What is the cheapest way to heat a greenhouse in winter?

Thermal mass is the cheapest option because it has no running cost: fill black barrels or tanks with water and place them where they catch direct sun, and the water absorbs heat all day and radiates it back out overnight. The rule of thumb from greenhouse-design specialists is roughly 4–5 gallons of water per square foot of greenhouse floor space. It won't fully replace a heater on the coldest nights, but it narrows the temperature swing a heater has to cover, so the heater runs less.

Do I need a heater if my greenhouse has double-layer poly?

Probably still yes for anything below freezing, but you'll run it less. A single layer of polyethylene film has an R-value around 0.85; adding a second layer with an inflation blower to create an air gap between them raises that to roughly R-1.25, and up to R-1.5 in some setups. That's a meaningful cut in heat loss, but it's still a thin film, not real insulation — on a hard freeze night you'll still need an active heat source to hold temperature.

Is it cheaper to heat a greenhouse with electricity or propane?

Propane delivers far more heat per dollar of fuel and works with no power at the structure, which makes it the better choice for large or off-grid greenhouses. Electric heaters cost more per BTU but are cleaner, safer to run unattended, and precisely thermostat-controlled — the better choice for a small hobby greenhouse that already has power run to it. Combustion heaters also add moisture and exhaust byproducts, so they need ventilation and a CO detector that electric heaters don't.

How cold can it get inside a greenhouse before plants die?

It depends entirely on the crop, not the structure. Cold-hardy greens like spinach and kale tolerate a light frost, but frost-tender seedlings and tropicals are damaged well above freezing, often in the mid-30s°F. An unheated greenhouse alone typically buys only a few degrees over outside air unless you add thermal mass or a frost blanket, which is why frost-tender crops need active heat, not just a covered structure, once nights drop toward freezing.

How to Heat a Greenhouse in Winter: 5 Methods Ranked by Cost (2026)

Quick Answer: The cheapest way to hold heat in a winter greenhouse is thermal mass — black water barrels at roughly 4–5 gallons per square foot of floor space — combined with double-layer poly film, which raises R-value from about 0.85 to 1.25–1.5. Neither one replaces an active heater on a hard freeze night, but together they cut how much a heater has to work, which is where most of the winter electric or propane bill comes from. For most hobby greenhouses, the winning combination is passive insulation plus a thermostat-controlled electric heater sized to your actual coldest night, not your average one.

Every winter greenhouse question comes down to the same tradeoff: free methods buy you a few degrees and cost nothing to run, while active heaters buy you as many degrees as you want but cost money every hour they’re on. Most growers end up layering both — insulation and thermal mass to shrink the gap, then a heater to close whatever’s left.

Winter greenhouse heating by the numbers:

4–5 gallons of water per square foot — the thermal-mass rule of thumb from greenhouse-design specialists for how much water storage a structure needs to meaningfully buffer day-to-night temperature swings; steel drums exchange heat faster than plastic ones, and black absorbs more solar heat than white.
R-0.85 to R-1.25–1.5 — the jump in insulating value from a single layer of polyethylene film to a double-layer inflation system with an air gap between the layers, per greenhouse-covering industry data. That’s close to doubling heat retention through the glazing alone.
~$0.26 per hour — what a 1,500-watt electric heater like the Bio Green Palma costs to run at the U.S. average residential electricity rate of roughly 17¢/kWh (EIA data), and only while it’s actually cycling on.
28°F of frost protection — the temperature buffer a spun polypropylene frost blanket provides when draped directly over plants, per horticultural-fabric manufacturer Agfabric’s ratings, while still passing about 85% of available light through.

The 5 ways to heat a greenhouse, compared

Method	Running cost	Heat added	Best for
Thermal mass (water barrels)	$0 to run	Small, steady buffer	Any structure, as a base layer
Double-layer poly / bubble wrap	$0 to run	Cuts heat loss ~30–40%	Reducing how hard a heater works
Frost blanket / row cover	$0 to run	~28°F of buffer over plants	Emergency one-off cold snaps
Electric heater	~$0.26/hr @ 1,500W	As much as you dial in	Small greenhouses with power
Propane heater	Higher BTU per $ of fuel	Most heat available	Large or off-grid structures

1. Thermal mass: free heat storage that already works during the day

The sun heats a greenhouse for free every clear day — the trick is capturing some of that heat before it escapes at night. Black water barrels or tanks placed where they get direct sun absorb heat all afternoon and release it slowly overnight as the water cools, narrowing the temperature swing between day and night. Specialists sizing solar greenhouses use a rule of thumb of roughly 4–5 gallons of water per square foot of floor space to make a real dent; steel drums perform better than plastic ones because steel conducts heat faster, exchanging it with the air more readily.

Thermal mass alone won’t carry a greenhouse through a hard freeze — it’s a buffer, not a heater. But it’s the only method on this list with zero running cost, and it keeps working every single night without a thermostat, a fuel tank, or a power bill.

2. Double-layer poly or bubble wrap: cut the heat loss before you add heat

Horticultural Bubble Wrap Insulation

Clear greenhouse-grade bubble insulation · attaches to glazing bars

Single poly film runs about R-0.85; a double-layer inflation system with an air gap between the layers raises that to roughly R-1.25–1.5.
Clear horticultural bubble wrap does a similar job on hard-panel kits, at the cost of roughly 10% of incoming light.
Cuts how often and how hard your heater has to run, which is where most of a winter heating bill comes from.

Check price on Amazon →

If your greenhouse is a soft-sided poly tunnel, a double-layer inflation kit — a small blower keeps a cushion of air trapped between two film layers — is the single biggest insulation upgrade available, roughly doubling the R-value of the glazing itself. Hard-panel kits (polycarbonate or glass) can’t take a second poly layer the same way, but clear horticultural-grade bubble wrap clipped to the inside of the glazing bars achieves a similar effect at a much lower cost than replacing panels. See our greenhouse insulation and greenhouse plastic picks for both routes.

Fast shipping matters here more than it seems — insulation you order the week before the season’s first hard freeze does no good sitting in a warehouse. Amazon Prime’s free two-day delivery is worth having on hand specifically for the week the forecast turns and you need bubble wrap or a frost blanket by Friday, not next month. Sponsored link — we may earn a fee if you sign up. No extra cost to you.

3. Frost blanket: the emergency backup for one bad night

A spun polypropylene frost blanket draped directly over plants — inside or outside a structure — buys roughly 28°F of protection above ambient air temperature while still letting about 85% of light through, per manufacturer Agfabric’s ratings. That’s enough to save a bed of frost-tender seedlings on a single unexpectedly cold night, but it’s not a substitute for a heater across an entire winter; it’s the tool you reach for when the forecast surprises you and there’s no time to set up anything else.

4. Electric heater: precise, clean, and thermostat-controlled

For a small hobby greenhouse that already has power run to it, an electric heater is the cleanest way to hold a setpoint through winter. A 1,500-watt unit on a standard 120V circuit — like the Bio Green Palma — costs roughly $0.26 per hour to run at the U.S. average electricity rate, and a built-in thermostat means it only cycles on when temperature actually drops below your target, which in most shoulder-season nights is a fraction of the time. No combustion means no moisture or exhaust byproducts to ventilate for, either.

5. Propane heater: the most heat per dollar, off-grid

When a greenhouse is too large for a 120V circuit to heat, or there’s no power at the structure at all, propane delivers far more BTU per dollar of fuel than any electric option. The tradeoff is that combustion adds both moisture and exhaust byproducts to the air, so a propane heater needs real ventilation and a carbon monoxide detector — non-negotiable safety gear, not an upsell. See our full best greenhouse heater roundup for specific electric and propane picks by size.

How to choose your winter heating setup

Start with thermal mass and insulation — they’re free. Water barrels and a double poly layer or bubble wrap cost nothing to run and shrink the job any heater has to do.
Know your coldest night, not your average one. Size a heater (or skip one) based on the worst night your area actually sees, not a mild-winter average — an undersized heater running flat-out on the one night that matters is the failure mode that kills a whole bench of seedlings.
Match fuel to your setup. Electric if you have power and want thermostat precision; propane if you don’t have power or need to heat a large structure for less per BTU.
Keep a frost blanket on hand regardless. It’s the cheapest insurance against the one surprise cold snap a forecast misses.
Put a greenhouse thermometer on the wall before you buy anything. You can’t size a heater — or know if your insulation is actually working — without knowing your real overnight low.

The bottom line

Free methods come first: black water barrels at roughly 4–5 gallons per square foot and a double poly layer or bubble wrap both cut into the temperature swing a heater would otherwise have to fight, at zero running cost. Keep a frost blanket in reach for the one night the forecast surprises you. Then size an active heater — electric for small, powered greenhouses at about $0.26/hour, propane for large or off-grid ones — to your coldest realistic night, not your average one. Combine all four and most hobby greenhouses stay frost-free through winter without an oversized heater running constantly. Growing a specific crop through the cold months? Our greenhouse for tomatoes, greenhouse for peppers, and greenhouse for cucumbers guides each break down the exact temperature window that crop needs to keep producing.