# How to Heat a Greenhouse: Cheapest to Most Reliable

Nine ways to heat a greenhouse ranked by cost, from a $26 row cover and $95 water barrels to a $1,157 vented gas heater, with extension-service heat-loss math.

- Canonical: [https://greenhouses.guide/guides/how-to-heat-a-greenhouse](https://greenhouses.guide/guides/how-to-heat-a-greenhouse)
- Markdown: [https://greenhouses.guide/guides/how-to-heat-a-greenhouse.md](https://greenhouses.guide/guides/how-to-heat-a-greenhouse.md)
- Published: 2026-09-20
- Updated: 2026-09-20
- Author: Anna Persson

## Quick answer

Checked September 20, 2026, the cheapest way to heat a greenhouse is not to heat the air at all: a $25.95 roll of AG-19 row cover over the plants buys up to 4 degrees of frost protection, and a $30 roll of AG-30 buys 4 to 6. Next come thermal mass, at $85 to $95 per new 55-gallon drum of water that releases about 4,500 BTU as it cools 10 degrees overnight, and a 20 watt seedling heat mat at $14.99 that costs about 9 cents a day. When you do heat the air, an 8x12 twin-wall polycarbonate house needs about 5,800 BTU per hour to hold 30 degrees above the outside low, which is a little more than one 1,500 watt heater makes. Electric costs about 27.5 cents per hour per 1,500 watts at the June 2026 US residential rate of 18.34 cents per kWh. Propane is cheaper per BTU at $2.674 a gallon, but every portable propane and kerosene heater is unvented, and the extension services we read say not to run one in a greenhouse without ventilation because of ethylene and carbon monoxide. The most reliable option for a big house is a vented gas unit like the Modine Hot Dawg HD30 at $1,157, and the long-term option is a climate battery at $2,000 to $5,000 for a residential house.

Checked **September 20, 2026**, the cheapest useful greenhouse heat is a $25.95 roll of row cover, and the most reliable is a $1,157 vented gas heater. Between them sit water barrels, bubble film, heat mats, compost, plug-in fan heaters and unvented propane, in roughly that order of cost, and not in that order of usefulness. This guide walks the ladder from the bottom, with what each rung costs today, what an extension service says it actually does, and where each one stops working.

We don't sell greenhouses. We save you from buying the wrong one, and from heating one wrong. The starting point is the heat-loss number for your house, because every method on this page either reduces that number or replaces it. If you only want the heater pick, go straight to [the best greenhouse heater, sized by square feet](/guides/best-greenhouse-heater). If you want the winter's total bill, [the heating cost guide](/guides/greenhouse-heating-cost) has it by climate.

## Quick Answer: The Ladder

| Rung | Method | Cost found September 20, 2026 | What it does | Where it stops |
|---|---|---|---|---|
| 1 | Row cover over the plants | AG-19 $25.95 per 83 in x 50 ft; AG-30 $30 | Up to 4F (AG-19) or 4 to 6F (AG-30) of frost protection at the leaf | Hard freezes; blocks 15 to 30 percent of light |
| 2 | Thermal mass, water barrels | $85 to $95 per new 55-gallon drum | About 4,500 BTU released per drum per 10F of cooling | Cloudy weeks; needs sun to recharge; 450 lb each |
| 3 | Insulate the north wall | R-10 rigid foam, two layers, as UGA did | Cuts loss through the one wall that gives no useful light | Does nothing for the roof, where most heat leaves |
| 4 | Seedling heat mat | $14.99, 20 W | Tray 10 to 20F above ambient for about 9 cents a day | Warms roots, not the house |
| 5 | Compost hotbed | Manure and labor | Soil at 75F for seed starting; core hits 141 to 155F in 24 to 48 hours | No published BTU or duration; smells; weeks not months |
| 6 | Plug-in electric heater | $34.57 (Lasko ceramic) to $149.99 (Bio Green Palma) | 5,118 BTU/h per 1,500 W; 27.5 cents per hour at full duty | One per standard outlet; short for anything over 8x12 |
| 7 | Unvented propane | $155.14 (Mr. Heater Portable Buddy, 4,000 or 9,000 BTU) | Cheap BTU, no wiring | Not recommended in greenhouses by any extension source we read |
| 8 | Vented gas unit heater | $1,157 (Modine HD30, 24,600 BTU output) | Heats a 12x16 single-glass house through a real winter | Gas line, vent pipe, 115 V, installer |
| 9 | Heat pump or climate battery | About $1,958 (MRCOOL DIY 12k); $2,000 to $5,000 (Ceres GAHT) | Efficient heat, or free stored ground heat run on fans | Capital cost; excavation; design plans |

## How These Numbers Were Built, and When

Prices were read from manufacturer or named retailer pages on September 20, 2026. Heat-loss math and U-factors come from the University of Alaska Fairbanks Extension, Rutgers and the University of Georgia. Thermal mass figures are from the University of Connecticut and UGA. Electricity is the EIA's June 2026 residential average. Propane is the EIA's last weekly print of the past heating season, March 30, 2026, and will be updated when the survey resumes in October. Home Depot pages blocked our fetch, so their prices are not used. No method here was tested by us in a greenhouse.

## Start With the Loss

Heat loss in BTU per hour equals glazed area in square feet, times the glazing U-factor, times the difference between inside and outside temperature in degrees F. Extension services publish the U-factors: about 1.1 to 1.2 for single glass or single polycarbonate, 0.7 to 0.8 for double poly film, 0.5 to 0.6 for twin-wall polycarbonate.

For an 8x12 house with a 5-foot eave and 7-foot ridge, the glazed surface is about 323 square feet. At 30 degrees colder outside than in, with twin-wall polycarbonate:

0.6 x 323 x 30 = **5,814 BTU per hour**

That is 1.7 kilowatts, so one 1,500 watt heater at 5,118 BTU per hour falls about 700 BTU short on that night. In single glass the loss doubles to about 11,600. Rutgers adds 10 percent for older leaky glass, and Georgia and Arkansas add an infiltration term for air changes. Every rung below either lowers the U-factor, lowers the area you have to heat, lowers the temperature difference at the plant, or supplies the BTU.

## Rung 1: Row Cover Inside the Greenhouse

The cheapest degrees you can buy. Agribon AG-19 is 0.55 ounces per square yard, passes 85 percent of light, and the maker rates it "up to 4 degree F frost protection", $25.95 for 83 inches by 50 feet. AG-30 is heavier, passes 70 percent of light, and is rated to protect to 26 to 28 degrees, or "4 to 6 F", at $30 for the same roll. Drape it over hoops or directly on the plants inside the house on frost nights and pull it off in the morning. It reduces the temperature difference at the leaf, which is the last term in the formula, without heating any air. It does nothing in a hard freeze and it is the first thing to try in zone 8 and 9. [Season extension without heat](/guides/season-extension-greenhouse) covers how many weeks this alone buys.

## Rung 2: Water Barrels

Water holds about 1 BTU per pound per degree F, three times what concrete holds by volume, according to the University of Connecticut's greenhouse engineering notes, which put usable stored solar heat at roughly 200 to 400 BTU per square foot of floor per day depending on region. A full 55-gallon drum weighs more than 450 pounds, so as it cools 10 degrees overnight it releases about 4,500 BTU. Against the 5,814 BTU per hour loss above, that is under an hour of heat per drum, which is why barrels moderate a swing rather than replace a heater. UGA's passive solar house stacked them two high along an insulated north wall and reported about a month of extra season over a plain high tunnel. New closed-top drums are $95 each, $85 at four or more, at Uline. Used food-grade drums are cheaper and unpriced here. Paint them black, put them where the winter sun hits, and accept the bench space they take. No extension source we found publishes a gallons-per-square-foot rule, so the 2 to 5 gallons per square foot of glazing that circulates online is unverified.

## Rung 3: Insulate What Does Not Need Light

The north wall gives almost no useful winter light in the northern hemisphere. UGA's passive house covered it with two layers of 1.5-inch R-10 rigid foam under black plastic. That drops the U-factor on that wall from 0.6 to a fraction, at the cost of nothing you would miss. Bubble film on the glazing is the version everyone asks about. The only R-value we could verify today is SolaWrap's R-1.7, a commercial bubble glazing sold by the $3,100 roll, not a hobby product. Hobby-scale bubble film prices and claims were on pages we could not read. What is certain is that twin-wall polycarbonate at U 0.5 to 0.6 already cuts single glass's loss in half, so if your house is single glass, [the glazing decision](/guides/glass-vs-polycarbonate-greenhouse) is worth more than any wrap.

## Rung 4: Heat Mats

A Vivosun 10 by 20.75 inch mat is $14.99, draws 20 watts, and raises the tray 10 to 20 degrees above the surrounding air. Twenty watts around the clock is 0.48 kWh a day, about 9 cents at 18.34 cents per kWh. Combine it with a row cover tent over the tray and you have a warm microclimate for seedlings in a house you never heat. The [seedling heat mat profile](/accessories/bootstrap-farmer-seedling-heat-mat) and [its controller](/accessories/bootstrap-farmer-heat-mat-controller) cover the details. This is how most hobby growers should start seeds in March, and it does nothing for the citrus tree in the corner.

## Rung 5: Compost and Hotbeds

Purdue's hotbed bulletin says to excavate 16 to 18 inches, tamp in fresh manure, cover it with 3 to 4 inches of soil and seed once the soil drops to 75 degrees. Nebraska specifies 8 to 12 inches of fresh manure with 5 to 10 percent straw. A compost pile's core reaches 141 to 155 degrees within 24 to 48 hours, per the University of Alaska Fairbanks master gardeners. What no extension source publishes is BTU output or how many weeks it lasts, so treat this as a seed-starting method, not a heating system.

## Rung 6: A Plug-In Electric Heater

The rung most people land on. A Lasko 754200 1,500 watt ceramic heater was $34.57 at Walmart; a splash-rated Bio Green Palma with a thermostat is $149.99. Both make 5,118 BTU per hour, which at the June 2026 rate is 27.5 cents per hour at full duty, about $3.30 per 12-hour night if it never cycles off. With a thermostat it usually runs half that. The catch is the outlet: 1,500 watts is the ceiling on a standard 120 volt circuit, so the second heater needs a second circuit and anything over an 8x12 twin-wall house needs 240 volts. The 2023 National Electrical Code requires GFCI protection on outdoor and accessory-building outlets, and a greenhouse is a wet location; the University of Florida's own greenhouse safety audit bans portable space heaters outright and allows only oil-filled radiators where any are permitted. Which units, and which wattage for which house, is [the heater guide](/guides/best-greenhouse-heater).

## Rung 7: Unvented Propane

A Mr. Heater Portable Buddy is $155.14, switches between 4,000 and 9,000 BTU, and runs 3 to 6 hours on a 1-pound cylinder. At bulk propane prices, 9,000 BTU per hour is about 26 cents an hour, comparable to electric; on 1-pound cylinders it is far more, and we could not verify that price. The problem is not cost. Alabama Extension: "unvented gas burners should not be used when growing greenhouse vegetables", because incomplete combustion produces "dangerous levels of ethylene, sulfur dioxide, and other gases", and plants are thousands of times more sensitive to ethylene than to carbon monoxide. The University of Alaska Fairbanks classifies every portable kerosene or propane heater as unvented and not recommended indoors. A 1983 New York State Flower Industries bulletin, still hosted by NC State, says of unvented kerosene in greenhouses: "DO NOT USE", and offers the general rule, "vent all heaters". The Buddy's oxygen sensor protects you from suffocating. It does not stop ethylene twisting your tomatoes or carbon monoxide building while you work in there. If you run one anyway, crack the roof vent, which throws away the heat you paid for.

## Rung 8: A Vented Gas Unit Heater

A Modine Hot Dawg HD30 is $1,157 at Greenhouse Megastore: 30,000 BTU input, 24,600 output, 80 percent efficient, power vented outdoors, natural gas or propane. At $2.674 a gallon and 91,452 BTU per gallon, that is about 88 cents an hour at full output. It covers a 12x16 single-glass house at a 30-degree gap with margin and a twin-wall house at 40 degrees easily. The combustion happens in a sealed exchanger and leaves through a pipe, which is what makes it the reliable answer for a heated house. You need a gas line, a vent through the glazing or wall, a 115 volt outlet and someone licensed to connect it, and a thermostat is extra. This is the rung where a heated greenhouse becomes a utility.

## Rung 9: Heat Pumps and Climate Batteries

A MRCOOL DIY 12,000 BTU mini-split heat pump runs on 115 volts, is rated to heat at minus 13 degrees, and was listed around $1,958. It moves more heat per kWh than a resistance heater and cools in summer, which no other rung does. The other route is a ground-to-air heat transfer system, the climate battery: fans push daytime greenhouse air through tubes buried up to 4 feet deep and pull the stored warmth back at night. Ceres prices a residential system under 1,000 square feet at $2,000 to $5,000 plus $750 for design plans. Atmos quotes about $7.14 per square foot in materials for a 30 by 96 foot example, excluding labor and electrical, with a 5 to 11 year payback and running cost of fans only. Both are decisions to make before the foundation is poured, which is the argument in [the foundation guide](/guides/greenhouse-foundation-base-guide).

## Which Rung for Which Grower

- **Hardy plants, zone 7 and warmer:** rungs 1 and 2. Row cover on frost nights and a few black barrels. No heater.
- **Seed starting in late winter:** rung 4 under rung 1. A mat and a tent.
- **Tender collection in a 6x8 to 8x12:** rung 6 on a thermostat, sized with the formula, plus rung 3 on the north wall so it cycles less.
- **A 10x12 or larger you want above 50 degrees all winter:** rung 8, or rung 6 at 240 volts with the loss math done first.
- **A new build in a cold climate:** price rung 9 before the slab. It is the only heat that gets cheaper every year.

## What We Could Not Verify

- A gallons-per-square-foot thermal mass rule from any extension source.
- Hobby-scale bubble film price and R-value; the pages were blocked or listed only foil barrier.
- Humidity output per gallon of propane burned; the UMass fact sheet returned an error.
- The price of 1-pound propane cylinders and the cost per BTU on them.
- BTU output or duration of a compost hotbed.
- Any solar-thermal hobby product with a price.
- The current-season propane price; EIA's weekly survey resumes in October 2026.
- We did not test any of these methods ourselves.

## Frequently Asked Questions

### Curious if bubble wrap works as insulation?

It lowers the glazing's U-factor somewhat and lowers light more. The only verified R-value we found is R-1.7 for a commercial bubble glazing. If the house is single glass, the useful comparison is that twin-wall polycarbonate halves the loss and bubble film on top of it is a small extra. Wrap the north wall with rigid foam first; that costs no light at all.

### Should I bubblewrap the sides and top inside my greenhouse?

The owner asking had a heater that could not keep up. Run the formula first. If the house loses more BTU than the heater makes on the coldest night, wrap slows the fall and does not stop it. The fix is a bigger or second heater, then insulation to make it cycle less.

### Can an unheated greenhouse in the U.K. provide enough heat to stay above freezing/frost?

Not on a clear, still, cold night. An unheated house can drop to or below outside temperature by radiative cooling. Row cover at the plant and water barrels along the north wall are the two unheated tools that reliably hold a few degrees; [the overwintering guide](/guides/overwintering-plants-in-a-greenhouse) lists which plants that is enough for.

### Is it worth the investment (initial cost + electric heating) or would it be better to just use the grow lights I already have in the basement?

For a handful of tropicals, the basement lights win. Heating a 6x8 above 40 degrees through a 30-night cold snap is roughly $50 in electricity plus the heater at the June 2026 rate, and that buys a greenhouse you can sit in. Three pots do not need that.

### Do I need grow lights and a heater?

In zone 9a, where the question came from, hardy plants need neither and tender ones need a thermostat-controlled heater on frost nights only. Lights matter for growth, not survival, in a bright winter.

## Sources

- US Energy Information Administration, [Electric Power Monthly, Table 5.6.A](https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a), June 2026; [Weekly US residential propane price](https://www.eia.gov/dnav/pet/pet_pri_wfr_a_epllpa_prs_dpgal_w.htm); [Energy units and calculators](https://www.eia.gov/energyexplained/units-and-calculators/).
- University of Alaska Fairbanks Cooperative Extension, [Controlling the Greenhouse Environment, HGA-00336](https://www.uaf.edu/ces/publications/database/gardening/controlling-greenhouse-environment.php) and [Space heater safety, EEM-00253](https://www.uaf.edu/ces/publications/database/housing/space-heater-safety.php); UAF Master Gardeners, [Compost-heated greenhouses](https://alaskamastergardener.community.uaf.edu/2017/11/27/heating-it-up-compost-heated-greenhouses/).
- Rutgers CCEA, [Environmental control of greenhouses](https://horteng.envsci.rutgers.edu/factsheets/envcontrolofghs.pdf); University of Georgia Extension, [B792, Greenhouses: Heating, Ventilation and Cooling](https://fieldreport.caes.uga.edu/publications/B792/greenhouses-heating-ventilation-and-cooling/) and [B1566, Constructing a Passive Solar Greenhouse](https://fieldreport.caes.uga.edu/publications/B1566/constructing-a-passive-solar-greenhouse-for-season-extension/).
- University of Connecticut IPM, [Heat storage for greenhouses](https://ipm.cahnr.uconn.edu/heat-storage-for-greenhouses/); Purdue Extension, [HO-53, hotbeds and cold frames](https://ag.purdue.edu/department/hla/extension/extension-publications-library/ext-pubs/ho-53-w.html); Nebraska Extension, [G1718](https://extensionpubs.unl.edu/publication/g1718/2007/html/view).
- Alabama Cooperative Extension, [Greenhouse vegetables and ethylene damage](https://www.aces.edu/blog/topics/ipm-farming/greenhouse-vegetables-and-ethylene-damage/); New York State Flower Industries, [Bulletin 149, unvented heaters, 1983](https://hortscans.ces.ncsu.edu/uploads/u/n/unvented_520a7e9420fd1.pdf).
- NEC 2023, [210.8(F)](https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=1427.1); Virginia Tech EHS, [Ground fault circuit interrupters](https://ehs.vt.edu/programs/occupational-safety/electrical-safety/ground-fault-circuit-interrupters.html); University of Florida EHS, [Greenhouse survey questions](https://www.ehs.ufl.edu/wp-content/uploads/2024/07/Greenhouse-Survey-Questions.pdf).
- Growers Solution, [Agribon AG-19](https://growerssolution.com/products/agribon-ag-19-floating-row-cover-frost-blanket); Arbico Organics, [Agribon AG-30](https://www.arbico-organics.com/product/agribon-ag-30-frost-protection/); Uline, [55-gallon closed-top drum](https://www.uline.com/Product/Detail/S-10757BLU/Drums/Plastic-Drum-55-Gallon-Closed-Top-Blue); SolaWrap, [bubble greenhouse plastic](https://www.solawrapfilms.com/bubble-wrap-greenhouse-plastic), accessed September 20, 2026.
- Walmart, [Lasko 754200](https://www.walmart.com/search?q=lasko+754200+ceramic+heater); Mr. Heater, [Portable Buddy](https://www.mrheater.com/portable-buddy-heater.html); Greenhouse Megastore, [Modine HD30](https://www.greenhousemegastore.com/products/modine-hd30-unit-heater); Vivosun, [heat mats](https://vivosun.com/en-US/Heat_Mat-c51); Mini Split Warehouse, [MRCOOL DIY 12k](https://www.minisplitwarehouse.com/product/mrcool-diy-12000-btu-115v-mini-split-heat-pump-ac); Ceres, [GAHT system](https://ceresgs.com/gaht-system/); Atmos, [Costs and ROI](https://atmosgreenhouse.com/costs-and-roi), accessed September 20, 2026.
- Reddit r/Greenhouses, [Curious if bubble wrap works as insulation](https://reddit.com/r/Greenhouses/comments/1omx6qf/curious_if_bubble_wrap_works_as_insulation_heres/), [Tips on keeping the greenhouse warm](https://reddit.com/r/Greenhouses/comments/1opaqov/tips_on_keeping_the_greenhouse_warm_in_the_hard/), [Can an unheated greenhouse in the U.K. stay above frost](https://reddit.com/r/Greenhouses/comments/1fybcua/can_an_unheated_greenhouse_in_the_uk_provide/), [Grow lights or greenhouse for wintering tropicals](https://reddit.com/r/Greenhouses/comments/18n8ptx/grow_lights_or_greenhouse_for_wintering_tropical/), [Overwinter greenhouse help 9a](https://reddit.com/r/Greenhouses/comments/1pbl3o6/overwinter_greenhouse_help_9a/).