Add Thermal Mass Materials to Your Off-Grid Greenhouse

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This greenhouse in Colorado, built by Smart Greenhouses LLC, has large water barrels along its back wall to collect, store, and distribute heat.
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These fiberglass barrels provide year-round climate control, and are as effective as they are attractive.
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Water walls should be exposed to light in winter, to absorb as much heat as possible during the day, and exposed to less light in summer.
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This water wall at Cheyenne Botanic Gardens in Cheyenne, Wyoming, heats up to 70 degrees Fahrenheit in summer and 60 degrees in winter.
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Thermal mass takes up a significant amount of space, but it’s low-cost and maintenance-free.

All greenhouses use the sun for heat during the day. At night, most greenhouses quickly lose that heat because of the poor insulating quality of their materials. The reason for this inefficiency has to do with some basic principles of traditional greenhouse design that focus on maximizing light. Glazing materials, such as glass or clear plastic, are good at letting in light, but they’re terrible at retaining heat.

Solar greenhouse design takes a different approach, finding a balance between glazing and insulation to create a structure that naturally resists overheating and overcooling without reliance on fossil fuels. Instead, the sun provides the energy, and the greenhouse collects and stores that energy to provide its own heating when required. The use of thermal mass materials is the oldest and simplest strategy for storing heat and naturally mitigating temperature swings.

How Thermal Mass Works

When light hits a material, some of it is absorbed and converted to heat. Thermal mass materials absorb this heat via conduction. Heat slowly conducts from the surface to the center of the mass, allowing the entire volume to heat up by a few degrees in a day. When the air temperature in the greenhouse drops at night, the mass slowly starts radiating this heat through conduction and by releasing short-wave infrared radiation. In this way, thermal mass regulates the temperature of the greenhouse. It absorbs energy from the sun during the day and slowly re-radiates it as heat at night (or whenever the air temperature drops below the temperature of the mass), which evens out daily temperature swings.

Because mass materials store heat, they’re often referred to as “heat sinks,” “thermal banks,” or “thermal batteries.” The idea is the same: to store as much heat as possible when it’s oversupplied, and slowly distribute it when it’s in demand. To make this possible, you must maximize the materials’ exposure to light in winter (or whenever greenhouse heating is needed). This will allow the sinks to charge as much as possible during the day so they can radiate heat later. Even if the materials aren’t directly illuminated, they’ll still absorb some heat from the surrounding air. This means they can have some effect on overcast days; however, the predominant driver of a sink’s ability to store heat is direct light absorption.

During summer, the goal of thermal mass is to help keep a greenhouse cool. To that end, the mass should be shaded during summer months. On summer days, it will still absorb some heat from the greenhouse as hot air moves across it. It will then radiate this heat at night, given a sufficient drop in temperature, and resume cooling the next day.

Choosing a Thermal Mass Material

All materials absorb and store heat to some extent, but some are much more effective than others. The amount of heat a material can store is called its “heat capacity” and is determined by two factors. First, the material’s specific heat is a property of the material, defined as how much energy is required to raise its temperature 1 degree Fahrenheit.

Density is the second factor that affects a material’s heat capacity. The more mass in a given volume, the more energy it can store. Most mass materials, such as stone and concrete, are effective because they’re dense.

When you multiply these two characteristics — specific heat and density — the result is a material’s “volumetric heat capacity” (check out the chart below for the volumetric heat capacities of common building materials). This is the most useful unit of measurement because it indicates the total heat stored in a given volume of the material. In a greenhouse setting, volume is an extremely important factor because mass materials can take up considerable space that could be otherwise used for growing.

Volumetric Heat Capacities of Common Building Materials

Material  Heat Capacity per Volume (Btu/ft3/F) 
Water* 62
Steel 59
 Rock or stone (30 percent air gaps)  25
Soil (damp, light soil) 25 
Concrete 25 to 32
Brick 23 to 35
Adobe 20
Wood (pine or fir) 18
Sand 18
Air 0.018
*Helpful conversion: 1 gallon of water has a volumetric heat capacity of 8.34 Btu.

Note that heat capacity is a different property than conductivity, the rate at which a material conducts or transfers heat. Thermal mass materials aren’t good insulators, generally speaking. They can store a lot of heat, but they easily conduct heat with their surroundings. This means they should be insulated from the exterior and shouldn’t be substituted for insulation.

Every greenhouse inherently contains some thermal mass in its structure, soil, water, and plants. While not insignificant, these sources of mass are usually not enough to provide year-round climate control. Thus, thermal mass materials are integrated in larger quantities, usually via water or masonry.

Water as Thermal Mass 

Water is far and away the most commonly used thermal mass in greenhouses. It has the highest heat capacity per volume of any readily available material, and it’s cheap. Stacking several large drums of water on the north wall of a greenhouse will create a “water wall,” a large, low-cost thermal battery.

Water walls should be exposed to light in winter and shaded in summer. A partially insulated roof can easily accomplish this if the water barrels are on the north wall of the greenhouse. The length of the insulation will determine when the water wall is exposed to light or shaded, based on the solar angles at your location (the University of Oregon offers a software tool to create sun path diagrams). To find the right length of insulation, sketch the greenhouse profile, the dimensions of the water barrels, and the solar altitude angles during the solstices (or equinoxes). By using a protractor or a program such as SketchUp, you can estimate the length of roof insulation that will allow the mass to be shaded in summer and fully illuminated in winter. (See illustration in slideshow).

We strongly recommend adding this section of roof insulation. Not only will it enhance the effect of the thermal mass, but it will also reduce heat loss through the roof, where heat loss is greatest.

Selecting Water Containers

Large containers will allow you to efficiently incorporate more water into a greenhouse space. They also have a lower surface area relative to their volume compared with small containers. Surface area determines how quickly a material can conduct heat with the surrounding air. To illustrate, let’s compare one 55-gallon barrel to 55 individual gallon milk jugs. While both contain 55 gallons of water, the large drum has a smaller surface area relative to its volume. Because it has a smaller area to conduct heat, the large drum takes much longer to heat up and cool down than the individual jugs. The large drum has a smaller effect on the greenhouse temperature in the short term, but it’s able to store heat over a longer period. Thus, larger containers are more useful for keeping a greenhouse above freezing over prolonged cold and cloudy periods. When water is the primary heat-storage mechanism, most growers go full-bore and use large containers. They’re easier to stack and store, allowing large volumes of water to be placed space-efficiently.

The most common containers are 55-gallon barrels, either plastic or steel. Because metal is more conductive than plastic, steel barrels are able to transfer heat more quickly — but they’re also prone to rusting, so they have shorter life spans. Barrels usually rust from the outside first, as water collects in the lid of the barrel and continually evaporates off the surface. If using steel, keep barrels off wet soil and consider using plastic coverings on top of the barrels.

Both plastic and steel barrels can be found cheaply and reused. Landscaping companies, your local transportation department, scrap yards, container companies, and Craigslist are good sources.

Water walls are simple in concept, but come with a serious need for caution. A full 55-gallon water barrel weighs almost 500 pounds — 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, either concrete or flagstone. Longtime solar greenhouse designers Penn and Cord Parmenter advise building a 2-foot-wide concrete slab under the barrels, while the rest of the floor can be open to the soil for planting. If you’re stacking barrels, use stable shelving between the layers. Penn and Cord go one step further and place wood strips across the barrels to ensure they don’t topple.

Other Tips for Using Water Barrels

Leave an air gap. Fill barrels onsite after installing them in your greenhouse. Leave an air gap because the water will expand slightly as it warms up during the day.

Avoid freezes. A large water container will likely break if it freezes completely and the water expands beyond the container’s capacity. A 55-gallon drum of water freezing is rare, but it can happen if the greenhouse is left open through winter — something to consider if you’re a three-season grower.

Paint drums a dark color. Darker colors absorb more heat, and maximizing heat absorption is a primary goal of thermal mass. Paint your greenhouse interior white to reflect light, but paint the water containers a dark color. Studies show that dark blue or red absorb almost the same amount of heat, and they reflect light in the blue or red spectrum, which is more useful for plant growth.

Incorporate thermal mass into your floor plan. A standard 55-gallon drum is 2 feet in diameter. Plan the location and spacing of mass when designing your greenhouse floor plan so you don’t end up with too little growing room.

Sizing Passive Mass

How much water does the average greenhouse need to stabilize temperature swings? A wide array of design variables means the amount required is different for each greenhouse. A general rule of thumb is to use 2 to 5 gallons of water per square foot of glazing area if thermal mass is your primary climate-control strategy. Colder climates should be on the higher end of this range and warmer climates on the lower end. This recommendation is relative to glazing area, not footprint, because glazing is a better indicator of the greenhouse’s heat gain and loss. A volume of concrete, for instance, has about half the heat capacity of a volume of water, so twice as much is needed to achieve the same effect.

This is a very general recommendation because so many factors go into sizing thermal mass: the climate, the design, the mass material used, the container it’s in, and where it’s used. But this rule has kept greenhouses above freezing in a range of climates. Most of the examples that rely on thermal mass are in cold and sunny climates, such as Colorado and Wyoming, but thermal mass has been successfully used in cloudy climates too.

Pros and Cons

The main drawback of thermal mass is that it will take up a large amount of potential growing room in a residential greenhouse. Its effectiveness is limited in cold and cloudy areas, such as parts of Canada or the northeastern United States, and it’s less useful in hot climates that don’t experience significant daily temperature fluctuations, such as the southern United States. Also, thermal mass won’t distribute heat very evenly, but will instead create a pocket of warm air directly around the mass. You can modify your planting plan to suit these growing zones, or use fans to help distribute the heat throughout the greenhouse.

Thermal mass materials do have plenty of advantages. They’re low-cost (in the case of water) and can serve other useful functions (in the case of masonry). Because they’re simple to install and maintenance-free, they’re good for people interested in DIY. Most importantly, they don’t require any electricity, helping facilitate a year-round passive solar greenhouse in an off-grid location.

Lindsey Schiller and Marc Plinke co-founded Ceres Greenhouse Solutions to research, design, and build energy-efficient greenhouses. This article is excerpted from their book The Year-Round Solar Greenhouse (New Society Publishers)

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