How to Insulate Cold Frames and Greenhouses: Essential Tips for Winter Protection

As winter approaches, insulating your cold frames and greenhouses becomes crucial to protect your plants from freezing temperatures and ensure they thrive throughout the colder months. Proper insulation helps maintain a stable environment inside, keeping your plants warm and healthy. In this guide, we’ll show you how to effectively insulate cold frames and greenhouses in November to create the ideal growing conditions for your winter garden.

Why Is Insulating Cold Frames and Greenhouses Important?

Insulating cold frames and greenhouses helps create a microclimate that shields your plants from the harsh effects of winter weather, including frost, wind, and freezing temperatures. By maintaining a consistent temperature, you can extend the growing season for certain crops, protect tender plants, and ensure that overwintering plants survive until spring. Proper insulation also helps reduce heating costs for greenhouses by improving energy efficiency.

Top Tips for Insulating Cold Frames and Greenhouses in November

1. Use Bubble Wrap for Greenhouse Insulation

Bubble wrap is one of the most popular materials for insulating greenhouses because it’s inexpensive, effective, and easy to install. The bubbles trap air, providing an additional layer of insulation that helps maintain a consistent temperature inside the greenhouse.

How to apply: Attach bubble wrap to the inside of the greenhouse panels using greenhouse clips or tape. Cover all sides, focusing on the north-facing wall and the roof, where the most heat escapes. Make sure there are no gaps, as even small drafts can affect the internal temperature.
Type of bubble wrap: Use UV-stabilized bubble wrap, as it’s designed to withstand sunlight without degrading. Opt for larger bubbles if possible, as they provide better insulation.

2. Add Insulation to Cold Frames

Cold frames are smaller, unheated structures that benefit greatly from added insulation in the winter months. Insulating the sides of the cold frame with materials such as straw, hay, or even cardboard can help trap heat inside.

How to insulate: Line the inside walls of your cold frame with straw or bubble wrap to insulate the structure. You can also place old blankets or garden fleece over the top at night to prevent heat loss when temperatures drop significantly. Remember to remove the covers during the day to allow light in.
Insulating the base: If your cold frame is on bare ground, you can further insulate it by adding a layer of straw or leaves at the base. This helps keep the soil warm and protects the roots of your plants.

3. Seal Gaps and Cracks

Any gaps or cracks in your greenhouse or cold frame can let in cold air, reducing the effectiveness of your insulation. In November, take the time to inspect your greenhouse or cold frame for any small openings that might allow drafts.

How to seal gaps: Use weatherproof sealant or caulking to fill any cracks around the frame, doors, or windows. Check that doors and vents close tightly to prevent drafts from entering. You can also use foam tape around the edges of doors and windows to improve the seal.

4. Install Thermal Curtains or Insulation Boards

Thermal curtains or insulation boards can provide an extra layer of warmth inside your greenhouse, especially on particularly cold nights. These materials work by trapping heat and preventing it from escaping through the walls or roof.

Thermal curtains: Hang thermal curtains or heavy-duty horticultural fleece inside the greenhouse, particularly on the north-facing side. These curtains can be drawn at night to help retain heat and pulled back during the day to allow sunlight in.
Insulation boards: Polystyrene or foam insulation boards can be attached to the lower half of greenhouse walls to help reduce heat loss. Be sure to leave the upper portion of the greenhouse uncovered so light can still enter.

5. Use a Thermal Mass to Retain Heat

Thermal mass is any material that absorbs heat during the day and releases it slowly at night, helping to maintain a stable temperature in your greenhouse. Adding objects with a high thermal mass, such as water barrels or large rocks, can help regulate temperatures without the need for additional heating.

Water barrels: Place large water containers, such as barrels or jugs, along the inside walls of your greenhouse. The water absorbs heat during the day and slowly releases it as the temperature drops at night. Make sure the barrels are placed where they will get the most sunlight.
Rocks or bricks: Similarly, large rocks or bricks can act as a thermal mass. Place them in sunny spots during the day to absorb heat, which will be radiated back into the greenhouse at night.

6. Use Fleece or Row Covers Inside

In particularly cold weather, even with insulation, your plants may need additional protection. Horticultural fleece or row covers provide an extra layer of warmth directly around your plants, creating a microclimate that keeps the plants insulated from frost.

How to use fleece: Drape horticultural fleece over delicate plants at night or on particularly cold days to trap heat and provide extra protection. The fleece is lightweight, breathable, and allows some light through, making it ideal for protecting plants inside greenhouses or cold frames.
Row covers: For larger areas, use row covers or cloches inside your greenhouse to insulate specific sections of plants. These covers help retain warmth around the plants and prevent frost damage.

7. Improve Ventilation on Sunny Days

While insulation is key to keeping your greenhouse warm, proper ventilation is equally important to prevent overheating and condensation buildup. On sunny days, temperatures can rise quickly inside a well-insulated greenhouse, even in winter.

Open vents: On mild, sunny days, open the vents or doors of your greenhouse or cold frame to let fresh air in and prevent the temperature from getting too high. Proper airflow also helps reduce condensation, which can lead to mold or mildew.
Monitor temperature: Use a thermometer to regularly monitor the temperature inside your greenhouse. Ideally, the temperature should stay above freezing but not rise too high, as fluctuations can stress the plants.

8. Consider Adding a Heater

In regions with extremely cold winters, insulating your greenhouse or cold frame might not be enough to keep temperatures above freezing. If you live in a particularly cold climate, consider adding a small heater to maintain a consistent temperature for your plants.

Types of heaters: Electric, gas, or paraffin heaters are commonly used in greenhouses. Choose a heater with a thermostat so that you can control the temperature and avoid overheating. Always follow safety guidelines when using heaters in enclosed spaces.
Supplementing insulation: Even with a heater, it’s important to insulate your greenhouse or cold frame well. Insulation reduces the amount of energy needed to heat the space, making it more cost-effective and environmentally friendly.

Most Common Questions and Answers About Insulating Cold Frames and Greenhouses

Q1: What is the best material for insulating a greenhouse?
A: Bubble wrap is one of the best materials for insulating greenhouses. It’s affordable, easy to install, and effective at trapping heat. Use UV-stabilized bubble wrap for long-lasting protection against the sun’s rays.

Q2: How do I keep my cold frame warm in the winter?
A: To keep your cold frame warm, line the sides with straw, bubble wrap, or insulation boards, and cover it with garden fleece or blankets at night. Additionally, insulate the base with straw or leaves to protect the soil and plant roots from freezing.

Q3: Should I insulate the roof of my greenhouse?
A: Yes, the roof is a major source of heat loss in a greenhouse. Insulating the roof with bubble wrap or thermal curtains helps reduce heat loss while still allowing sunlight to enter during the day.

Q4: How can I prevent frost in my greenhouse?
A: To prevent frost, insulate the walls with bubble wrap, use horticultural fleece over sensitive plants, and consider adding a small heater if necessary. Water barrels or other thermal mass objects can also help maintain heat overnight.

Q5: How do I stop condensation in my greenhouse?
A: Proper ventilation is key to preventing condensation. Open vents or doors on sunny days to allow moisture to escape. You can also use a dehumidifier if condensation becomes a persistent problem.

Q6: Can I insulate a greenhouse with polystyrene?
A: Yes, polystyrene boards can be used to insulate the lower sections of greenhouse walls. They provide excellent insulation and help reduce heat loss. Be sure to leave enough space for light to enter from the upper sections.

Q7: How can I maintain airflow in a well-insulated greenhouse?
A: While insulating your greenhouse, make sure to maintain proper ventilation by opening vents or doors during the day. Good airflow prevents overheating, reduces condensation, and helps control humidity.

Q8: Should I heat my greenhouse in winter?
A: Heating a greenhouse in winter can be necessary in very cold climates, especially if you’re growing tender plants or want to extend the growing season. Use a small electric, gas, or paraffin heater, and combine it with proper insulation to minimize heating costs.

Q9: How do I insulate small cold frames?
A: Insulate small cold frames by lining the sides with straw, bubble wrap, or foam boards. You can also cover the frame with old blankets or garden fleece on cold nights and remove them during the day to let sunlight in.

Q10: Can I use horticultural fleece inside a greenhouse?
A: Yes, horticultural fleece is ideal for providing an extra layer of protection around plants inside a greenhouse. It traps heat and protects plants from frost while still allowing some light and air to pass through.

By insulating your cold frames and greenhouses in November, you create a stable environment that helps your plants thrive throughout the winter. Using materials like bubble wrap, thermal mass, and horticultural fleece ensures that your garden remains productive, even in colder temperatures, while reducing energy costs and protecting tender plants.