Skip to Content

How To Seal The Bottom Of A Greenhouse In 3 Steps

Sealing the bottom of your greenhouse is very important because it keeps cold air from leaking in and destroying your plants. The trick is to seal the gaps where the foundation meets the frame, using a high-quality caulk or silicone. How to seal the bottom of your greenhouse depends on what type of material the greenhouse is made of. It would be best if you determined which sealant to use that will work on your greenhouse material.

How to seal the bottom of your greenhouse in 3 steps:

  • Step 1 – Prepare your greenhouse floor with greenhouse fabric (weed mat).
  • Step 2 – Use a sealing method or sealant that is suited for the material of your greenhouse.
  • Step 3 – Seal gutter ends, corners, and nearby frame joints.

This article will discuss each of these steps in detail to provide you with a clear guide on how to seal the bottom of your greenhouse to ensure that your plants can thrive on the lovely warm air all year round!

How Do You Seal The Bottom Of Your Greenhouse?

Being able to grow plants and crops in extremely cold weather conditions is absolutely marvelous! However, if the bottom is not sealed airtight, it defeats the whole purpose of having a greenhouse.

The following steps will guide you on how to go about sealing the bottom of your greenhouse to prevent any cold air from getting in and ruining your plants.

Step 1 – Prepare Your Greenhouse Floor With Greenhouse Fabric (Weed Mat)

Before you seal your greenhouse, you need to lay down a weed mat to keep weeds from growing and using up the warmth and air intended for your plants or crops.

Roll your greenhouse fabric out the whole length of your greenhouse. Cut as many fabric sheets as you need to cover the entire surface of the greenhouse, leaving about 6 inches at both ends of each fabric sheet and overlapping 6 inches at the lengths.

  • Tuck the greenhouse fabric under the greenhouse to the outsides so that the edges of the greenhouse are on top of the greenhouse fabric.
  • You can use a staple gun to secure the fabric to the greenhouse outside by turning the extra inches of fabric up and stapling it to the greenhouse frame.
  • You can also use Aluminum Tape to secure the fabric to the frame.
  • Make sure it adheres properly by applying enough pressure on the tape, using your fingers.
  • Walk around the greenhouse and check if all the tape is secure and there are no gaps between the fabric and the frame.

Some people even use Saran wrap on top of the tape around the entire greenhouse to ensure it is sealed correctly. Staple the overlaps of the sheets on the inside of the greenhouse, probably.

Once your fabric floor is secured and sealed, you can place your plants directly onto the fabric (weed mat) floor, or you can place concrete slabs to put them on or as a walking surface or pathway.

Different Types Of Flooring For Your Greenhouse

  • Limestone
  • Tamped down soil
  • ¾ inch crushed gravel (excellent drainage)
  • Stained concrete bricks
  • Combination of gravel and paving stones
  • Wood planks (sealed/vanished/waterproofed) – limestone works well to seal wood.
  • Tiles (great option for a level foundation)
  • Mulch (inexpensive and effective)

Base Styles

  • Slabs
  • Brick base
  • Solid concrete base – requires adequate drainage.
  • Concrete plinth – concrete flooring just around the edge of the greenhouse
  • Tarmac
  • Block paving
  • Bare earth

Here’s a helpful video showing how to prepare a greenhouse floor:

Step 2 – Choose A Sealing Method Or Sealant To Use For Your Greenhouse

There are different ways to seal various types of greenhouses. You must use the correct sealant or sealant method for your type of greenhouse. These include;

Polycarbonate Greenhouses

Sealing around the panels where the frame and the polycarbonate panels connect is imperative. If you don’t seal the tiny spaces around each panel, plenty of air will be able to pass through, cooling down the warm air inside of the greenhouse.

There is no point in using polycarbonate panels if you don’t seal them properly. These panels are hollow inside, and the cold air can get into the panels causing them to fail warm insulating air.

It also keeps dust and dirt from entering the flutes of the panels. Flutes (also called ribs or channels) are long, narrow, rectangular cells that run the length of the sheets.

The sheets’ flutes trap air, create an excellent thermal barrier, and provide incredible extra strength and long-spanning capabilities, making multi-wall polycarbonate a versatile glazing material.

There is one downside to the cellular structure of multi-wall polycarbonate. The flutes can trap dirt, insects, dust, and other debris. Fortunately, this is an easy problem to fix and can be done by using edge sealing tape or U-profile (u-shape edging).

To seal the panels, you can use metal (aluminum) tape. This tape is durable and waterproof, and it adheres strongly to polycarbonate. Make sure you seal all of the sides of the panels to keep any air from getting into the flutes.

I recommend getting an Aluminum Tape Here (link to Amazon) this type of tape is used for professional HVAC installation.

  • I recommend sealing both the top and bottom ends of the sheets.
  • Make sure you use high-quality tape that can expand and contract with the natural movement of the sheet.
  • A solid aluminum foil coated tape with an all-weather adhesive works well to seal off the top edge of your multi-wall polycarbonate sheet.
  • To seal off the bottom, use an aluminum vent tape made from a strong material.
  • Vent tape helps moisture control and helps to maintain sheet clarity.
  • Small drops of condensation in the flutes are normal, and a high-quality tape will allow the drops to get out while preventing dust from getting in.

There are a few things you should do before sealing your polycarbonate sheets. Make sure the flutes are clear of dirt, dust, or insects. Check that the sheets don’t have any sharp points on their edges; make them smooth before sealing.

Pull back a few inches of the protective plastic on the sheet to make way for the tape. Run your tape along the entire length of the edge top and bottom and fold the tape over each side, making sure it covers the whole length.

Smooth out the tape using your fingers by applying enough pressure. Another way to seal multi-wall polycarbonate sheets is by using U-profiles (aluminum or polycarbonate). When you attach the u-profiles to the top and the bottom of the sheet, make sure the long edge of the U-profile is on the inside of the panel.

Curtain Style Greenhouses

Use a heavy piece of metal or block of wood to hold down the bottom of the greenhouse plastic (curtains). The metal or wood needs to be the entire length of the sides of the greenhouse to cover the entire length of the plastic.

  • Fold the bottom piece of greenhouse plastic curtain inward and place the wood or metal bars on top of the plastic (all around the greenhouse) to secure and seal the plastic firmly to the ground.
  • Next, run Velcro strips from the top to the bottom of each corner of the frame (do not separate the two halves of the Velcro strip).
  • Make sure it has adhered properly by using enough pressure when you apply the strips to the frame.

Peel off the backing of the Velcro strips and carefully press the plastic curtain against the whole length of the adhesive backing. Once again, apply enough pressure to ensure that the curtain won’t separate from the adhesive strip.

To open the curtains, all you need to do is gently peel the Velcro (separating the two halves of the strip), and the plastic curtain will separate from the frame.

Glass Greenhouses

Glass greenhouses have frames that are similar to window frames. It is important to seal the areas where the glass meets the frame. You will need Pro Glaze sealant and Tremco 830 or a good quality caulk to fill the gaps in the glass frames.

Types Of Sealants

Some sealants won’t work on certain materials, therefore knowing which sealant to use is vital. There are various types of sealants, all intended for different purposes. These include;

Silicone Caulk

Building materials expand and contract in changing weather conditions. Tiny cracks usually appear where the greenhouse frame meets the foundation of the greenhouse. You can use silicone caulk to seal the small cracks.

It works especially well with a concrete floor and a wooden frame but can seal various materials. Silicone caulk lasts for several years and can withstand the humidity and heat inside a greenhouse.


Use caulk to fill any small cracks or gaps on the outside of the greenhouse.

Expandable Foam

Silicone caulk works well to seal small cracks but isn’t as effective on larger cracks. Expandable foam (dispensed from aerosol cans) seals these big cracks. The foam flexes as the materials contract and expand, so you don’t have to replace it every year.

Weatherproofing Repair Tape

This tape is great for sealing polycarbonate panels on long cracks along the foundation of your greenhouse. It is extremely durable and water-resistant.


Make a sloppy mix of cement and sand, and using rubber gloves, pack the cement mixture into the gaps using a stick to push it in to fill all the gaps properly. This is an inexpensive yet highly effective way to seal the bottom of your greenhouse.

Silicone (UV Stabilized, All-Weather, and Transparent)

Check for small gaps in the frame at the eave corners, base corners, and at the end of each ridge where the corner bars meet. Apply silicone on the inside of the building to help keep any leaks to a minimum.

Be careful not to block the ends of any internal gutters while you apply the silicone, as this will increase leaks instead of preventing them. Apply silicone on the outside of corners where the two base corners meet.

Bitumen-Based Sealant

Use black bitumen-based sealant or paint the joints with bitumen paint.

Step 3 – Seal Gutter Ends, Corners, and Frame Joints

  • The corners of a greenhouse are where 90% of leaking originates.
  • Make sure that the gutter ends and nearby joints are sealed with High-Quality Silicone (link to Amazon).
  • Use a black gutter sealer under the frame; it moves with any frame shift and is airtight and waterproof.

Important Greenhouse Features

Make sure that the following features are not affected when you seal the bottom of your greenhouse;


A greenhouse should always have vents to allow air to circulate inside. If it is completely sealed, your plants can die from mildew and mold. The only time to close the vents is at nighttime when the temperature is below freezing to prevent frost forming on your plants.


It is important to remember that greenhouses are not designed to be 100% watertight; a small amount of water should enter. A greenhouse floor needs to have good drainage. Floors with good drainage include;

  • Brick – allows water to drain through the gaps into the ground.
  • Dirt (bare earth) – natural drainage
  • Sand – great filtration and drainage
  • Stone or cement slabs – gaps between the slabs allow water to exit the greenhouse and creates a walking surface.
  • Gravel floor – a great choice for a greenhouse and could be used in conjunction with a weed barrier (weed mat). These weed barriers keep weeds from growing through the gravel, and the gravel provides a great drainage system for your greenhouse andd creates a tight enough seal that prevents air from entering the greenhouse.


Managing the humidity inside a greenhouse is vital because condensation in a greenhouse is not a good thing. If there are condensation droplets that form on the leaves of your plants, it can cause mildew to form. Therefore, sealing the bottom and the sides of the greenhouse is of utmost importance.


It’s one thing to seal the bottom of your greenhouse, but you also need to keep the warmth from the sunlight inside. Plastic or glass is great for light emission, but they don’t keep the heat very well. After you have sealed your greenhouse foundation, incorporate materials such as brick or stone to hold the heat longer.

You can do this by laying a brick or stone walking surface or pathway inside of the greenhouse. Although they slow down the healing process in your greenhouse, they take a long time to cool and will release the heat slowly throughout the night.


To keep your expenses (and problems) to a minimum, proper maintenance is necessary. Once a month, inspect the greenhouse inside and check out for cracks and holes. Regular inspection and maintenance help you stay on top of things and lowers the risk of plant damage or loss.

In Conclusion

If not properly sealed, stormy weather can wreak havoc on your greenhouse (source). Cracks around the foundation (bottom) of a greenhouse can make it difficult to maintain warm temperatures. Sealing the bottom ensures that the plants thrive in the correct levels of humidity and heat.

Sharing is caring!