Window Draughty
Doors & Windows › Doors & Windows
Window Draughty
Identify the cause of a draughty window and fix it for good.
A draughty window is most commonly caused by a failed or perished rubber seal around the frame. Other causes include gaps in external silicone, a warped or ill-fitting sash, or cracks in the surrounding masonry. Most fixes are straightforward DIY jobs.
What’s Causing This?
Failed or perished window seal — The most common cause of a draughty window. The rubber compression seal that runs around the frame deteriorates over time, losing its elasticity and allowing cold air through the gap. See our guide on how to replace a window seal to sort this quickly.
Gaps in external silicone or mastic — The bead of sealant between the window frame and the surrounding masonry or render can crack, shrink, or fall away entirely, leaving an open channel for wind. This is especially common on older uPVC frames. You can find full guidance in our seal a leaking window guide.
Warped or poorly fitting sash on a timber window — Wooden sashes move with seasonal moisture changes. If the sash no longer sits flush against its frame or staff bead, draught will get in even when the window is fully closed. Our draught proof sash windows guide covers this in detail.
Broken or misaligned uPVC window handle or locking mechanism — If the handle does not pull the sash tightly against the frame, the compression seal cannot do its job. A faulty handle or worn multi-point locking strip leaves a consistent cold-air gap. See fix a uPVC window handle for a step-by-step fix.
Cracks in the surrounding masonry or render — Gaps where the window frame meets external brickwork or render allow wind to channel into the reveal and into the room. This is less common but worth checking, particularly on older properties. Our guide to rendering an outside wall covers remedial external repairs.
How to Diagnose the Exact Cause
- Hold your hand around the frame on a windy day. Move slowly around all four edges of the closed window. Cold air will be noticeably stronger at the point of entry — this pinpoints whether the draught is coming from the seal, the frame–wall junction, or the glazing unit itself.
- Inspect the rubber compression seal. Run a finger along the full perimeter of the rubber seal. Look for sections that are flattened, brittle, cracked, or missing entirely. A healthy seal should be soft, springy, and continuous.
- Check the external silicone bead. Go outside and inspect the joint between the window frame and the surrounding masonry on all sides. Look for cracks, gaps, or sections where the sealant has pulled away completely.
- Test the handle and locking mechanism. On a uPVC window, close the window and engage the handle fully. If the handle feels loose, fails to reach the locked position, or the frame does not pull tight against the seal, the mechanism is failing and reducing compression.
- Check for frame movement on timber windows. Push gently on the closed sash. If it moves or rattles, the sash is not pressing firmly against the draught-proofing strip. Look for gaps between the sash edge and the surrounding frame or bead with a piece of thin card — if it slides through, air will too.
- Inspect the external masonry reveal. Look for cracks, loose mortar, or gaps where the window frame meets the brick or blockwork. Use a torch at night with a light inside the room to see if any light escapes — gaps large enough to admit light will admit wind.
How to Fix It
Failed window seal: Remove the old seal by pulling it free from its groove or cutting it away if it is bonded in. Clean the channel thoroughly and press in a new matching compression seal, cutting it to length at the corners. Full instructions in our replace a window seal guide.
Failed external silicone: Cut away the old sealant with a trimming knife, clean the joint with a dry cloth, and apply a fresh bead of frame sealant in a single continuous pass. Smooth with a wet finger. Full steps in our seal a leaking window guide.
Draughty sash window: Fit draught-proofing pile strip or compressible foam seal around the sash rebate, and add a brush seal to the bottom rail. Our draught proof sash windows guide covers this in full, including fixing warped frames.
Faulty uPVC handle or lock: Replace the handle or adjust the multi-point locking keeps to increase compression. See fix a uPVC window handle for step-by-step guidance. If the locking strip is worn, the entire mechanism strip may need replacing — a straightforward swap on most uPVC units.
Cracked masonry or render around the frame: Rake out loose material, undercut the crack slightly, and fill with an exterior flexible filler or repoint with matching mortar. For larger areas of failed render, see our guide to rendering an outside wall. Finish with a bead of frame sealant at the frame–masonry junction.
When to Call a Tradesman
Most draught-proofing and resealing work is well within DIY capability and does not require a tradesman. However, you should call a qualified window installer if the window frame itself is structurally rotten, cracked, or visibly bowed — attempting to seal around a failing frame will not resolve the underlying problem and may mask progressive damage. If a timber frame has significant rot, you will need a joiner to carry out repairs before any sealing work makes sense; see our repair a wooden window frame guide or consult a FENSA-registered installer. If the window unit needs full replacement, a FENSA or CERTASS registered installer is legally required to certify the installation under Building Regulations Part L (energy efficiency) in England and Wales — this is not optional and affects your ability to sell the property. Do not attempt to replace a full window frame yourself without understanding your obligations; check requirements on gov.uk.
How to Prevent It Happening Again
- Inspect seals annually in autumn. Check the full perimeter of every window seal before the heating season starts. Catching a section that is beginning to compress or crack means a quick seal replacement rather than a full re-seal of the frame.
- Re-run the external silicone bead every 7–10 years. Silicone sealant has a finite service life. Mark when you last resealed each window and plan to refresh the external joint before it begins to fail rather than waiting for visible cracking.
- Keep uPVC mechanisms lightly lubricated. Apply a small amount of general-purpose lubricant to hinges and locking keeps once a year. A smooth-operating mechanism maintains full compression against the seal and extends its working life.
- Address masonry cracks promptly. Small cracks in the brick or render around window reveals are far easier to fill when caught early. Left untreated, water ingress accelerates the gap and can undermine the frame fixing.
Frequently Asked Questions
Why is my window draughty even when it is fully closed?
The most likely cause is a perished rubber compression seal that no longer creates an airtight contact between the sash and the frame. A broken handle or worn locking mechanism that fails to pull the sash tight against the seal will produce the same result.
Can I draught-proof a uPVC window myself?
Yes. Replacing a compression seal or resealing the external silicone joint are both straightforward DIY tasks requiring only basic tools and materials available from any builders’ merchant or DIY retailer.
How much does it cost to replace a window seal?
Replacement compression seal is inexpensive — typically a few pounds per metre for standard sizes. The main cost is your time. If you hire a window company to do it, expect to pay a call-out charge plus labour, so DIY is almost always worth attempting first.
Do I need Building Regulations approval to draught-proof my windows?
No. Draught-proofing, resealing, or replacing a seal does not require Building Regulations approval. Approval is only needed when you replace the entire window unit, in which case the installer must be FENSA or CERTASS registered, or you must apply for a Building Regulations certificate separately.
My sash window rattles and lets in cold air — do I need to replace it?
Not necessarily. Rattling and draughts on sash windows are very commonly fixed by fitting new draught-proofing pile strip around the sash rebate and a brush seal at the bottom rail. Full frame replacement is rarely needed unless the timber is structurally rotten.
When fitting new compression seal on a uPVC window, always start at a bottom corner and work continuously around the frame without joining mid-rail — a joint anywhere other than a corner creates a potential cold bridge that the new seal will not cure. Leave the final corner join slightly proud and trim to a clean mitre once the seal is fully seated in its groove.
Sources
- FENSA — Building Regulations and window replacement requirements — fensa.org.uk
- Energy Saving Trust — Draught-proofing your home — energysavingtrust.org.uk
- gov.uk — Building regulations approval: when you need it — gov.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
