This is one of the most-asked questions we get, and it deserves a straight answer rather than a brochure one: a greenhouse stays warmer than the outdoors, but it will not stay frost-free through a hard winter night on sunlight alone. For true year-round growing in a cold climate, you pair the right greenhouse with a modest amount of supplemental heat. The good news is that a well-built one makes that heat go remarkably far.
Let me explain what's actually happening, so you can decide what your winter really needs.
What "warm" means during the day vs. at night
On a sunny winter afternoon, a greenhouse can be shirt-sleeves warm inside while it's near freezing outside. That's the greenhouse effect doing its job — sunlight comes through the glazing, warms everything inside, and the structure holds that heat in. Even in January, a bright day can push the interior well above the outdoor temperature with no heater at all.
The challenge is night. Once the sun goes down, there's no incoming heat, and whatever warmth you banked slowly bleeds back out through the glazing. How fast it bleeds — and whether your plants dip below freezing before morning — depends on three things: how well your greenhouse holds heat, how cold your nights get, and whether you're adding any heat of your own.
So "do greenhouses stay warm in winter?" really becomes two questions: how much heat does mine hold, and how much do I need to add?
How much heat does the greenhouse itself hold?
This is where the structure earns its money, and it comes down mostly to glazing.
- 10mm twin-wall polycarbonate insulates best. Its two walls trap a layer of air, and that air is what slows heat loss overnight — the difference between a heater that cycles occasionally and one that runs all night.
- A hybrid setup — tempered glass walls, polycarbonate roof — keeps heat in through the roof (where most of it escapes) while giving you the clear glass look at eye level.
- Single-pane glass is the classic look and perfectly good in milder winters, but it holds less heat than twin-wall polycarbonate.
Our roof material guide breaks the glazing options down in detail, and if winter is genuinely serious where you are, the cold-climate guide covers the structural side — snow load, frame, and foundation depth.
A quick, honest word on sealing: heat also leaks through gaps, so a tight, well-assembled structure on a proper level foundation holds warmth far better than a racked, drafty one. Getting the foundation right isn't just about standing up to frost heave — it's about not leaking your heat into the ground all winter.
How much heat do you need to add?
For most people who want to grow tender plants or start seeds through a real winter, the answer is some — and usually less than they expect. A few ways buyers handle it:
- An electric or gas greenhouse heater on a thermostat is the most common setup. The thermostat only fires the heater when the temperature drops toward your set point, so a well-insulated greenhouse sips power instead of guzzling it.
- Thermal mass — a few black barrels or jugs of water inside — soaks up heat during the day and releases it slowly at night, flattening out the overnight low for free.
- Grouping tender plants together and covering them on the coldest nights buys you a few degrees when a cold snap arrives.
How much heat you'll actually need depends entirely on your climate and what you're growing. Keeping a greenhouse just above freezing to overwinter dormant plants is a light lift. Growing tomatoes in Minnesota in January is a bigger one. A good place to start is the USDA Plant Hardiness Zone Map — it tells you your zone, which shapes both what you can grow and how much supplemental heat is realistic.
Don't forget to vent — even in winter
It sounds backwards, but ventilation still matters in the cold. On a bright winter day a sealed greenhouse can overheat and cook plants, and stale, damp air invites mold. Roof vents and a louvered intake let you dump excess heat and manage humidity, and automatic vent openers do it hands-off while you're away. Our ventilation guide covers how to set that up.
The honest bottom line
A greenhouse won't magically stay tropical through a Montana January on its own — no honest seller will tell you it does. What a quality greenhouse does is hold heat so well that a modest, thermostat-controlled amount of supplemental warmth turns it into a genuine four-season growing space. Choose your glazing for your climate, seal it up on a proper foundation, add the right heater, and year-round growing stops being a dream and becomes a Tuesday in February.
Every Janssens greenhouse is backed by a 15-year warranty and built in Lier, Belgium — the kind of structure that makes decades of winter growing possible.
Want to grow through your winter? Call us or send your zip code and we'll match your glazing, foundation, and heating plan to your actual climate and the plants you want to keep alive. We do this every week — there's no wrong question here.