Best Weatherstripping, Door Seals and Draft-Proofing Kits

The leak you are paying to heat

A greenhouse loses heat two ways. It conducts it through the glazing, which is what bubble insulation addresses, and it loses it bodily as warm air escapes through gaps and cold air pours in to replace it. The second one is called air infiltration, and in a typical hobby greenhouse it is a bigger share of the total than most owners assume.

It is also the cheapest thing to fix by a wide margin. Insulating a small greenhouse costs a mid-range purchase. Sealing it costs a few dollars of foam tape and an hour. Doing the insulation first and the sealing never is like putting a sweater on and leaving the front door open.

This is why draft-proofing sits ahead of insulation and well ahead of heating in the fall changeover sequence. Every degree you stop losing is a degree you do not have to buy.

Finding the leaks in ten minutes

Pick a bright day, close the door and every vent, and stand inside. Leaks announce themselves as lines and pinpoints of daylight. Work around the whole structure methodically: door frame, threshold, hinge side, latch side, every vent surround, the eaves line where roof meets side, the base rail where the frame meets the ground, and every glazing overlap.

For anything you cannot see, use your hand on a windy day, or light a stick of incense and watch which way the smoke pulls. Smoke finds gaps that daylight does not, particularly around a base rail sitting on uneven ground.

The door is most of the problem

Doors are the biggest single leak in almost every greenhouse, for structural reasons rather than bad manufacturing. They are the only moving panel large enough to matter, they carry the most wear, and sliding doors in particular run in channels that were never designed to seal.

Hinged doors take conventional foam or brush weatherstrip around the frame, plus a brush sweep on the bottom edge. Check the door still closes and latches after fitting — a compressed foam strip on all four edges can be enough to stop a light aluminium door latching properly, and a door held ajar by its own draft-proofing is worse than no strip at all.

Sliding doors are harder. The runner gap cannot be sealed without stopping the door sliding, so the approach is brush strip along the leading edge and the jamb it closes against, plus a brush at the bottom that rides above the runner rather than in it. Accept that a sliding door will never seal as well as a hinged one.

Both types benefit from a heavy clear curtain hung inside the doorway, creating an air lock. It is crude and it works remarkably well, particularly on a door you use daily in winter.

Vents, and the autovent problem

Automatic vent openers work on a wax cylinder that expands with heat. In cold weather the cylinder contracts and the vent closes — but often not tightly, because the mechanism has no closing force beyond gravity and a light spring. A vent sitting two millimetres open all winter is a permanent chimney at the highest point of the structure, which is precisely where you least want one.

Two fixes. Fit foam strip around the vent frame so the lid closes onto a compressible seal rather than metal onto metal. Or, on vents you genuinely will not use until spring, remove the autovent cylinder entirely for the winter and latch the vent shut. Keep at least one vent operable, always — a fully sealed greenhouse is a gray mold problem waiting to happen, as covered in autumn condensation and gray mold.

Glazing overlaps and the base rail

Panes in an aluminium frame are held by clips and can creep. Over a summer of thermal cycling, an overlap that was 15mm can become 5mm, and a pane can lift out of its channel entirely at one corner. Walk the house and push every pane back into position, then add or replace clips where they are missing. This is free and it is routinely the largest single leak after the door.

The base rail is the other big one. Very few greenhouses sit on perfectly level ground, and the gap under the rail runs the entire perimeter. Soil banked against the outside of the rail seals it effectively and costs nothing. A strip of foam or a bead of exterior sealant works where the rail sits on a hard base.

What actually seals, and what fails by February

• Closed-cell foam tape — the workhorse. Compresses well, resists water, survives a season outdoors. Buy closed-cell, not the open-cell foam sold for indoor draughtproofing, which soaks up water and freezes.
• Brush strip — the right answer for anything that slides or rubs. Survives movement that would shred foam.
• EPDM rubber profile — the durable option for door frames, several seasons where foam gives one or two. More expensive, more fiddly to fit, better in the long run.
• Exterior silicone or frame sealant — for permanent gaps you will never need to open. Not for anything that moves.
• Duct tape — will not survive a winter of UV and condensation, and leaves adhesive residue that takes a season to clean off. Skip it.

What to buy

Do it in this order

1. Re-seat every pane and replace missing clips. Free, and often the biggest win.
2. Seal the base rail with banked soil or sealant.
3. Strip the door frame and fit a bottom sweep. Check it still latches.
4. Strip vent surrounds; latch shut the vents you will not use, leaving at least one operable.
5. Only now fit insulation film, onto a dry, sealed, clean frame.

The sequence matters because film fitted over an unsealed frame hides the leaks rather than fixing them, and because clips and sealant are much easier to apply before there is a layer of bubble film in the way. Once sealing and insulation are done, the heater sizing conversation becomes a much smaller one — see shoulder-season heaters.

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Common questions