Yes, most modern double glazed windows have weep holes. These are small, inconspicuous openings at the bottom of the external frame, built to drain condensation and rainwater that collects in the glazing channel. The British Fenestration Rating Council (BFRC) confirms weep holes are a standard requirement in sealed window units to prevent moisture build-up and frame damage (BRE, 2026).
Weep holes are not optional, they are a functional necessity for any double glazed window that meets current building regulations. Without them, trapped water can cause the glass seal to fail, leading to misting between panes and accelerated decay of the timber or uPVC frame. They are typically 5-8mm wide and may be covered by a small plastic cap or mesh to deter insects.
Weep holes are mandatory in double glazed windows
Building Regulations Part L requires that all replacement windows include adequate drainage for condensation and rainwater (GOV.UK, 2026). Weep holes fulfil this requirement. The Fenestration Self-Assessment Scheme (FENSA) also mandates that installers fit windows with weep holes to pass compliance checks (FENSA, 2026). If your windows lack weep holes, they are likely non-compliant and may invalidate your warranty.
Weep holes are not a sign of a window fault
Many homeowners mistake weep holes for damage or poor workmanship. In reality, they are a deliberate design feature. The Energy Saving Trust notes that weep holes are essential for maintaining the thermal efficiency of double glazing by preventing moisture-related seal failure (Energy Saving Trust, 2026). A blocked weep hole can cause water to pool inside the frame, leading to rot in timber windows or distortion in uPVC.
Weep holes must be kept clear for proper drainage
Debris, dirt, or paint can block weep holes over time. Check them annually, especially after storms or painting work. Use a pipe cleaner or soft brush to clear obstructions, never drill or enlarge the holes, as this can void the manufacturer’s warranty. The Glass and Glazing Federation advises that blocked weep holes are a common cause of window misting and can lead to costly repairs (MCS, 2026). If you see water inside the sealed unit, inspect the weep holes first.
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). This figure assumes mid-range uPVC frames with A-rated glazing from a FENSA-registered installer. The Energy Saving Trust estimates that upgrading from single glazing to A-rated double glazing saves a semi-detached home around £195 per year on heating bills. With an upfront cost of £6,500, the payback period works out at about 33 years. Over a 25-year product lifetime, the total energy savings would reach roughly £4,875, though the windows will likely last longer than the payback period. If the home is heated with mains gas and has loft insulation already in place, the savings are more predictable. Homeowners can check Energy Saving Trust for personalised figures based on property type and location.
| Item | Figure |
|---|---|
| Upfront cost after grants | £6,500 |
| Yearly savings | £195 |
| Payback period | 33 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is believing weep holes indicate a broken or poorly sealed window. This misunderstanding leads homeowners to block them up, causing costly damage. Here are the three biggest errors people make with weep holes.
- Blocking weep holes with sealant or paint Homeowners often seal weep holes thinking they are gaps or cracks. Blocking them traps moisture inside the frame, which can cause the double glazing seal to fail within months, leading to misted panes and a replacement cost of £300 to £500 per window.
- Assuming weep holes are only for rainy weather Many think weep holes are only needed during storms. In reality, condensation forms daily inside the glazing channel, especially in winter. Without weep holes, this moisture has nowhere to go, and the window frame can rot or warp, voiding the manufacturer’s warranty.
- Ignoring blocked weep holes during cleaning Dirt, spider webs, and debris can clog weep holes over time. A blocked weep hole is just as bad as a sealed one, it prevents drainage and leads to water pooling in the frame. Cleaning them twice a year with a pipe cleaner or compressed air costs nothing but can extend window life by years.
Quick reference
- Weep holes are required by Building Regulations Part L for all replacement double glazed windows in the UK.
- Weep holes are typically 5 to 8 millimetres wide and located on the outside of the window frame only.
- Blocking weep holes can void your window warranty and lead to misted glass that costs £300 to replace per pane.
- Weep holes should be cleaned twice a year with a pipe cleaner or compressed air to prevent blockages.
- If your windows were installed after 2002 and lack weep holes, they are likely non-compliant with FENSA standards.
Frequently Asked Questions
Yes, most modern double glazed windows have weep holes. The British Fenestration Rating Council confirms they are a standard requirement to prevent moisture build-up.
No, you should never block weep holes. Blocking them traps water inside the frame, which can cause seal failure and void your warranty according to FENSA guidelines.
Weep holes are small slots, typically 5-8mm wide, located at the bottom of the external window frame. They may have a plastic cap or mesh cover to stop insects.
