TL;DR
- Diagnose the problem with the paper test, daylight test and hand test before spending anything. Most draughty doors fail in specific, identifiable spots that point to the right fix.
- Worn compression seals and threshold sweeps are the most common culprits and the cheapest to address, with DIY repairs typically £20 to £150 and installer-level work £150 to £400.
- Replace the door when the slab is rotten, swollen or warped, the frame has moved, or the door fails the paper test in three or more places. Repair if the slab and frame are sound.
- Aim for a whole-door U-value of 1.4 W/m²K or better, specify quality compression seals around the full perimeter, and keep the FENSA or CERTASS certificate with your home documentation.
A surprising amount of the heat your boiler works to produce never makes it past the front door. The losses are rarely dramatic, just a thin draught at ankle height, a hallway that always feels two degrees cooler than the rest of the house, a radiator that runs harder than it should from October to March. Individually, none of these symptoms is alarming. Cumulatively, they are why so many UK homeowners pay more to heat their homes than they need to.
The good news is that most door-related heat loss is genuinely fixable, often without replacing the door at all. The harder part is knowing what kind of problem you are looking at: is this a worn seal worth £20 to fix, an alignment issue that needs an installer, or a tired door that has reached the end of its life? This guide walks through the diagnostic steps in plain English, sets out the right fix for each cause, and gives you a clear framework for deciding when a repair is enough and when replacement is the smarter long-term call.
Quick checklist: signs your door is letting heat out
Run your front or back door against this short list. The more boxes you tick, the stronger the case that heat is escaping in a way worth addressing.
You can feel a draught around the door when it is closed. The hallway or room nearest the door is noticeably colder than the rest of the house. You can see daylight around the slab or threshold when the door is shut. The seals look hardened, torn, compressed flat or have come loose from their channel. The door sticks, scrapes or needs a firm shove to close cleanly. The lock has become stiff, sloppy or no longer pulls the door tight against the seals. Condensation forms on the inside face of the slab or around the frame in cold weather. Heating bills have crept up despite no other changes to the home. The door is more than 15 to 20 years old and looks like it every day.
One or two ticks usually point to a simple repair. Three or more, particularly if the slab itself or the frame is involved, point to a deeper issue worth diagnosing properly before deciding what to do.
How to test your door for heat loss
Before reaching for sealant or quoting a replacement, test the door methodically. Each of these checks takes a few minutes and tells you something specific about where the heat is going.
The hand test
On a cold, breezy day, close the door and run the back of your hand slowly around the perimeter, about 25mm from the slab. You will feel cold air movement at any point where the seal is failing. Pay particular attention to the threshold (the most common failure point), the lock side at handle height, and the top corners.
The paper test
Close a sheet of A4 paper in the door at various points around its perimeter and try to pull it out. If the paper holds firmly against gentle resistance, the seal is compressing properly at that point. If the paper slides freely, the seal at that point is no longer doing its job. Repeat at the threshold, the hinge side, the lock side, and at the top corners. The paper test is the single most useful diagnostic on a draughty door.
The daylight test
Stand inside the closed door on a bright day with the hallway lights off. Any daylight visible around the perimeter or under the threshold is a direct heat loss path. Even hairline lines of light represent measurable air leakage over a heating season.
The smoke or incense test
On a still day, light an incense stick or a smoke pencil and hold it slowly around the door perimeter. Air movement deflects the smoke clearly, showing exactly where the leakage is happening. This test reveals subtle leaks that the hand and paper tests can miss.
Thermal imaging
If you have access to a thermal camera or a smartphone thermal attachment, the same checks become visual. Cold blue lines around the perimeter or threshold show heat loss paths in real time. Some installers offer a thermal survey as part of a quote, particularly on properties where the homeowner suspects significant losses but cannot pin them down.
What each sign usually means
The same draught can have several different causes, and the right fix depends on which one applies. Here is what each common sign typically points to.
Draughts at the threshold
The most common single source of door-related heat loss in UK homes. The cause is usually a worn threshold sweep (the brush or rubber strip on the underside of the slab), a compressed bottom seal, or blocked drainage channels in the threshold profile that have allowed the seal to deform. All three are repairable without replacing the door.
Draughts around the slab perimeter
Almost always failed compression seals, which sit in a channel around the frame and engage with the slab when the door is closed. Seals harden, compress permanently or split with age, particularly on doors more than ten years old. Replacement seal sections are inexpensive and quick to fit.
Visible daylight around the slab
Either the slab has dropped slightly on its hinges, the frame has moved out of square, or the slab itself has warped. Hinge adjustment resolves dropped slabs. Frame movement on older properties usually requires reseating the frame, which is an installer’s job. Slab warping is generally not repairable and indicates the door has reached the end of its life.
Cold hallway or room nearest the door
Cumulative heat loss through the door, frame and any glazing combined. If the door itself tests well on the paper and daylight checks, the issue may be the frame seal to the surrounding masonry, or the glazing units losing their thermal performance. A whole-door investigation is needed.
Sticking, scraping or a hard-to-close door
Usually, the slab has dropped on its hinges, swollen due to moisture, or warped because of temperature exposure. A sticking door rarely seals properly, so heat loss and difficulty closing are often the same problem expressed in different ways. Hinge adjustment, available on most modern multipoint locks, resolves dropped slabs.
Stiff or sloppy lock
Usually, because the slab has dropped and the locking points are no longer engaging cleanly with the strike plates. The door cannot pull tight against the seals when the lock will not throw fully, which means the seals are not compressing as designed. Forcing a stiff lock makes the problem worse. Address the alignment.
Condensation on or around the door
Internal condensation forming on the inside face of the slab or frame indicates that the surface is dropping below the dew point of the room air, which means the door is not insulating well. Modern doors with insulated cores and good seals typically eliminate this condensation by keeping the inside face closer to room temperature. Persistent condensation around an older door is usually a sign that thermal performance has dropped and the door is approaching end of life.
Rising heating bills
The cumulative effect of all of the above, plus seasonal weather. A door that has lost its airtight closure can be responsible for a meaningful portion of unwanted heat loss in a house that is otherwise well-sealed. The Energy Saving Trust reports that draught-proofing measures reduce unwanted cold air entering through gaps and can lower heating use noticeably.
Repair or replace? A decision framework
If your checks suggest the door itself is past a simple seal or hinge repair, it may be worth comparing modern external door options for your home. This can help you weigh up insulation, security, glazing and style before deciding whether replacement is the right long-term fix.
The honest decision is rarely obvious from the outside. Some draughty doors are genuinely worth saving with a few inexpensive repairs. Others have run their course and pour money into a tired product. The framework below should resolve most cases.
Repair is usually the right answer when the slab is structurally sound, the frame is in good condition, and the issue is localised to seals, hardware, threshold sweeps or hinge adjustment. A failed seal, a worn threshold sweep, a dropped slab, or a sticky lock on an otherwise sound door are all sensible repairs. Total spend usually lands between £20 and £200, depending on what is needed and whether you can do the work yourself.
Installer-level repair is the right answer when the slab is sound, but the frame has moved, multiple seal sections need replacement, the threshold needs replacing, or the multipoint lock needs realignment. Expect £150 to £400 for a service call covering this kind of work.
Replacement is usually the right answer when the slab itself is compromised (rot, swelling, warping), the frame has rotted or moved significantly, multiple problems are present at once, the door is more than 20 years old and showing its age generally, or the cumulative cost of repairs is approaching half the cost of a quality replacement. Replacing one component on a door that is failing in several places typically delays the inevitable rather than solving it.
The clearest single test: if the door fails the paper test in three or more places and the slab itself shows visible age (yellowed uPVC, peeling timber, swollen base), replacement is usually the better long-term value. If the door fails the paper test in one or two specific spots and the rest of the door looks sound, repair is almost always the right call.
Best fixes by problem
Once you know what is causing the heat loss, the right fix follows.
Worn or failed compression seals
Replace the failed seal sections. The seals push into a channel around the frame and are inexpensive parts, typically £5 to £15 per metre. A confident DIY job for the bottom and lock side, or an installer can fit the whole perimeter in under an hour for £100 to £200. Seals are the most common single cause of door-related heat loss and the cheapest fix on the list.
Worn threshold sweep
Replace the threshold sweep, the brush or rubber strip on the underside of the slab. Universal sweeps are available at most builders’ merchants for £10 to £30, fit with screws or adhesive, and resolve floor-level draughts on most doors. For doors with proprietary thresholds, ask the original installer or manufacturer for the matching part.
Dropped slab on hinges
Most modern multipoint locks include a hinge adjustment that allows the slab to be lifted slightly back into alignment. The adjustment is usually a small screw on the hinge body, accessed with an Allen key. A locksmith or your installer can usually correct this in under an hour. Forcing a stiff lock without addressing alignment makes the problem worse.
Frame seal failure to the surrounding wall
The gap between the door frame and the brickwork or render needs to be sealed externally with low-modulus silicone in a colour matched to the frame or surrounding masonry. Internally, the gap should be filled with expanding foam, trimmed back, and finished with a plaster bead or architrave. Failed external sealant can be raked out and reapplied for £30 to £80 by a competent DIYer or £150 to £250 by a tradesperson.
Failed double-glazed units
Misted or cloudy glazing units indicate that the seals on the unit itself have failed. The unit is no longer thermally performing as designed and needs replacement. Glazing units can be replaced individually on most doors as sealed cassettes, typically £150 to £400 per panel depending on size and specification.
Letterbox and keyhole leakage
Older letterboxes without restrictors and keyholes without escutcheons can be surprising sources of draught. Internal letterbox restrictors with brush seals cost £10 to £25 and fit in minutes. Keyhole escutcheons are similar cost and similarly quick to fit. Both also have security benefits beyond the thermal improvement.
Tired slab beyond economic repair
If the slab itself is rotten, swollen, warped or the door is generally past its useful life, full replacement is the right answer. Realistic 2026 fitted ranges: uPVC £1,200 to £2,200, composite £1,800 to £3,500, aluminium £2,500 to £5,000, engineered timber £2,500 to £5,500.
Choosing an energy-efficient replacement door
If the diagnosis points to replacement, the next decision is which door to fit. Material matters less than most marketing suggests; specification matters more.
Composite doors are the mainstream choice for UK homes in 2026, achieving whole-door U-values of 1.2 to 1.5 W/m²K with insulated cores and quality frames. They deliver strong all-round performance on appearance, security, thermal numbers and longevity. For most replacement situations on typical properties, composite is the sweet spot for value.
uPVC doors typically achieve 1.4 to 1.8 W/m²K, depending on the profile and core specification. Quality uPVC with reinforced and insulated profiles can comfortably meet Part L. Cheap uPVC with hollow sections sits closer to the regulatory limit and is rarely the best long-term value.
Aluminium doors with thermal-break construction reach 1.3 to 1.6 W/m²K on solid configurations. The thermal break is the part to ask about, since poorly designed thermal breaks can be a meaningful weak point. Aluminium suits contemporary properties and large glazed openings.
Engineered timber doors achieve 1.0 to 1.4 W/m²K and look correct on period properties in a way no other material matches. The trade-off is maintenance: refinishing every three to five years.
For glazed doors, the glazing specification dominates the thermal performance. Modern double-glazed units with low-emissivity coatings and warm-edge spacer bars typically achieve glass U-values around 1.1 W/m²K. Triple glazing reaches 0.6 to 0.8 W/m²K but adds cost and weight, and is more relevant on heavily glazed configurations or premium specifications.
The honest framing for energy efficiency: aim for a whole-door U-value of 1.4 W/m²K or better, ensure the seals are quality compression seals around the entire perimeter, specify a properly detailed threshold with drainage channels, and ensure laminated rather than only toughened glass on any glazed panels.
UK regulations and standards to know
Replacement external doors fall under Approved Document L of the Building Regulations, which sets the minimum thermal performance for any door fitted as part of a replacement. The current edition incorporates the 2023 amendments and applies to dwellings in England.
The limiting whole-door U-value for replacement windows and doors in existing dwellings is 1.6 W/m²K. Modern compliant doors typically perform comfortably better than this. Doors with significant glazed area (over 60 per cent glass) are subject to a stricter target, typically 1.4 W/m²K.
For new builds and significant extensions, the thermal targets are stricter again. Most reputable installers fit doors that meet or exceed these requirements as standard.
Certification is handled by FENSA or CERTASS through registered installers under the Competent Person Scheme. A registered installer self-certifies the work and provides a certificate confirming Building Regulations compliance, usually within four to six weeks of installation. Conveyancers ask for this certificate when you sell. Without it, buyers can demand a retrospective Building Control inspection, which is costly and avoidable.
Listed buildings and conservation areas have separate considerations. Some thermal upgrades are restricted on listed properties to preserve original detail, and engineered timber is usually the preferred replacement material. Discuss the project with the local planning authority before ordering if your property falls into either category.
Real homeowner scenarios
Generic guidance only goes so far. Here are the situations we see most often, with the practical answer for each.
Victorian terrace with a draughty timber door
The original timber door is leaking heat from every edge, the seals have failed, and the bottom of the slab is starting to soften. The hallway never warms up. The right answer is a quality replacement: a six-panel composite door with an insulated core, decorative glazing, and a TS 007 three-star cylinder lands at £2,500 to £3,000 fitted. The thermal improvement is dramatic, the security upgrade is significant, and the kerb appeal lifts materially. For listed properties or conservation areas, engineered timber in a sympathetic style is usually the required material.
1990s uPVC door with worn seals
The slab and frame are sound, but the seals have compressed permanently and the threshold sweep has worn. The paper test fails at the threshold and on the lock side, but holds at the hinge side and top. This is a clear repair situation. New seal sections at £80 to £150 fitted, plus a new threshold sweep at £20 to £40, transform the door’s thermal performance for a fraction of the cost of replacement. The door has another five to ten years in it before replacement makes sense.
New build with a poor threshold seal
The door is less than five years old but a draught comes from the threshold. Often this is a builder-grade installation where the threshold drainage was not properly cleared during fitting, or the threshold sweep was the basic specification rather than the upgraded option. Contact the original installer if the door is still under warranty; if not, a service call to clear the drainage and fit an upgraded sweep typically costs £100 to £200 and resolves the issue completely.
Period cottage with a timber door
The door is period-correct but draughty, and conservation area restrictions limit the replacement options. The right answer is usually careful restoration: refinishing the slab, replacing the seals, renewing the threshold sweep, and adjusting the hinges. A specialist heritage restorer can typically bring a sound timber door back to working condition for £400 to £900, far less than the cost of a new compliant timber door, and with the original character preserved. Check listed building consent requirements before any work that affects appearance.
Family home with cumulative heat loss
The hallway is cold, the radiator nearest the door runs harder than the rest of the house, and heating bills have crept up over five years. Multiple paper test failures around the perimeter, plus a slab that has visibly dropped on its hinges. The cumulative repair cost approaches £400 to £600. Combined with a door that is approaching twenty years old, replacement at £2,200 to £2,800 fitted typically delivers better long-term value, particularly factoring in the thermal performance upgrade.
Maintenance tips to stop heat loss returning
Once a door is sealing properly, a small amount of routine attention keeps it that way. The total time involved is genuinely modest.
Clean the slab and frame two or three times a year with warm soapy water and a microfibre cloth. Dirt and pollution accumulate on seals and accelerate hardening over time, so a clean door is a longer-lasting door.
Check the seals visually each spring and autumn. They should look intact, continuous around the perimeter, and slightly compressed when the door closes. Replace any sections that have torn, hardened or compressed permanently before they cause draughts.
Lubricate the hinges and lock mechanism annually with silicone spray (not WD-40, which is a water displacer rather than a lubricant). Apply graphite powder to the lock cylinder. A few minutes a year keeps the door pulling tight against its seals year after year.
Keep the threshold drainage channels clear. A blocked drainage channel allows water to back up against the door, accelerating wear on the bottom seal. Clear with a soft brush every few months.
Run the paper test annually as a quick health check. Any new failures caught early are cheap repairs. The same failures left for three or four years usually become more involved jobs.
Frequently asked questions
1. How do I know if my front door is letting heat out?
Run the paper test, the daylight test and the hand test around the closed door. Any seal section where paper slides freely, any visible daylight, or any cold airflow you can feel with the back of your hand indicates active heat loss. Most draughty doors fail at least one of these tests in at least one location.
2. Can I fix a draughty door myself?
Often yes. Replacing seal sections, fitting a new threshold sweep, adding a letterbox restrictor and adjusting hinges are all confident DIY tasks for most homeowners. Frame reseating, multipoint lock realignment and structural issues are usually installer work. Total cost for DIY-level fixes typically lands between £20 and £150.
3. When should I replace instead of repair?
Replace when the slab itself is rotten, swollen or warped, when the frame has moved or rotted, when multiple problems are present simultaneously, or when the door is more than 20 years old and showing its age generally. The clearest test: if a buyer viewed your property tomorrow, would the door read as a strength or as a job they would need to do?
4. What is the warmest type of front door?
Premium composite, engineered timber and insulated steel doors all achieve excellent thermal performance, with whole-door U-values around 1.0 to 1.4 W/m²K on quality products. The differences between materials at the same price point are usually less than 0.3 W/m²K. Specification (core, frame, glazing, seals) matters more than material category.
5. Are composite doors more energy efficient than uPVC?
Generally yes. Quality composite doors typically achieve U-values of 1.2 to 1.5 W/m²K, while quality uPVC doors land at 1.4 to 1.8 W/m²K. The difference is real but modest. Cheap uPVC with hollow sections performs noticeably worse than either category at full quality.
6. Do glazed doors lose more heat?
Yes, slightly. Glass conducts more heat than an insulated core, regardless of how well the unit is engineered. The effect is small for decorative glazing and significant for heavily glazed doors or sidelights. Modern double glazing with low-emissivity coatings keeps the impact modest. For most UK homes, some glazing is worth the small thermal compromise for the natural light it brings.
7. What U-value should a replacement door have in the UK?
Aim for a whole-door U-value of 1.4 W/m²K or better. The regulatory minimum is 1.6 W/m²K. New builds and significant extensions are subject to stricter targets. Premium doors achieve below 1.0 W/m²K but at meaningful cost premium.
8. Will a new door reduce condensation?
Yes, in most cases. Modern doors with insulated cores keep the inside face closer to room temperature, which reduces the surfaces that drop below the dew point. Persistent condensation on an older door is usually a sign that thermal performance has dropped, and a replacement typically resolves the issue.
9. Why can I see daylight around my door?
Either the slab has dropped on its hinges, the frame has moved, or the slab itself has warped. Hinge adjustment resolves dropped slabs in most cases. Frame movement and slab warping are deeper issues, usually requiring an installer or full replacement.
10. How much does draught-proofing a door cost?
DIY-level draught-proofing typically costs £20 to £150 for seal replacement, threshold sweeps and letterbox restrictors. Installer-level work covering full perimeter seals, threshold replacement and lock realignment typically costs £150 to £400. Full replacement starts at £1,200 fitted for uPVC and £1,800 for composite.
11. How long should an external door last?
uPVC doors typically last 20 to 25 years, composite 25 to 30 years, aluminium 30 to 40 years, and engineered timber 30 years or more with proper maintenance. Hardware, seals and glazing often need attention sooner than the slab itself.
12. Can shutters or thermal blinds help with heat loss around doors?
Yes, modestly. Internal thermal blinds and shutters add insulation layer when closed, particularly on doors with significant glazed area. The effect is smaller than fixing the underlying seal or slab issue, so address the door itself first. Shutters are a useful supplementary measure on cold north-facing entrances or doors with large glazed panels.