How to Insulate a Greenhouse for Winter: Walls, Roof, Base and Floor

To insulate a greenhouse for winter, work from the outside in. Seal the air leaks first, then line single glass with horticultural bubble insulation, put rigid foam on the north wall and below the benches, insulate the base so cold can't creep in from the frozen ground, and close a thermal curtain at night. On the real greenhouse we walk through below, these steps cut the winter heating bill by more than half. If you want a greenhouse that is truly insulated all year, the biggest gain comes from the glazing you choose when you buy.

We are a small, woman-owned greenhouse business in Georgia. Every fall, customers from mild Southern winters to snowy mountain towns ask us the same question: how do I make my greenhouse warmer without doubling my power bill? This guide is our answer, with diagrams, a shopping list and a real example you can copy.

If you are getting your greenhouse ready for winter as a whole (cleaning, heater checks, watering), start with our greenhouse winterizing checklist. This post goes deeper into one part of it: insulation.

Where does a greenhouse lose its heat?

A greenhouse loses heat in four ways. Each one has its own fix, and the steps in this guide are numbered to match.

• Through the glazing. Glass and polycarbonate are thin, so heat passes straight through them. This is the biggest loss in almost every greenhouse.
• Through air leaks. Warm air escapes through gaps at doors, vents, panel joints and the base, and cold air is pulled in to replace it. Wind makes this much worse.
• Into the ground. Frozen soil around the edge of the greenhouse pulls heat out of your beds and floor.
• Out to the night sky. On clear, still nights, a greenhouse radiates heat upward. This is why clear nights are often the coldest inside.

Every glazing material has a U-value, which is how much heat it lets through. The lower the number, the better it insulates. These are the values our greenhouse heating cost calculator uses:

*BTU per hour, per square foot, per °F of difference between inside and outside. Typical values; exact figures vary by brand and frame.

Which steps matter most for your greenhouse?

Find your greenhouse below and start with the steps listed for it. Every greenhouse benefits from sealing (Step 1) and keeping the air dry (Step 8).

What you'll need to insulate a greenhouse

These are the materials we use in this guide. Most come from a garden center or hardware store; the heaters, shelves and vent openers are in our shop. Each step below shows where each one goes.

Horticultural bubble insulation

Clear, UV-stabilized bubble sheet for lining glass. Look for large bubbles and a UV rating of 3 years or more.

Rigid foam board

1 to 2 in XPS or foil-faced polyiso, about R-5 to R-6.5 per inch. For below the benches, the north wall and the base.

Foam weatherstrip tape

Self-adhesive foam or EPDM rubber strip. Seals the base, door frames and vent edges.

Glazing-bar clips

Plastic clips that hold bubble insulation to aluminum glazing bars, so nothing sticks to the glass.

Thermal curtain

Insulating fabric or a second layer of bubble wrap on rings and wires. Pull it closed at dusk.

Min/max thermometer-hygrometer

Shows the coldest temperature and the humidity overnight, so you know if the insulation is working.

Dark water barrels

Thermal mass. Dark barrels under the benches soak up the sun by day and release heat at night.

Seedling heat mat

Warms the roots of a few trays without heating the whole greenhouse. Use it with a thermostat.

Step 1: Seal the air leaks

Insulation can't help if warm air is pouring out through gaps. Seal first, then insulate.

• Find the leaks. On a windy day, walk around inside with a lit incense stick or a strip of tissue. Wherever the smoke or tissue moves, you have a leak.
• Doors: replace worn brush strips and rubber seals, and adjust the latch so the door pulls tight.
• Vents and louvers: clean the tracks and make sure they close fully. Louvers are often the leakiest part of a greenhouse, so many growers cover them from inside with bubble insulation for the winter and use a roof vent instead. An automatic roof vent opener, like the Janssens automatic roof window opener, closes the vent by itself when the evening cools.
• Panel joints: replace cracked or slipped panes and fill small gaps with greenhouse-safe silicone or clear glazing tape.
• The base: seal the gap between the frame and your foundation with foam tape or EPDM rubber strip. Cold air creeps in at ground level.

Our guide to keeping a glass greenhouse warm in winter has a full draft-proofing list for glass houses.

Step 2: Line the walls and roof with bubble insulation

Horticultural bubble insulation is the classic way to insulate a glass greenhouse. It works like a second pane: it traps still air against the glass, while still letting most of the light through.

• Buy the right kind. Use clear, UV-stabilized horticultural bubble wrap, not packing bubble wrap. Packing wrap turns yellow and brittle in one season and blocks more light. Larger bubbles generally insulate better.
• Clean the glass first. Winter light is precious, and dirt under the wrap stays there until spring.
• Fix it to the frame, not the glass. Clips or pins made for aluminum glazing bars hold it in place. Leave a small air gap between the wrap and the glass: that pocket of still air does most of the insulating.
• Tape the seams with clear tape so warm air can't slip behind the wrap.
• Keep one roof vent free so you can still let out damp air on sunny days.
• Number each piece with a marker as you cut it. Next fall, it goes back up in an hour.

Does bubble wrap help on polycarbonate? On twin-wall polycarbonate it helps less, because the panels already have an air layer built in. It's worth adding in very cold zones, or on single-wall polycarbonate, but do leaks, the base and a night curtain first.

Step 3: Use rigid foam where light doesn't matter

Some parts of your greenhouse don't need to be clear in winter. Insulate those with rigid foam board, which holds heat about five to seven times better than bubble wrap.

• Below the benches. Plants on benches don't need light from below. A band of foam board around the lower walls, up to bench height, is one of the cheapest upgrades there is.
• The north wall. In the northern US, the north wall gets almost no direct winter sun. Covering its lower half (or all of it, in cold zones) cuts heat loss without costing you much light.
• Behind a lean-to. If your greenhouse leans on a house or garage wall, that wall already gives you heat. Don't waste it by leaving gaps where the greenhouse meets the building.

Use 1 to 2 inch extruded polystyrene (XPS) or foil-faced polyiso board. Face the foil side or white paint toward the inside, so it bounces light back onto your plants instead of soaking it up. Wedge the boards behind the glazing bars or hold them with the same clips as the bubble wrap.

Step 4: Insulate the base and perimeter

This is the step most people skip, and it's the one that keeps your soil warm. In winter, cold seeps sideways from the frozen ground outside into your beds and floor.

• Perimeter insulation: dig a narrow trench around the outside of the base, 12 to 24 inches deep, and stand 1 to 2 inch rigid foam board in it against the foundation. Backfill, and the foam acts as a wall between frozen ground and your growing space. If you're building a new foundation, add it at the same time.
• A solid, sealed base: a greenhouse on a proper foundation frame, timber base or stem wall is far easier to seal than one sitting on loose ground. Riga owners can use the Riga foundation frame, which anchors in a shallow trench without a slab.
• A stem wall: a low masonry wall under the glass raises your greenhouse and is easy to insulate. See our guide to building a greenhouse on a stem wall.

Planning a new base? Our greenhouse foundation guide compares the options, and the foundation calculator works out the materials for your size.

Step 5: Insulate the floor and lift your plants

• Get plants off the ground. A bare concrete or soil floor is the coldest place in the greenhouse. Put pots on benches or shelves, such as the slatted shelves for Royal Victorian greenhouses, or at least on a piece of foam board.
• Insulate raised beds. Line the inside walls of raised beds with foam board so the soil holds its warmth.
• Store the sun's heat. Brick, stone or pavers on a gravel bed soak up the sun by day and give heat back at night. Dark water barrels under the benches do the same job. Our guide to heating a greenhouse without electricity explains how much thermal mass you need.

Step 6: Close a thermal curtain at night

Most heat is lost at night, when there is no sun to replace it. A thermal curtain (also called an energy curtain or thermal screen) is a layer of fabric or bubble insulation you pull across the greenhouse at dusk and open again in the morning.

• Hang it at eave height. Run wires along the length of the greenhouse and hang the curtain on rings so it slides open and closed.
• Use it on the walls too. In a glass house, drop-down curtains along the walls make a second skin at night.
• Open it every morning. Winter light is too scarce to lose, and a closed curtain on a sunny day traps damp air.

A night curtain is the best upgrade for greenhouses that already have twin-wall polycarbonate, because it tackles the night sky loss that the panels can't.

Step 7: Heat a smaller space

The less air you heat, the less you pay. Hang a bubble-wrap or plastic partition across one end of the greenhouse, group your tender plants there, and heat only that zone. Let the rest drop to just above freezing for hardy greens. For a few trays of seedlings, a heat mat under the trays costs less to run than heating the whole house.

Step 8: Don't let an insulated greenhouse turn damp

A well-sealed, well-insulated greenhouse holds in moisture as well as heat. Damp, still air leads to mold, mildew and botrytis, which kill more winter plants than cold does.

• Run a small circulation fan all winter. It mixes warm air down from the roof and keeps leaves dry. Browse our greenhouse fans, and use the fan and ventilation calculator to size one.
• Open a roof vent for an hour or two around midday on sunny days.
• Water in the morning, and only when the top inch of soil is dry.
• Aim for about 50 to 70% humidity. A cheap thermometer-hygrometer with a min/max memory tells you what happened overnight.

Real example: insulating a Royal Victorian VI34, step by step

Here is the whole plan on one real greenhouse, the Janssens Royal Victorian VI34. It's 10 x 15 ft with 4 mm tempered glass, one of our most popular glass greenhouses. We'll put it in zone 7, keep it at 45°F at night for overwintering citrus and winter greens, and size everything for a 12°F night.

1. Measure it

2. Make the plan

• Step 1, seal: new brush strip on the doors, foam tape along the 50 ft base, and an automatic opener on a roof window so it shuts itself at dusk.
• Step 2, bubble insulation: line all the glass above bench height, roof included, because zone 7 nights regularly drop into the teens.
• Step 3, foam: a 2.5 ft band of 1.5 in foam board all the way round, below the benches.
• Step 4, base: foam board buried 2 ft deep around the outside of the base.
• Step 5, floor: slatted shelves so pots sit off the cold floor, with two dark water barrels underneath.
• Step 8, air: a small clip-on fan running all winter, and a vent opened on sunny afternoons.

3. Shopping list

4. The result

Sealing, bubble insulation and foam together cut this greenhouse's heating bill by more than half, about $270 every winter. It also changes which heater you need. Without insulation, the VI34 needs about 21,900 BTU on a 12°F night, more than the DR-988's roughly 19,100 BTU. With those three steps, it needs about 9,800 BTU, so the DR-988 covers it with room to spare and cycles on far less often. The Bio Green Palma, at about 5,100 BTU, keeps the greenhouse above freezing as a backup if the main heater fails. Perimeter foam, the floor steps and a night curtain (Steps 4 to 6) would save more again; we left them out of the numbers to keep them conservative.

Your greenhouse insulation checklist

Screenshot or print this list and tick it off as you go. Most people can do it over two weekends in early fall.

Measure your own greenhouse

Use the same math as the VI34 example to work out what to buy. All you need is a tape measure.

Then put your greenhouse into the heating cost calculator to see your own savings, and the heater size calculator to pick a heater. Most of our greenhouses are already in the list, so the sizes fill in for you. For real costs by greenhouse size, see how much it costs to heat a greenhouse in winter.

Want a fully insulated greenhouse? Start with the glazing

Everything above makes an existing greenhouse warmer. But if you're buying a greenhouse for year-round growing in a cold climate, the glazing you choose matters more than any add-on. Insulation that's built in works every night, with nothing to put up or take down.

• Hoklartherm Riga greenhouses: German-made 8 mm twin-wall polycarbonate on the main body and 10 mm on the end walls, with a rounded shape that sheds snow. Hoklartherm builds the Riga for growing vegetables in the middle of winter, not just for season extension.
• Hoklartherm Santini II: 16 mm triple-wall polycarbonate, one of the best-insulating panel types you can buy for a backyard greenhouse.
• Hoklartherm Livingten: 22 mm dual-pane insulated glass, so you get the clarity of glass with far less heat loss than single panes. It is the greenhouse to choose if you want to grow tender plants all winter without putting up insulation every fall.
• Nordic garden rooms: double-glazed, Low-E glass rooms built to be used year-round, for people who want a garden room as much as a greenhouse.

Not sure which glazing suits your climate? Our glass vs polycarbonate buyer's guide compares them side by side.

Frequently asked questions