How to Check Window and Door Installation Quality

Windows and doors sit at the boundary between inside and outside. Get them wrong and you get draughts, water ingress, condensation, heat loss, security problems, and - in a loft conversion - a failure of your emergency escape route. They account for around 9% of recorded building defects, but the regulatory landscape around them has changed significantly in recent years, and a lot of installers haven't caught up.

Since June 2022, trickle vents are mandatory on replacement windows. Since 2023, minimum U-values have tightened. Fire door requirements have been strengthened following Grenfell. This guide covers what you should be checking and what the installer should be providing.

TL;DR Checklist

• FENSA or CERTASS certificate received for all replacement glazing
• Trickle vents present and operational (mandatory since June 2022)
• Escape windows: 0.33m² clear opening, no dimension under 450mm
• Continuous draught seals around all opening lights - no gaps
• U-value of 1.4 W/m²K or better for replacement windows
• All opening mechanisms work smoothly without binding or sticking
• Weather bar fitted to bottom of external doors
• Fire doors: FD30 rated, self-closing, intumescent strips on all three edges

The Full Guide

FENSA or CERTASS certification

This is the non-negotiable starting point. If you're having replacement windows or external doors installed in England and Wales, the work must comply with Building Regulations - and the way that's typically demonstrated is through a competent person scheme. FENSA and CERTASS are the main ones.

When your installer is registered with FENSA or CERTASS, they self-certify the work and notify your local authority's building control. You receive a compliance certificate, which goes on the building control register. This certificate confirms the installation meets the Building Regulations requirements for thermal performance, ventilation, and safety.

Without this certificate, you have uncertified work. When you come to sell, your conveyancer will flag it, and your buyer's surveyor will raise it as an issue. You'll either need retrospective building control approval (which means an inspector examining work that may now be covered up) or indemnity insurance, which isn't ideal.

Trickle vents

Since the June 2022 update to Approved Document F, trickle vents are mandatory on all replacement windows. This was a significant change - previously, replacement windows only needed trickle vents if the originals had them. Now, all replacements must have them.

Trickle vents are small openable slots built into the window frame (usually at the top) that provide background ventilation when the window is closed. The minimum equivalent area (the measure of how much air they let through) is 5,000mm² for habitable rooms and 2,500mm² for wet rooms (bathrooms, kitchens, utility rooms).

Check that the vents are actually there, that they open and close, and that they're the right size. Some installers skip them to save cost or because they don't want to cut the frame. Others install undersized vents that don't meet the minimum requirement. Both result in a non-compliant installation.

Emergency egress

In rooms where the window is the means of escape in a fire - typically bedrooms, and especially loft conversion bedrooms - the window must meet specific opening requirements. This is governed by Approved Document B.

The clear openable area must be at least 0.33m², with no dimension less than 450mm. "Clear openable area" means the actual gap you could climb through, not the overall window size. The bottom of the openable area should be between 800mm and 1100mm above the finished floor level - high enough that you don't fall out accidentally, low enough that you can get through it in an emergency.

This is particularly important in loft conversions, where the escape window is often the only alternative exit. Check the actual opening, not just the window specification. A window that looks big enough but only opens to a restricted angle might not provide the required 0.33m² clear opening.

Draught seals

Draught seals are the rubber or brush strips that run around the edges of opening windows and doors, forming a seal between the opening light and the frame when closed. They're the primary barrier against air infiltration.

Check that seals are continuous around all four edges of every opening light. Run your finger along the seal - it should be smooth, intact, and spring back when pressed. Look for gaps at corners, compressed sections that don't spring back, and missing strips (particularly at the bottom of casement windows where water collects and degrades the rubber).

On a windy day, hold your hand around the edges of a closed window. You shouldn't feel any draught. If you can feel air movement, either the seals are inadequate or the window isn't closing tightly against them - which usually means the hinges need adjusting.

U-value compliance

The thermal performance of a window is measured by its U-value - the lower the number, the better the insulation. For replacement windows in existing dwellings, Approved Document L requires a whole-window U-value of 1.4 W/m²K or better.

"Whole-window" is important - it's the U-value of the complete unit including the frame, not just the glass. Double-glazed units with a low-emissivity coating and argon gas fill typically achieve 1.2 to 1.4 W/m²K depending on the frame material and profile.

Check the energy rating label on the window - it should be displayed or available from the installer. The U-value should also be recorded on your FENSA certificate. If the installer can't tell you the U-value of the windows they're fitting, that's a significant concern.

Operation and hardware

Every opening light should operate smoothly. Open and close each window and door, testing all mechanisms - handles, espagnolette locks (the multi-point locking bars), friction stays (the hinges on casement windows), and tilt-and-turn mechanisms.

Things to check: handles should turn without excessive force. Locking points should engage and disengage smoothly. Friction stays should hold the window open at any position without the window dropping or swinging. Tilt-and-turn windows should switch between tilt and turn modes cleanly.

For external doors, check the threshold. A weather bar at the bottom of the door prevents rain driving under the door. The threshold should be level and the door should seal against it without binding. Wheelchair-accessible thresholds (Part M) have a maximum 15mm upstand.

Fire doors

If your project includes fire doors - and it should if you're converting a loft, creating a room above an integral garage, or altering a three-storey house - they need specific features to perform their function.

An FD30 fire door provides 30 minutes of fire resistance. It must have intumescent strips on all three closing edges (the two sides and the top). These strips expand in heat to seal the gap between the door and the frame, preventing smoke and fire from passing through. Cold smoke seals (brush strips) are also usually required.

The door must self-close fully into the frame from any open position. Test this - open the door to 90 degrees and let go. It should close completely under its own power and latch shut. Self-closing mechanisms that can't fully close the door, or that have been propped open, defeat the purpose entirely.

Check the gap between the door and the frame on the closing edges - it should be no more than 3mm. Any more than that and the intumescent strip may not seal properly in a fire. Also check that any door furniture (viewers, letterboxes, hinges) is fire-rated. Standard letterboxes and viewers are not fire-rated and will compromise the door's performance.

Common Problems

Missing trickle vents on replacement windows. This is now one of the most common compliance failures. The June 2022 requirement caught many installers off guard, and some are still fitting windows without vents. Without them, the installation doesn't comply with Part F and the FENSA certificate shouldn't have been issued.

Fire doors propped open. Fitted correctly, then the self-closer is removed or the door is wedged open because it's inconvenient. If a fire door is required, it must self-close. The only acceptable alternative is an automatic door release held open by an electromagnet that releases when the fire alarm activates.

Condensation between double-glazed units. Misting inside the sealed unit means the seal has failed and moisture has entered the cavity between the panes. This is a manufacturing or handling defect, not an installation issue. The failed unit needs replacing - it can't be repaired.

Draughts around the frame-to-wall junction. The gap between the window frame and the masonry is typically sealed with expanding foam and finished with silicone or mastic. If the foam is incomplete or the external seal has gaps, you'll feel draughts and may get water ingress around the frame. Check both inside and outside.

Questions to Ask Your Builder

• "Are you FENSA or CERTASS registered, and can I have your registration number?" - Verify it online before work starts. If they're not registered, they'll need to arrange building control approval, which adds cost and time.
• "Are you fitting trickle vents to these replacement windows?" - The answer must be yes. If they say "only if the old ones had them," they're working to the pre-June 2022 rules.
• "What's the whole-window U-value of the units you're installing?" - They should know this. If they can't answer, they may not know whether the windows meet Part L.
• "Will the escape window in the loft give me 0.33 square metres of clear opening?" - If it's a loft conversion, this is critical. Ask them to demonstrate the clear opening with the window fully open.
• "Is the fire door FD30 rated, and does it have intumescent strips?" - Check the door edge yourself. The strips should be visible recessed into the door edge on the top and both sides.