof will determine when the water wall is exposed to light (heating season) or shaded (cooling season) based on the solar angles at your location. To find the right length of insulation, sketch the greenhouse profile, the dimensions of the water barrels, and the angle of the sun during the summer and winter solstices (and / or equinoxes). You can then estimate the length of the roof insulation that allows the mass to be shaded in the summer and fully exposed to light in the winter.
Large water containers are usually 55-gallon barrels, either plastic or steel. Steel barrels are better for energy performance: steel is more conductive than plastic, and can exchange heat with the air at a faster rate. Yet steel is also prone to rusting. Barrels usually rust from the outside first, due to constant exposure to humidity and condensation inside the greenhouse. To reduce rust, keep barrels off of wet soil and use plastic lids on top of the barrels to prevent water from collecting in the lid.
Most water walls are made out of plastic barrels that can be found cheaply and re-used. Landscaping companies, your local roads / highway department, scrap yards, and Craigslist are good sources for low-cost barrels.
Typically water walls are made by stacking several drums, as this allows for the most space-efficient way to incorporate a large volume of water. Keep in mind that a full 55-gallon water barrel weights almost 500 lbs. and is heavy enough to crush you if it were to fall. Water walls need to be stable and structurally supported. They should be on a level surface. Longtime proponents of water as thermal mass Penn and Cord Parmenter advise building in a 2’ wide concrete slab on which to stack barrels. The rest of the floor can be open to the soil, or a flooring of your choice. If not concrete, consider other ways to create a level floor like pavers or flagstone. If you are stacking drums on top of one another be sure to use a strong and level surface between layers.
Freezing a 55-gallon drum of water in a year-round greenhouse is rare; as this requires a very long period of freezing temperatures (which would likely kill plants). However, this can happen if the greenhouse is un-used and open through the winter. Freezing a barrel will likely break it. If you do not plan to use the greenhouse year-round (three-season growers), and live in a climate with harsh winters, you may need to drain the barrels, a cumbersome task. Thus, water walls are best suited for year-round growers.
An additional tip is to leave a small air gap at the top of the barrel when filling it. This is because water expands when it warms up during the day. After barrels are installed in the greenhouse, fill them on site (since they are too heavy to lift) leaving a small gap at the top.
Dark colors absorb more heat than lighter colors. Conventional wisdom says to paint drums black. This comes with the drawback that now the greenhouse has a large black wall, which can detract from the feeling of an open green space. Some studies show that blue or red colors absorb roughly the same amount of heat, and reflect light in the red and blue spectrum that plants need for growth.
There is also the option to be creative with water barrels. The year round greenhouse at The Cheyenne Botanic Gardens in Cheyenne Wyoming, for example, incorporates tall light blue columns of water which bring both passive climate control and a beautiful aesthetic to the passive solar greenhouse. The blue color comes from a fungicide added to the water to prevent algae growth. If using a translucent container, consider using a fungicide, as algae will likely occur when water is exposed to light.
As mentioned above, the major drawback to large water containers is that they take up considerable room in the greenhouse. It is critical to plan the location and spacing of mass when designing your greenhouse floor plan so you don’t end up with too little growing room.