Frost Cloth vs Bubble Insulation: What Each One Actually Does

They are not competing products

This comparison exists because people buy the wrong one, not because one is better. Frost cloth and bubble insulation solve different problems at different scales, and a greenhouse in a cold climate generally wants both.

Frost cloth works at the plant. It is a lightweight fabric laid over a bed or draped on hoops, and it works by interrupting radiant heat loss from the soil to the sky. It is an artificial cloud. It goes on and comes off in minutes and it protects a specific bed.

Bubble insulation works at the structure. It is a film fixed to the inside of the frame, and it works by adding trapped air between the inside of the greenhouse and the cold outside. It is a semi-permanent winter fitting, up in October and down in March, and it reduces heat loss from the whole building.

Buying frost cloth to insulate a greenhouse means you have bought a blanket for a house. Buying bubble film to protect seedlings from a frost means you have insulated the walls while the plants freeze under them.

Where frost cloth wins

Unheated greenhouses. If you are not adding heat, insulating the structure only slows the loss of heat you do not have. Covering the plants directly is a far better use of money, because it works on the small volume immediately around them.

Occasional protection. If you get half a dozen threatening nights a year, a cloth you deploy on those nights costs almost nothing and blocks no light for the other 359.

Young sowings and tender plants. A bed of autumn seedlings is exactly the scale frost cloth is designed for, and the plants that need protection most are usually in one place.

Hardening off in reverse. Plants moving between conditions in autumn benefit from a removable layer you can adjust daily, which insulation cannot offer.

Where bubble insulation wins

Heated greenhouses. If you are paying for heat, insulation directly reduces the bill every night it is up. This is where it pays for itself, and it is the main reason to fit it at all.

Sustained cold. Through a long cold spell you cannot deploy and retrieve cloth daily, and a permanently deployed heavy cloth blocks so much light that plants suffer. Insulation on the structure works continuously without touching the plants' light at bed level.

Protecting the structure and its contents. Water butts, stored tubers, a pot of overwintering fuchsias — anything that benefits from a warmer building rather than a warmer bed.

Reducing condensation on the glazing. A warmer inner surface means less water running down the glass, which helps with the gray mold problem covered in autumn condensation.

The light problem they share

Both cost you light, and light is the binding constraint in winter growing. Below roughly ten hours of daylight, plants stop growing regardless of temperature, and every layer you add pushes them further under that ceiling — see the 10-hour light rule.

This produces the correct strategy for both products:

• Insulate selectively. North side, end walls and door heavily; south side lightly or not at all.
• Deploy cloth tactically. Medium weight to live on the bed, heavy weight for warning nights only, off again in the morning.
• Clean the glazing first. Free, and it recovers more light than either product costs you.

The verdict

If you buy one thing and the greenhouse is unheated: frost cloth. Two weights, plus hoops and clips. It protects what you actually care about, costs the least, and does not darken the house all winter.

If you buy one thing and the greenhouse is heated: bubble insulation on the north side, end walls and door. It reduces the running cost every night, which is the expense you are actually trying to control.

If you buy both, which most cold-climate growers should: insulate the structure selectively in early October, keep a medium frost cloth on the beds through the season, and add a heavy cloth on the handful of nights the forecast turns hard.

And before either: seal the drafts. Air leaking out through an unsealed door costs more heat than a bubble layer saves, and it costs a few dollars to fix — see weatherstripping and door seals.

What to buy

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Common questions