Heating a Greenhouse: A Homeowner’s Guide

When winter approaches, it’s a good idea to start thinking about how you’ll protect your plants from freezing. Even on a sunny winter afternoon, temperatures can quickly drop once the sun goes down, leaving your plants at risk of frost.

This leaves you with two options: Either bring your most sensitive plants indoors or heat your greenhouse to keep your plants happy.

But how do you heat a greenhouse? Will you need to buy extra equipment, and if so, how much will it cost?

In this guide, we'll walk you through all you need to know about heating a greenhouse in the winter, from calculating your heating requirements and comparing different systems to keeping costs down.

Does Every Greenhouse Need Heating?

Not every greenhouse needs supplemental heating. It depends on your local climate, what you're growing, and how warm you need the greenhouse to stay.

There are a few things to think about.

How Warm Does an Unheated Greenhouse Stay at Night?

Greenhouses capture plenty of warmth on sunny winter days, but once the sun goes down, that heat starts escaping. In fact, an unheated greenhouse can eventually cool to nearly the same temperature as the air outside.

How quickly it cools depends on several factors:

• Glazing and insulation: Better-insulated coverings slow heat loss.
• Outdoor temperature and wind: Colder, windier nights can increase heat loss.
• Air leaks: Gaps around doors and panels let warm air escape.
• Stored daytime heat: Water barrels and other heat-retaining materials can release some warmth overnight.

When Should You Have Supplemental Heat?

If your greenhouse is likely to get colder than your plants can tolerate, you'll need a protection plan.

Check your plants’ thresholds.

For example, kale can survive temperatures around 20°F, but bush beans can be killed by even a light frost.

Now, you might not necessarily be heating your greenhouse to produce vegetables. It might just be to protect your dormant plants, in which case you’ll want a different plan.

Choose Your Winter Temperature Target

Keeping a few plants alive until spring vs. continuing to grow vegetables through January are two very different goals, and they'll greatly affect how much heat you need.

Four Common Winter Growing Goals

Seed starting is a good example of why this is such an important consideration.

For instance, tomato seeds germinate best when the growing medium is around 75 to 85°F, while established greenhouse tomatoes generally need nighttime temperatures around 60 to 65°F for productive growth.

Find Your Minimum Acceptable Temperature

First, research the temperature requirements of your specific plants. Look at the difference between the lowest temperature they can survive and the temperature they need to grow well.

Once you've chosen a minimum indoor temperature, you can compare it with your area's expected winter lows.

Keep that temperature in mind, as it’ll be an important part of calculating how much heating capacity your greenhouse needs.

Why Do Greenhouses Lose Heat So Quickly?

Once you have a temperature target, the next step is figuring out how much heat your greenhouse is likely to lose. There are several things that can affect heat loss, including your greenhouse’s materials, how tightly it’s sealed, its shape, and its size.

The Greenhouse Covering

You'll often see two measurements used to describe insulation performance:

• U-value: Measures how easily heat passes through a material (the lower the number, the less heat escapes)
• R-value: Measures how well a material resists heat flow (the higher the number, the better the insulation)

We'll use these values later when calculating your greenhouse's heating requirements.

Note that glass, polyethylene, and polycarbonate all transfer heat at different rates, but the number of layers and the space between them also make a difference. For instance, twin-wall polycarbonate generally insulates better than single-pane glass, while double-pane glass can insulate better than single-layer polycarbonate.

Drafts and Air Leaks Add to the Problem

If cold air keeps getting in, well-insulated panels won’t do you much good. Check for gaps around doors, loose glazing panels, and poorly sealed joints, all of which can allow warm air to escape.

Wind

Wind can push cold air through small openings, increasing heat loss.

So if your greenhouse is in an exposed location, you may need more heating capacity than if it were in a sheltered area.

Design

While size is important, your floor area isn’t everything.

Two greenhouses with identical footprints can have different heating requirements if one has taller walls, a larger roof, or less efficient glazing. That's why you’ll use the total exposed surface area, along with other heat losses, when sizing your heater.

Reducing Heat Loss Before Buying a Bigger Heater

If you're looking for the cheapest way to heat a greenhouse in winter, start by making sure you're not losing more heat than necessary. You’d be surprised by how much a few improvements could do for your heating.

Here are a few things to put on your to-do list:

• Seal air leaks: Check around doors and glazing for gaps that let cold air in.
• Repair loose panels and vents: Make sure the panels fit securely and ventilation shutters close completely.
• Consider insulated glazing: If you're buying or upgrading a greenhouse, twin-wall polycarbonate provides better insulation than single-pane glass. Vista Greenhouses' residential greenhouse collection includes several models with insulated panels.
• Install thermal curtains: These create an extra insulating barrier between your growing area and the cold greenhouse covering.
• Check the foundation: Address exposed or uninsulated perimeter areas where heat can escape.

Thermal curtains can be particularly helpful in larger greenhouses, since keeping a large interior space warm can get expensive.

Greenhouse Heating Calculator - How to Estimate Heater Size

When heating a greenhouse, you need a heater powerful enough to replace the heat it loses on a cold night. But unlike sizing a ventilation fan, you can't calculate this using just greenhouse volume.

Here's how to get a rough estimate.

Step 1: Find the Temperature Difference

First, find your area's winter design temperature, which is a low outdoor temperature used to plan heating capacity. This isn't necessarily your average winter low or the coldest temperature ever recorded. Just look for a local heating or winter design temperature that represents the typical cold-weather condition your heater should be able to handle.

Subtract that number from the temperature you want to maintain inside your greenhouse.

Step 2: Calculate Heat Loss Through the Covering

Next, calculate how much heat escapes through your greenhouse walls and roof.

The formula is:

For our example, imagine an 8 × 12 greenhouse with 6-foot sidewalls and an 8-foot peak. Its exposed wall and roof area is approximately 363 square feet, not its 96-square-foot floor area.

We'll assume it's covered in separate double-layer polyethylene film with an insulating air space between the layers, which has a documented U-value of approximately 0.70.

That's the estimated heat escaping through the covering.

Step 3: Account for Air Leaks

Cold air entering through gaps creates additional heat loss.

A simplified heat-loss formula is:

Our example has a volume of 672 cubic feet. Assuming one natural air change per hour:

Step 4: Add Everything Together

Combine the covering and air-leakage losses. You'll also need to account for foundation or perimeter losses where applicable.

For our example, the University of Georgia's calculation method uses an uninsulated perimeter heat-loss coefficient of 0.8 BTU per linear foot per °F. Our 8 × 12 greenhouse has a 40-foot perimeter, so:

Step 5: Convert BTU/hr to Watts if Needed

If you’re comparing electric heaters, divide your required BTU/hr by 3.412 to estimate the equivalent heating wattage.

That’ll give you a starting point, but make sure to check the manufacturer's actual heating output and your available electrical capacity before you buy.

Types of Greenhouse Heating Systems

There are several types of greenhouse heating systems, and they all work differently.

Before we dig into each one, here's a quick look at your options.

You can also use a hybrid approach.

For example, you might use root-zone heating to warm seedlings while a separate heater maintains a safe air temperature during cold nights.

Electric Heating for Greenhouses

The great thing about electric heaters is that they don't require fuel storage or produce combustion gases, but you'll need to account for electricity costs and the power that’s available to your structure.

How Electric Greenhouse Heaters Work

Electric resistance heaters convert electricity into heat. There are a few common designs:

• Fan-forced heaters: Push warmed air throughout the greenhouse.
• Radiant heaters: Transfer heat directly to nearby plants and surfaces.
• Oil-filled heaters: Use an electrically heated fluid to warm the heater's surface, which then releases heat into the surrounding air.

When to Choose Electric Heating

Electric heating is a viable choice for a small greenhouse, especially if you have an adequate electrical supply.

Many systems use thermostat automation, meaning they switch on automatically when the temperature goes below a certain threshold.

The main thing to look out for is operating cost. Your bill will depend on your local electricity rates and how often you run your heater.

Checking Your Electrical Capacity

Before you buy a small greenhouse heater, check the following:

• Wattage and voltage: Does the heater require a standard 120V outlet or a 240V connection? If the latter, you might need an upgrade.
• Circuit capacity: Can the circuit safely handle the heater alongside any other connected equipment?
• Moisture protection: Are the heater and electrical connections suitable for their installation environment?
• Mounting and clearance: Can you install the heater according to its manufacturer's instructions, away from plants and combustible materials?

Don't use extension cords or power strips to run portable electric heaters. And if your greenhouse needs a new circuit or electrical upgrade, have a qualified electrician install it.

Propane and Natural Gas Greenhouse Heating

If your greenhouse needs a lot of heat or doesn't have a suitable electrical supply, you can consider fuel-fired heating. Propane and natural gas heaters can deliver plenty of warmth, but you'll need to account for fuel availability, installation, and ventilation requirements.

How Do Propane Greenhouse Heaters Work?

A propane greenhouse heater burns fuel to generate warmth. A natural gas heater works similarly, but it typically connects to a gas supply line rather than a refillable propane tank.

There are two main configurations:

• Vented heaters: Direct combustion gases outside through an exhaust system.
• Unvented heaters: Release combustion gases and moisture directly into the greenhouse.

The downsides of unvented equipment include potential toxic gas buildup and humidity-related disease, so you really have to manage airflow and combustion carefully. While they’re often less expensive, they require more thought.

If you decide to go ventless, Vista Greenhouses' RSI Propane Radiant Heating System includes an oxygen-depletion shutoff system that automatically turns the heater off if oxygen levels drop too low. It also features a built-in thermostat and circulation fan, although proper ventilation and regular maintenance are still essential.

Ventilation and Combustion Safety

Fuel-fired heating requires particular care.

Unvented heaters release combustion products and moisture directly into the greenhouse while operating. If the heater doesn’t get enough oxygen to burn fuel completely, or if ventilation is inadequate, carbon monoxide can create a serious safety hazard, while pollutants such as ethylene can damage plants.

Only install equipment approved for the intended application and follow manufacturer requirements, along with local codes. While you should always use appropriate carbon monoxide detection, never treat an alarm or oxygen-depletion sensor as a substitute for proper installation.

Other Greenhouse Heating Methods Worth Considering

Beyond electric and propane heaters, many growers use more targeted systems that warm specific areas instead of the entire greenhouse.

Root-Zone Heating

Starting seeds? You may not need to warm every inch of your greenhouse.

Root-zone heating uses electric seedling mats or warm-water tubing beneath growing benches to heat the soil or potting mix directly. It keeps young plants' roots at a suitable temperature.

Just remember that warm roots don't necessarily protect leaves and stems from freezing. You may still need supplemental air heating when temperatures drop too low.

Hydronic Heating

Hydronic systems heat water using a boiler or another heat source, then circulate it through pipes or tubing to distribute warmth.

They can provide consistent heating throughout your greenhouse, but the installation process is more involved. They're generally worth considering if you have a large greenhouse or more elaborate growing setup.

Radiant Heating

Instead of primarily heating the surrounding air, radiant heaters transfer energy directly to plants and nearby surfaces.

They run on electricity or gas, and how effective they are depends on where they’re placed.

Wood and Other Fuel Systems

Another possibility is having a wood-burning system. It’s a more demanding option, however, since you have to consider fuel storage and regular maintenance.

Plus, any combustion-based setup needs to meet manufacturer requirements and local regulations, especially if there’s fire safety involved.

Can You Heat a Greenhouse With Solar Power?

Absolutely! However, if you're thinking about using a solar greenhouse heater, it's important to understand how these systems capture sunlight, store heat, and use that heat to generate electricity.

Passive Solar Heating

During the day, sunlight passes through your greenhouse's glazing and warms the plants and soil.

This process is called passive solar heating. It's particularly effective in greenhouses designed to capture winter sunlight, but the amount of warmth available depends on the weather, how many hours of daylight there are, and your greenhouse construction.

How Thermal Mass Works

Water barrels, stone, concrete, and soil can absorb heat during the day and gradually release it as temperatures fall.

This is called thermal mass.

For example, placing water barrels where they'll receive direct sunlight can help moderate overnight temperature changes. The main difference here is that thermal mass doesn't generate heat. If a barrel doesn’t get much sunlight, it won’t store much warmth to release.

What About a Solar-Powered Electric Heater?

Solar panels produce electricity, which can power a heater you can use at night.

For this, you’ll need enough electricity generation and, for an off-grid setup, sufficient battery storage or another power source. The battery capacity must account for how much power the heater draws and how long it needs to operate.

The main thing to note here is that solar production varies with the season and cloud cover, so it’s not the most reliable source.

How to Heat a Greenhouse in Winter for Free

If you're researching how to heat a greenhouse without electricity, passive solar heating should be on your radar.

Sunlight is free. And it can be very useful for heating a greenhouse. But even purpose-built solar greenhouses may need backup heating during extended cloudy periods or extreme cold.

For most backyard growers in cold climates, solar energy is best treated as supplemental.

Thermostats and Automatic Heating Controls

Beyond having a properly sized heater, you need to have controls in place to decide when it turns on and when it shuts off.

With thermostat switches, your heater automatically turns on when the temperature drops below a chosen setpoint and shuts off once it reaches that temperature. Some heaters have built-in climate control thermostats, while others use a separate external controller. If you use an external controller, check the heater’s specifications to confirm that the controller matches its voltage and is rated to handle its electrical load in amps or watts. Just matching it to the voltage isn't enough.

For example, a 1,500-watt heater running on 120V draws about 12.5 amps, so a 120V controller that is only rated for 10 amps would still be undersized even though the voltage matches.

The Importance of Air Circulation and Humidity Control

Even with the right heater, temperatures won't necessarily be the same throughout your greenhouse. Warm air naturally rises, meaning the area near the roof can be much warmer than the air around your plants.

Good air circulation helps distribute that warmth more evenly and prevents cold pockets from developing around your growing area.

Keep Warm Air Moving With a Circulation Fan

Circulation fans gently move air around the greenhouse, helping distribute heat from your heater.

You can position fans to create a continuous airflow pattern through the growing area without blasting a strong stream of air at your plants.

A Heated Greenhouse Still Needs Ventilation

Then there’s ventilation, which exchanges stale indoor greenhouse air with fresh air from outside.

If you're wondering how to keep your greenhouse warm in winter without creating moisture problems, proper ventilation is key. It helps manage humidity alongside temperature.

Plants release moisture into the air, which can condense on cold glazing and leaves overnight. Persistent moisture can create favorable conditions for fungal diseases.

Explore Vista Greenhouses’ Ultimate Guide to Greenhouse Ventilation to learn more.

How Much Does It Cost to Heat a Greenhouse?

It depends on more than the price of the heater itself, as well as electricity or fuel, installation, and how often the equipment runs throughout winter.

Here's a simple way to estimate your expenses.

Estimating Electric Heating Costs

If you have electric heating, start by looking at the heater's wattage and your electricity rate.

Remember that 1,000 watts equals one kilowatt, so a 1,500-watt heater uses 1.5 kW when operating at full power.

So, let's assume electricity costs $0.20 per kWh, and your heater runs for 10 hours.

That's the cost for 10 hours of operation, not necessarily a full 10-hour overnight period. If you have a thermostat, it may switch the heater on and off, which can reduce its total running time.

Estimating Propane Heating Costs

Beyond your local price per gallon, the cost of propane depends on how much fuel your heater burns and how efficient it is.

One gallon of propane contains approximately 91,452 BTU of energy before accounting for heater efficiency.

To estimate consumption:

1. Divide the required delivered heat by the heater's efficiency.
2. Divide that result by 91,452 to estimate gallons.
3. Multiply the gallons by your propane price.

For a fair comparison, let's calculate the cost of delivering the same amount of heat as our electric example.

Not every heater is as efficient as the next, so look at your equipment’s specs before comparing fuel costs on an equal delivered-heat basis like this approach, if you're looking for the cheapest way to heat a greenhouse in winter.

A few other variables you can add to the equation include:

• Purchase price
• Professional installation
• Maintenance
• Fuel delivery costs

The Most Common Greenhouse Heating Mistakes

A problematic heating system can be a disaster for growers. If you know what to look out for ahead of time, you can avoid potential problems.

A few mistakes beginner greenhouse owners make:

• Choosing their heater based only on floor area: An 8 × 12 greenhouse with single-pane glass will have very different heating requirements than one with insulated polycarbonate. There are many other factors to consider when determining heat capacity, including glazing, outdoor temperatures, heat loss, etc.
• Setting the temperature too high: Every degree of unnecessary heating increases the temperature difference between your greenhouse and the outdoors. Base your heating target on your plants' needs.
• Ignoring air leaks: Check doors, glazing joints, baseboards, vents, and framing gaps to see if your heat is escaping.
• Using the wrong electrical supply: Confirm that your circuit can handle the heater's wattage and voltage requirements, especially if you have other equipment on the circuit.
• Relying on extension cords: Always plug portable electric heaters directly into suitable wall outlets, never into extension cords or power strips.
• Not considering combustion safety: Unvented fuel heaters introduce combustion gases and moisture. Check the equipment's approved use and local regulations before installation.
• Putting the thermostat beside the heater: A sensor that’s exposed to direct heat may shut the system off before the growing area reaches its target temperature.
• Skipping backup planning: Check fuel reserves, prepare for power outages, and use low-temperature alerts so you can catch equipment failures when they’re happening.

Frequently Asked Questions

What temperature should I keep my greenhouse at in winter?

It depends on what you're growing. But for the hardiest dormant plants, a minimum of 35°F to 45°F is a good starting point. If you have seedlings or are growing tropical species, 50°F to 55°F. However, don’t assume these starting points will work for your plants. Research your plants' minimum temperature requirements, then choose a setpoint that supports your growing goals.

What's the cheapest way to heat a greenhouse in winter?

Once you’ve done all you can to reduce heat loss, such as sealing air leaks or adding insulated glazing, passive solar heating is the cheapest option. This method works by trapping heat naturally. You can also use water barrels to absorb sunlight during the day to continue radiating heat once it’s cold.

Can I heat a greenhouse without electricity?

Yes. If you're researching how to heat a greenhouse without electricity, you can use passive solar heating or certain fuel-fired systems.

Just note that some propane heaters still require electricity for fans or controls.

Is propane or electric heat cheaper for a greenhouse?

Many people think propane is cheaper, but it ultimately depends on your local electricity and propane rates. The best way to determine what's most cost-effective is to calculate the cost of delivering the same amount of heat with each system, rather than comparing electricity and fuel prices directly.

Can a solar greenhouse heater work through the night?

Yes, provided there's enough stored energy to meet your overnight heating requirements.

Find the Right Heating Setup For Your Greenhouse

With the right heating setup for your climate, plants, and winter conditions, you can help keep your greenhouse warm through the colder months.

If you’re ready to explore your options, browse Vista Greenhouses' heating equipment to find products that suit your setup. If you have questions about specifications or compatibility, get in touch with our team.