To keep a glass greenhouse warm in winter, first stop the heat from leaking out, then add just enough heat back. Seal the gaps, add a layer of insulation and use a thermostat-controlled heater sized for your coldest night. Most of our customers find that sealing and insulating saves more money than any heater upgrade.
✅ Quick answer
- Single-pane glass loses heat about twice as fast as double-wall polycarbonate or insulated glass.
- Seal drafts first. Check door seals, vents and the base before anything else.
- Line the glass with clear bubble wrap to add a trapped layer of air, like double glazing.
- Size your heater with this formula: surface area × U-value × temperature gap.
- Keep it at 40 to 45°F for hardy greens. Every degree warmer adds to your bill.
- Buying new? Insulated glass or a polycarbonate roof cuts winter heating a lot.
We are a small, woman-owned greenhouse business in Georgia, and we answer every question personally. "How do I keep my glass greenhouse warm?" comes up every fall. Here are the nine tactics we share most, in the order we suggest you try them.
Why is a glass greenhouse hard to keep warm in winter?
Glass lets in lots of light, which is why we love it. But a single pane does little to hold heat in. On a clear, cold night, warmth escapes through the glass, the frame and every small gap.
Engineers rate this heat loss with a "U-value." A lower number means less heat escapes. Here are common planning values used by greenhouse growers and university extension services:
| Glazing | U-value | Heat loss vs. single glass |
|---|---|---|
| Single-pane glass | 1.25 | 100% (baseline) |
| Double-wall polycarbonate | 0.7 | About 56% |
| Double-layer insulated glass | 0.7 or better | About 56% or less |
So the goal is simple. Make your single glass act more like a double layer, and keep cold air out. The tactics below do exactly that.
How do I stop cold drafts in my glass greenhouse?
Tactic 1: Seal every gap
Drafts are the cheapest heat loss to fix. Walk around your greenhouse on a windy day and feel for moving air. Then work through this list:
- Doors: replace worn door seals and brush strips. Make sure doors close tight.
- Vents and louvers: check that they close fully. Clear leaves and grit from the tracks.
- Glass panes: replace cracked or slipped panes. Clear weatherproofing tape can seal small joints.
- The base: seal gaps between the frame and your foundation. Cold air loves to creep in at ground level.
How your glass is held in matters too. The Royal Victorian VI34 uses 4mm tempered glass held by rubber seals, with single large panes instead of overlapping pieces. Fewer overlaps mean fewer places for cold air to slip through.
What is the best way to insulate a glass greenhouse?
Tactic 2: Line the glass with bubble wrap
Clear horticultural bubble wrap is the classic winter fix for glass. It traps a layer of air against the glass, a bit like double glazing. It still lets light through, though you lose some.
- Clean the glass first so you keep as much winter light as you can.
- Fix the wrap to the inside of the frame, not to the glass. Clips made for greenhouse glazing bars work well.
- Leave a small air gap between wrap and glass. That pocket of still air is what does the insulating.
- Tape the joins so warm air can't slip behind the wrap.
- Leave vents free so you can still let out damp air on sunny days.
💡 Our tip: if you only want to protect a few plants, wrap the walls first and the roof last. The roof is where most of your winter light comes in, so many growers leave it clear.
Tactic 3: Insulate the base and the north wall
The north side of a greenhouse gets little winter sun. Rigid foam board on the lower part of that wall cuts heat loss without costing you much light. Foam along the inside of the base also helps keep cold soil from chilling the air.
A solid, sealed foundation makes a big difference here. Our greenhouse winterizing checklist walks through the full fall prep.
What size heater does a glass greenhouse need?
Tactic 4: Use a heater sized for your coldest night
In most of the US, a glass greenhouse needs some heat to stay frost-free all winter. The trick is to size the heater for your coldest night, not your average one. Use this formula:
📐 Greenhouse heater formula
BTU per hour = Surface area × U-value × Temperature gap
Surface area = walls + roof in sq ft · U-value = 1.25 for single glass · Gap = inside °F minus coldest outside °F · Watts = BTU ÷ 3.412
🧮 Worked example: Cross Country Parkside 8x12 in glass
- Walls: about 40 ft around × 5 ft high = 200 sq ft
- Roof (12/12 pitch) plus the two gable ends: about 168 sq ft
- Total surface area: about 368 sq ft
- Keep it 50°F inside on a 14°F night: a gap of 36°F
368 × 1.25 × 36 = about 16,600 BTU per hour. Add 20% for safety and you get about 19,900 BTU, or roughly 5,800 watts. The manufacturer's own figure for the same conditions is 18,547 BTU, so our quick math lands close.
That number tells you a single 1,500-watt plug-in heater (about 5,100 BTU) is not enough for this house on a 14°F night. It would need a 240V heater, or a smaller heater plus the insulation tactics above. Here is how the heaters we carry compare:
| Heater | Power | Heat output | Best for | Approx. price* |
|---|---|---|---|---|
| Bio Green Palma | 110V, 1,500 W | About 5,100 BTU | Small houses, mild winters, backup | From about $149 |
| Milkhouse heater | 120V, 1,300 or 1,500 W | Up to about 5,100 BTU | Small houses, backup heat | From about $160 |
| Dr. Infrared DR-988 | 240V, 5,600 W | About 19,100 BTU | 8x12 glass houses in cold areas | From about $220 |
*Prices checked in October 2026. Prices can change and sales may apply, so click the model name to see today's price.
We also carry ceiling-mounted and propane options in our greenhouse heater collection. For monthly running costs by size, see how much it costs to heat a greenhouse in winter.
⚠️ Safety first: a 240V heater like the DR-988 needs a dedicated 30-amp circuit. Have a licensed electrician run it, and use GFCI protection in a damp greenhouse. Any fuel-burning heater needs fresh air and a carbon monoxide detector. Always follow the heater's manual.
Can I keep a greenhouse warm without a big heater?
Tactic 5: Add thermal mass
Thermal mass stores the sun's free heat by day and gives it back at night. Dark-painted water barrels under benches are the easiest version. Stone, brick and concrete paths help too.
Mass won't replace a heater in a cold climate. But it smooths out temperature swings and lets your heater run less. Our guide to heating a greenhouse without electricity covers ten passive ideas.
Tactic 6: Heat a smaller zone
You don't always need to heat the whole greenhouse. Hang a bubble-wrap curtain to close off one end. Group tender plants there, and heat only that space.
For a few trays of seedlings, a heated propagator or heat mat is cheaper still. It warms the roots, which is where young plants need it most.
Tactic 7: Lift plants off the cold floor
Cold air sinks, so the floor is the coldest spot in your greenhouse. Benches and shelves raise pots into warmer air. They also let air move around the plants, which helps prevent mold.
Should I ventilate a greenhouse in winter?
Tactic 8: Move the air and control damp
Yes, a little. A closed, heated greenhouse gets damp fast. Damp, still air invites mold and mildew on winter plants.
- Run a small circulation fan. It stops warm air from pooling under the roof, so your heater cycles less.
- Open a vent briefly on sunny days, around midday, then close it well before the sun goes down.
- Water in the morning, so leaves and soil surfaces dry before night.
- Use a min-max thermometer, ideally one with a remote alarm, so you know if the heater fails.
Which glass greenhouse stays warmest in winter?
Tactic 9: Choose better glazing when you buy
If you are shopping for a new greenhouse and plan to grow all winter, glazing is the biggest choice you will make. You have three main paths:
| Option | Example | Winter heat loss | Best for |
|---|---|---|---|
| Single tempered glass | Royal Victorian, Parkside | Highest | Mild winters, frost protection |
| Hybrid: glass walls, polycarbonate roof | Royal Victorian or Parkside hybrid option | Lower | The glass look with a warmer roof |
| Insulated (dual-pane) glass | Livingten | Lowest of the three | Serious year-round growing |
The Livingten uses 22mm dual-pane glass (4mm glass, a 14mm gas gap, 4mm glass). Its aluminum profiles are thermally insulated, so the frame doesn't act as a cold bridge. It comes in 163, 195 and 225 sq ft sizes.
Love the classic look? The Royal Victorian range and the Cross Country Parkside 8x12 both offer a polycarbonate roof with glass side walls. You keep the clear glass walls and lose less heat through the roof.
📌 About prices: manufacturers update their prices during the year, and we often run sales and promotions. The price on the product page is always the current one. Prices in this article are a guide from when we wrote it (October 2026), not a quote or guarantee. Found a lower price for the same model at an authorized retailer? We offer a Price Match Guarantee - please check the rules first. Call us and we will confirm today's price and any current deals.
How warm should a glass greenhouse be in winter?
Set your thermostat for what you grow, not for comfort. Lower settings save the most money.
- Frost-free (around 40°F): overwintering hardy plants, pots and bulbs.
- 40 to 45°F: lettuce, spinach, kale and other hardy winter greens.
- 55 to 60°F at night: tomatoes, peppers and tropical plants. This costs the most to keep.
🌱 Put it together: seal first, insulate second, then add a right-sized heater on a thermostat. Thermal mass and a smaller heated zone trim the bill further. That order gives you the warmest greenhouse for the least money.
Not sure which tactics fit your greenhouse and climate? Give us a call. We are happy to help, we offer free shipping to the continental US, and our assembly crews can set up your new greenhouse for you.
Frequently asked questions
How do I keep my glass greenhouse warm in winter?
Seal drafts at doors, vents and the base. Line the glass with clear bubble wrap, add thermal mass, and run a heater on a thermostat sized for your coldest night.
Will a glass greenhouse stay warm without heat?
On sunny days, yes. On clear, cold nights, single glass loses heat quickly and can drop close to the outside temperature. In mild areas, insulation and thermal mass may be enough for hardy plants.
Does bubble wrap really work in a greenhouse?
Yes. It traps a layer of still air against the glass, which slows heat loss. It does cut some light, so use clear horticultural bubble wrap and keep the glass clean.
Is glass or polycarbonate warmer in winter?
Double-wall polycarbonate holds heat better than single glass, roughly halving heat loss. Insulated dual-pane glass, like the Livingten's, performs at least as well as double-wall polycarbonate.
Can a 1,500-watt heater heat a glass greenhouse?
A 1,500-watt heater gives about 5,100 BTU per hour. That suits a small, well-sealed glass greenhouse in a mild climate. An 8x12 glass house in a cold area usually needs a 240V heater.
Should I open greenhouse vents in winter?
Briefly, on sunny days. A short airing around midday lets out damp air and helps prevent mold. Close the vents well before evening to keep the day's heat in.
Want a warmer greenhouse this winter?
Tell us your greenhouse size, where you live and what you want to grow. We will help you pick the right heater, glazing or new greenhouse, and let you know about any current sales.