How to Stop Condensation in Multiwall Polycarbonate Roofing Sheets (Complete Guide)

How to Stop Condensation in Multiwall Polycarbonate Roofing Sheets (Complete Guide)

Multiwall polycarbonate sheets are designed to prevent moisture build-up, but condensation can still happen — especially during cold months or when the sheets were installed with the wrong tapes, incorrect ventilation, or without the right accessories. The good news is that condensation inside polycarbonate sheets is completely preventable once you understand the airflow requirements and the correct installation method.

This guide explains exactly why condensation forms in multiwall polycarbonate sheets, how to stop it, and how to fix it if moisture has already appeared.

Internal links included:

What Is Condensation in Multiwall Polycarbonate?

Condensation occurs when warm, moist air meets the cool interior chambers of multiwall polycarbonate sheets. This creates tiny water droplets inside the flutes.

Condensation is often mistaken for leaks, but the causes are completely different:

  • Leaks = Water entering through gaps

  • Condensation = Moisture forming from trapped air and poor ventilation

If addressed early, condensation can be fixed — but if moisture gets inside the sheet chambers due to incorrect taping, long-term clarity issues may occur.

1. The Most Common Cause: Incorrect Tapes on the Sheet Edges

Every multiwall polycarbonate sheet must have:

  • Aluminium tape on the top (high end)

  • Breather tape on the bottom (gutter end)

If these are reversed, missing, or blocked, condensation will form inside the chambers.

Why aluminium tape is used at the top

It stops wind-driven rain and snow from entering the chambers.

Why breather tape is used at the bottom

It allows trapped moisture to escape naturally.

Breather tape contains micro-perforations that let air out while stopping insects and dust entering the sheet.

Fix: Replace the incorrect tape

  1. Remove the bottom drip trim

  2. Remove incorrect tape

  3. Clean the edge of the sheet

  4. Apply new breather tape

  5. Refit drip trim or U-profile

Replacement tapes:
https://polycarbonatex.co.uk/collections/multiwall-accessories

If the top tape is also incorrect, repeat the process using aluminium tape.

2. Condensation Caused by Blocked Breather Tape

Even if the correct tape is used, condensation can still form if:

  • Dirt blocks the breather tape

  • Insects clog the holes

  • Moss grows near the gutter edge

  • Drip trim is fitted too tightly and blocks airflow

Fix: Restore ventilation

  1. Remove drip trim

  2. Clean tape edge gently

  3. Ensure ventilation gap at the bottom

  4. Reinstall trims with a 1–2mm breathing gap

If the breather tape is damaged or mouldy, replace it entirely.

3. Missing or Incorrect Drip Trim

A drip trim directs water downwards and helps ventilation flow through the bottom chambers.

If the drip trim is:

  • Missing

  • Fitted incorrectly

  • Sloping backward

  • Too tight over the tape

…condensation will build up and stay trapped.

Fix: Install or adjust drip trim

A correctly installed drip trim allows:

  • Moisture to escape

  • Chambers to ventilate

  • Water to drain directly into the gutter

View drip trims in accessories:
https://polycarbonatex.co.uk/collections/multiwall-accessories

4. Glazing Bars Not Sealing or Ventilating Correctly

Glazing bars keep the sheets in place and provide weather protection, but they must be fitted with the correct gaskets and end caps.

Glazing bars can cause condensation if:

  • End caps are missing

  • Gaskets are cracked

  • Bars are overtightened

  • Bars are twisted due to heat or poor installation

  • Water enters from the top of the bar

Fix:

  1. Inspect glazing bar end caps

  2. Replace any missing or loose end caps

  3. Check gaskets for gaps or shrinkage

  4. Loosen overtightened screws

  5. Replace old bars if warped

Replacement end caps:
https://polycarbonatex.co.uk/collections/multiwall-accessories/products/corotherm-multiwall-polycarbonate-sheet-glazing-bar-end-cap

5. Incorrect Roof Pitch (Too Flat)

A polycarbonate roof must have at least a 5° pitch — ideally more.

If the roof is too flat:

  • Moisture cannot drain

  • Warm air sits below the sheets

  • Condensation increases

  • Water can run backwards into joints

Fix:

Increase roof pitch if possible.
If not:

  • Ensure top aluminium tape is fully sealed

  • Ensure bottom ventilation is clear

  • Re-tape any areas compromised by trapped moisture

6. Temperature Differences Inside the Structure

Condensation naturally occurs when:

  • A hot greenhouse meets cold outside air

  • A warm conservatory roof meets winter conditions

  • A pergola has no airflow underneath

Fix: Improve airflow

  • Add ventilation gaps

  • Ensure underside of roof isn’t completely sealed

  • Avoid blocking airflow with heavy beams

  • Consider small roof vents for enclosed structures

Ventilation is the #1 long-term defence against condensation.

7. Water Already Inside the Chambers

If water is visible inside the multiwall chambers (not just misting), this usually means:

  • Wrong tape was used

  • Tape failed

  • Tape lifted over time

  • The edge of the sheet was cut incorrectly

  • End closures missing

Once water enters, it rarely comes out fully.

Fix: Replace the sheet

Order a replacement to the exact size you need:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

If replacing the sheet, always install:

  • Aluminium tape at the top

  • Breather tape at the bottom

  • End closures or U-profiles

  • Proper drip trims

8. Cold Bridging from Fixings

Driving screws directly through polycarbonate sheets is not recommended.

Doing so can cause:

  • Internal condensation

  • Hairline cracks

  • Drips from screw holes

  • Heat loss increasing condensation

Fix:

  • Remove incorrect fixings

  • Replace with glazing bar system only

  • Use rubber washer fixings only when appropriate

  • Seal holes using LM roofing sealant

If sheets are cracked at fixings, replacement is required.

9. Moisture from Surrounding Environment

Some environments naturally cause more condensation:

  • Greenhouses

  • Hot tubs under pergolas

  • Roofs above cooking/eating areas

  • Humid garden rooms

Fix:

  • Increase roof pitch

  • Add ventilation

  • Use correct tapes and trims

  • Install drip trim to promote drainage

Correct accessories:
https://polycarbonatex.co.uk/collections/multiwall-accessories

10. Poor Cleaning Practices

Using pressure washers or harsh chemicals can cause:

  • Tape damage

  • Water forced behind sheets

  • Gasket deterioration

  • Internal moisture build-up

Fix:

Clean using:

  • Warm soapy water

  • Soft sponge

  • Light pressure from a hose

Never spray water directly up into the sheet edges.

How to Prevent Condensation Long-Term

To keep your sheets clear and dry:

✔ Use the correct tapes every time

Top = Aluminium
Bottom = Breather

✔ Keep ventilation gaps open

Never block the tape area.

✔ Install drip trims properly

Allows water and moisture to escape.

✔ Maintain glazing bars and gaskets

Replace if worn.

✔ Keep the roof clean

Avoid pressure washers.

Accessories for long-term prevention:
https://polycarbonatex.co.uk/collections/multiwall-accessories

When You Should Replace the Sheets

Replace sheets when:

  • Water is trapped inside chambers

  • Sheets are cloudy or mouldy inside

  • Fixings have caused cracks

  • Sheets are bowed from heat

  • Tape failure has allowed dirt to enter

Order new cut-to-size sheets here:
https://polycarbonatex.co.uk/products/cut-to-size-multiwall-polycarbonate-sheet

Conclusion

Condensation in multiwall polycarbonate sheets is almost always caused by incorrect taping, poor ventilation, or worn accessories. With the right tapes, trims, glazing bars and airflow, your roof can remain clear, dry and long-lasting.

To get compatible accessories and replacement parts for a professional, watertight finish, visit:

With the correct installation and maintenance approach, your multiwall polycarbonate roof can stay condensation-free for 20+ years.

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