U-values are the single number that tells you how well a door keeps heat inside your home. They appear on every external door specification sold in the UK, they sit behind building regulations, and they are widely misunderstood. This guide explains what the number means, which doors it applies to, what the regulations actually ask for in each UK nation, and how to read a manufacturer figure without being misled by it.
Doors2Floors has supplied doors and flooring to homes and businesses across the UK. We list external doors, composite external doors and glazed external doors, alongside internal doors. Where U-values matter and where they do not is set out plainly below.
What a U-value actually measures
Start with the definition, because a lot of confusion downstream comes from getting this part slightly wrong.
The formal definition
A U-value, properly called thermal transmittance, is the rate of heat transfer through one square metre of a building element when there is a one kelvin temperature difference between the two sides. It is expressed in watts per square metre kelvin, written W/m2K.
Why kelvin and not Celsius
A one kelvin difference and a one degree Celsius difference are the same size of interval, so the two are interchangeable here. Kelvin is used because it is the SI unit for temperature difference.
Lower is better
A door rated at 1.2 W/m2K loses heat more slowly than one rated at 1.8 W/m2K. There is no upper limit to how bad a U-value can be, and a single glazed timber door with no seals can be several times worse than a modern doorset.
U-value is not R-value
R-value is thermal resistance and is the reciprocal of the U-value. Higher R is better, lower U is better. R-values appear in North American product literature and in insulation datasheets, so do not compare the two numbers directly.
How U-values are calculated for doors
Doors and windows are assessed under the BS EN ISO 10077 series, which covers the calculation of thermal transmittance for windows, doors and shutters. The calculation combines the frame, the panel, any glazing and the linear thermal bridging where they meet.
Why the whole door figure is the one that counts
A glazing unit can have an excellent centre pane figure while the door as a whole performs far worse, because the frame and the edges of the glazing unit lose more heat. Always ask for the whole door or whole doorset U-value.
Door leaf versus doorset
A door leaf figure covers only the door itself. A doorset figure covers the leaf and the frame together, which is what actually sits in your wall. Doorset figures are usually slightly worse and always more honest.
What U-value does not tell you
It says nothing about air leakage, nothing about solar gain, nothing about acoustic performance and nothing about security. A door with a good U-value and bad seals will still feel cold and draughty.
Which doors U-values apply to
This is the part that saves most readers a great deal of confusion, and it is missing from almost every guide on the subject.
External doors
Any door forming part of the thermal envelope of a heated building is covered. That means front doors, back doors, French doors and bi-fold doors onto the garden. Browse our external door range.
Doors to unheated spaces
A door between a heated room and an integral garage, an unheated porch or an unheated conservatory is also part of the thermal envelope and is treated accordingly.
Internal doors between heated rooms
These have no U-value requirement under building regulations, because both sides are inside the heated envelope. If a retailer quotes you a U-value for a standard internal door, it is a marketing number rather than a regulatory one. Our internal doors are specified on style, construction, fire rating and acoustic performance instead.
Loft hatches and cellar doors
A hatch into an unheated loft or a door into an unheated cellar is part of the envelope and does matter, even though people rarely think about it.
Fire doors
Fire performance and thermal performance are separate specifications. An FD30 door is certified for fire resistance and may or may not have a meaningful U-value. See our FD30 fire doors and our fire door overview.
Commercial and non domestic buildings
Non domestic buildings are covered by a different volume of the guidance with different targets. If your project is commercial, get the figures from your building control body or your designer rather than from a retail page.
What the UK regulations ask for
Building regulations are devolved. England, Wales, Scotland and Northern Ireland each set their own energy standards, and they are revised periodically.
England
Energy efficiency in England is covered by Part L of the Building Regulations, with practical guidance in Approved Document L. Volume 1 deals with dwellings and Volume 2 with buildings other than dwellings. Replacement external doors in existing dwellings have a limiting standard applied to them, and at the time of writing a figure of 1.4 W/m2K is the commonly quoted limit for replacement doors.
Wales
Wales has its own version of Part L, published by the Welsh Government, with its own Approved Document and its own revision timetable. The numbers are similar in nature but you should never assume they are identical to England.
Scotland
Scotland uses the Building Standards system rather than Approved Documents. Energy is covered by Section 6 of the Technical Handbooks, with separate Domestic and Non Domestic handbooks.
Northern Ireland
Northern Ireland uses Technical Booklets, with conservation of fuel and power covered by Technical Booklet F.
Why we are not printing a full table of numbers
Because the numbers change. Energy standards across all four nations have been revised repeatedly and further revisions have been signalled. Any table printed on a retail blog will eventually be wrong, and a wrong number here could cause a genuine compliance failure on your project. Confirm the current figure with your local building control body or your installer before you order.
Notifiable work
Replacing an external door in a dwelling is normally notifiable work. It is usually handled either through a building control application or through a competent person scheme registration by your installer, who then issues a certificate.
Keep the paperwork
The compliance certificate for a replacement door is asked for during conveyancing. Losing it can hold up a sale years later. Our exterior viewing guide covers what buyers and surveyors look at.
Listed buildings and conservation areas
Historic buildings often have relaxations available where compliance would unacceptably alter character, but relaxations are decided case by case. Speak to your conservation officer and building control body together, before ordering anything.
What determines a door U-value
Four things do almost all of the work: the core, the glazing, the frame and the seals.
The core material
An insulating foam core in a composite door is the single biggest contributor to a good figure. Solid timber is a moderate insulator. A hollow core is a poor one, which is why hollow core doors are internal products only. Our hollow core versus solid core guide explains the construction difference.
Thickness
More insulation depth means a lower U-value, all else being equal. This is why external doorsets are thicker than internal doors.
Glazed area
Glass almost always performs worse than the solid part of a door. A fully glazed door will have a worse whole door figure than a solid one of the same construction. Compare our glazed external doors against solid options.
Glazing specification
Single glazing is very poor. Sealed double glazing is the standard. Triple glazing improves further but adds significant weight, which affects hinges and frame fixing.
Low emissivity coatings
A low emissivity coating is a microscopically thin metallic layer on one glass surface that reflects long wave heat back into the room. It improves the glazing figure substantially at very little cost and is standard on modern units.
Cavity gas fills
Argon is the common fill and performs better than air. Krypton and xenon perform better again and cost far more, so they appear mainly in slim cavity units for heritage applications.
Warm edge spacer bars
The spacer bar that separates the panes used to be aluminium, which conducts heat straight around the edge of the unit. Warm edge spacers in polymer or stainless steel reduce that edge loss and are a meaningful upgrade.
Frame material
Timber and uPVC frames are reasonable insulators. Aluminium conducts heat extremely well, which is a disadvantage here, so aluminium frames need a thermal break, an insulating barrier inside the profile that interrupts the heat path.
Thermal bridging at junctions
Heat takes the easiest route. Where the frame meets the wall, where the glazing meets the frame and where hardware passes through the door, heat can bypass the insulation. Good detailing matters as much as good materials.
Seals and weatherstripping
Compression seals around the perimeter and a properly detailed threshold control air leakage. Air leakage is not part of the U-value calculation, but it is very much part of how cold your hallway feels. Our draught excluder guide covers this in full.
Installation quality
A doorset with an excellent laboratory figure, fitted with gaps stuffed with offcuts instead of properly insulated and sealed, will underperform badly. Fitting is not a detail, it is half the result.
Letter plates and hardware penetrations
Every hole through the door is a potential path for heat and air. Brushed and flapped letter plates reduce it. Our door hardware products include the fittings involved.
Materials compared
A realistic ranking, with the caveat that a well made example of a weaker material beats a poor example of a stronger one.
Composite doorsets
Generally the best combination of thermal performance, security and low maintenance for a domestic front door. Our composite external doors are supplied as complete doorsets with frames.
Timber doors
Timber is a genuine insulator, roughly comparable in behaviour to an insulating masonry unit, and far better than metal. Its weakness is movement and the maintenance of the finish. Our wooden front door care guide covers upkeep.
uPVC doors
Multi chambered uPVC profiles perform well thermally at low cost. The weaknesses are appearance and the fact that a foiled finish cannot be repainted.
Aluminium doors
Slim sightlines and excellent durability, but only acceptable thermally with a properly engineered thermal break. Never buy a non thermally broken aluminium door for a heated space.
Steel doors
Security first products. Thermal performance depends entirely on the core and on whether there is a thermal break.
Bi-fold and sliding doors
Large glazed areas and many frame junctions make these the hardest elements to get right. Ask specifically for the whole door U-value at the actual size you are ordering, not a standard test size. Our bi-fold mechanism guide and bi-fold cost guide cover the rest.
French doors
Two leaves meeting in the middle means an extra junction and usually a slightly worse figure than a single door. Our French doors guide explains the configurations.
Stable doors
The horizontal split adds a junction and a seal line. Well made stable doors handle it, cheap ones do not. See our stable doors guide.
Reading a manufacturer specification honestly
Specifications are written to sell. Here is how to read one properly.
Ask which figure it is
Centre pane, whole glazing unit, door leaf or complete doorset. These are four different numbers and the difference between them can be large.
Ask what size it was tested at
U-values are quoted at standard test sizes. A different size changes the ratio of frame to panel and therefore changes the result.
Ask whether glazing is included
A quoted figure for a solid version of a door tells you nothing about the glazed version of the same door.
Watch for best case configurations
A headline figure often applies to the least glazed, most insulated variant in a range. Confirm the figure for the exact specification you are buying.
Ask for third party verification
Simulated figures calculated to the standard are normal and acceptable. Figures with no stated basis are not.
Energy rating labels
Window energy rating style labels combine U-value with solar gain and air leakage into a single letter band. They are useful for comparison but are not the same thing as a U-value and are not what building control asks for.
Be sceptical of savings claims
Any figure claiming a specific percentage saving on your heating bill from a door upgrade should be treated with caution. Savings depend on your fuel, your tariff, your heating pattern, your insulation elsewhere and the weather. No supplier, including us, can honestly promise you a number.
Putting a door upgrade in context
Doors matter, but they are one part of a much larger picture, and it is worth being realistic about scale.
Relative area
A typical front door is roughly two square metres. Your walls, roof and windows are far larger. Fabric heat loss is proportional to area as well as to U-value, which is why loft and wall insulation usually deliver more than a door does.
Where a door upgrade genuinely wins
Replacing a single glazed, ill fitting, unsealed old door with a modern doorset makes a large difference to comfort in the hallway, because you fix air leakage and cold radiation at the same time.
Where it delivers less
Replacing a reasonably modern door that already meets current standards with a marginally better one rarely pays for itself. Spend the money on whatever element is worst.
Comfort versus cost
Comfort improvements are real and immediate even where the bill saving is modest. A hallway that is no longer cold gets used differently.
Condensation and cold surfaces
Cold internal surfaces attract condensation and eventually mould. Raising the internal surface temperature of a door reduces that risk.
Order of works
Insulate first, seal second, then upgrade glazing and doors, then size the heating system. Doing it in that order avoids paying for heating capacity you no longer need.
Get advice for your own property
For a whole house retrofit plan, speak to a qualified retrofit assessor or energy assessor rather than relying on any single product page.
Room by room, where heat actually leaves the house
U-values are calculated per element, but heat loss is experienced room by room. Walking the house in this order shows you where a door upgrade will be felt.
The front hallway
The coldest room in most UK homes, because it contains the front door, often has a letter plate, and is frequently unheated. Fixing the door here changes how the whole ground floor feels.
The porch
An unheated porch acts as an airlock and is genuinely useful. If you heat it, it stops being a buffer and becomes another cold surface to insulate.
The kitchen and the back door
Back doors are often the oldest and worst fitted in the house. Check the seals and the threshold before looking at anything else.
The utility room
Frequently unheated and frequently between the kitchen and the garden. A solid, well sealed internal door here is worth more than most people expect. See our unglazed doors.
The integral garage
The door between house and garage is part of the thermal envelope and often has a fire rating requirement too. Our fire doors cover certified options and our fire door materials guide explains construction.
The conservatory
If the conservatory is unheated and separated by external quality doors, it is a buffer. If you remove those doors, the conservatory becomes part of the heated envelope and the rules change entirely. Our conservatory flooring guide covers the same space from the floor up.
Living rooms with garden doors
Large glazed openings are where most modern heat loss sits. Compare bi-fold doors and our bi-fold ideas guide.
Bedrooms
Usually only external walls and windows. Internal bedroom doors have no thermal requirement, so choose them on style and acoustics. Browse oak doors, white primed doors and grey doors.
Bathrooms
Humidity plus cold surfaces equals condensation. Ventilation matters more here than any U-value. Never block ventilation serving a gas or solid fuel appliance.
The loft hatch
An uninsulated, unsealed loft hatch is a genuine weak point and one of the cheapest things in the house to fix.
Cellars and basements
Often unheated, often damp. If you are seeing damp as well as cold, speak to a qualified surveyor or damp specialist before insulating anything.
Landings and stairwells
Warm air rises and collects here. A door at the bottom or top of the stairs changes the convection pattern significantly. Our room dividers and pocket doors suit tight spaces.
Improving the doors you already have
If replacement is not on the cards, these are the measures that actually work.
Replace failed seals
Perimeter seals harden and stop compressing. Replacing them is cheap and is usually the highest impact thing you can do.
Fit a threshold seal or draught excluder
The gap under the door is often the largest single opening in the whole envelope. Our draught excluder guide explains the options and how to fit them correctly.
Letter plate covers
A brushed internal cover on a letter plate stops a surprising volume of cold air.
Keyhole covers and escutcheons
Small, cheap and effective on older doors with through cylinders.
Heavy curtains and portieres
A lined curtain over a front door reduces radiant loss and draught noticeably. It is a traditional solution because it works.
Adjust the hinges and keeps
A door that does not compress its seals evenly is not sealing at all. Adjusting the keeps or packing the hinges often fixes it. Our hinge guide and door hinges cover the hardware side.
Fix sticking and dropped doors
A dropped door leaves a wedge shaped gap. Our swelling and sticking guide and planing guide deal with the causes.
Secondary glazing on glazed doors
Where the door itself must be retained for heritage reasons, secondary glazing on the glazed panel is often permitted and is very effective.
Draught proof the internal doors too
Internal doors have no U-value requirement, but closing off a cold hallway from a heated living room reduces the volume you are heating. A solid internal door does this far better than a hollow one. Browse our fully finished internal doors.
Choosing a replacement door on thermal performance
A practical order of decisions rather than a wish list.
Confirm the current requirement first
Ask your building control body or your installer what figure applies to your property in your nation, today. Start there rather than with a product.
Decide how much glazing you need
Light into a dark hallway is worth a slightly worse U-value. A north facing hall with no other light source is a good reason to accept glazing.
Pick the material
Composite for performance and low maintenance, timber for heritage and repaintability, aluminium for slim modern sightlines with a thermal break.
Get the whole doorset figure in writing
On the quotation, for the exact configuration and size you are ordering.
Check the fitting method
Ask how the gap between frame and structure will be insulated and sealed, inside and out. This is where performance is won or lost.
Measure the structural opening
Composite doorsets are supplied at fixed sizes. Our measuring guide shows the method, and each product page lists the exact dimensions.
Budget realistically
Our black composite doorsets run from £845.00, for example the BCD Black Composite Lever Door with White Frame at £845.00 and the BCD Black Long Bar version at £895.00. Our front door costs guide breaks down the other elements.
Consider colour and appearance last
Get the performance right, then choose how it looks. Our black front door guide, grey composite door guide and front door colour ideas cover the options.
Common myths about door U-values
These come up in almost every conversation we have with customers, and most of them come from articles written for other countries.
Myth: every door has a legal U-value requirement
Only doors forming part of the thermal envelope do. Internal doors between two heated rooms do not. Our fully finished internal doors are specified on other criteria entirely.
Myth: solid wood is a poor insulator
Timber is a reasonable insulator and far better than metal. Its problem is movement and finish maintenance, not thermal conductivity. See our oak door value guide.
Myth: triple glazing is always the right answer
It is heavier, more expensive and reduces useful solar gain. It suits some properties and is overkill in others.
Myth: a good U-value means no draughts
Air leakage is measured separately. A door can have an excellent U-value and still whistle. Our draught guide is the fix.
Myth: the door is the biggest source of heat loss
It rarely is, because of area. Walls, roof and windows are far larger.
Myth: dark doors are colder
The opposite is marginally true in sunlight, because dark surfaces absorb more solar radiation. Neither effect changes the calculated U-value. Our black front door guide covers heat absorption properly.
Myth: replacing a door does not need approval
Replacing an external door in a dwelling is normally notifiable. Skipping it causes problems at conveyancing.
Myth: the manufacturer number is the number you will get
Laboratory or calculated figures assume correct installation. Poor fitting can undo the difference between a good and a mediocre door.
Myth: fire doors and thermal doors are the same thing
Entirely separate specifications and separate certifications. Our fire door overview and glazed fire doors cover the fire side.
Myth: you can improve a U-value with paint
You cannot. Insulating paints and coatings do not meaningfully change the thermal transmittance of a door. Our door painting guide is about appearance and protection, not thermal performance.
Myth: heavier means better insulated
Weight usually means density, and dense materials often conduct heat better. Insulation is about trapped air and low conductivity, not mass.
Myth: the numbers never change
They change regularly across all four UK nations. That is precisely why this article points you at building control rather than printing a table.
Frequently asked questions
What is a good U-value for a front door
Better than the current limiting standard for your nation, whatever that is when you buy. Ask building control rather than relying on a number printed in an article.
Do internal doors need a U-value
Not between two heated rooms. A door onto an integral garage, unheated porch or unheated conservatory does count.
Is a lower U-value always better
Thermally yes, but there are trade offs. Triple glazing is heavy, and very low U-value doors cost more. Match the specification to the property.
What is the difference between U-value and R-value
They are reciprocals. Lower U is better, higher R is better. Do not compare them directly.
Does the door colour affect the U-value
No. Colour affects solar absorption at the surface, not the calculated thermal transmittance.
Does a thicker door always perform better
Usually, but only if the extra thickness is insulation rather than dense timber or a wider cavity with no gas fill.
Do I need building control approval to replace my front door
Normally yes. It is typically handled by a competent person scheme installer who issues a certificate, or by a building control application.
What if my house is listed
Relaxations may be available where compliance would harm the character of the building, but that is a decision for your conservation officer and building control body together.
Will a new door cut my heating bill
It will reduce heat loss through that element. How much money that saves depends on far too many variables for anyone to promise you a figure honestly.
Are composite doors better than timber thermally
Generally yes, because of the insulating core. Timber wins on heritage appearance and on repairability.
What does thermally broken mean
An insulating barrier inside a metal frame profile that interrupts the path heat would otherwise take straight through the metal.
What is a warm edge spacer
A low conductivity spacer bar between the panes of a sealed unit, replacing the older aluminium spacer and reducing edge heat loss.
Does a letter plate affect performance
It affects air leakage rather than the calculated U-value. A brushed cover helps.
Can I improve an old door instead of replacing it
Yes. Seals, threshold draught excluders, letter plate covers and secondary glazing all work and cost a fraction of a new doorset.
Where do I find the U-value of a door I already own
The original certificate, the installer paperwork or the manufacturer if you know the model. If none of those exist, you will have to estimate from the construction.
Glossary of terms you will meet on a quotation
Quotations use a lot of shorthand. Here is what the common terms mean in plain English.
Thermal transmittance
The formal name for a U-value. Same thing, different words.
Thermal resistance
The reciprocal of thermal transmittance, quoted as an R-value.
Thermal bridge
A path through the construction where heat travels more easily than through the surrounding material.
Thermal break
A deliberate insulating interruption placed in a conductive profile to stop a thermal bridge.
Low emissivity glass
Glass with a thin metallic coating that reflects long wave heat back into the room.
Sealed unit
Two or more panes of glass separated by a spacer and sealed at the edge, with air or an inert gas in the cavity.
Doorset
A door leaf and its frame supplied together as a matched assembly.
Limiting standard
The worst performance the regulations will permit for an individual element, as opposed to the better performance normally targeted.
Notifiable work
Work that must be reported to building control, either directly or through a competent person scheme.
Competent person scheme
A scheme allowing registered installers to self certify that their work complies, issuing a certificate instead of a building control application.
Air permeability
A measure of uncontrolled air leakage through the building fabric, tested separately from U-values.
Fabric first
The principle of reducing heat loss through the building fabric before increasing heating capacity or adding technology.
Where to read more
Our door materials guide, finishes guide and modern homes door guide cover the specification side, and our clearance doors are worth checking before you commit.
Bringing it together
A U-value is a simple idea wrapped in a lot of confusing detail. Lower is better, the whole doorset figure is the one that matters, glazing makes it worse, fitting quality makes or breaks it, and internal doors between heated rooms are outside the whole conversation. The regulations differ across England, Wales, Scotland and Northern Ireland and they change, so confirm the current requirement for your property before you order rather than trusting any number you read online, including ours.
When you are ready to choose, start with our external doors and composite doorsets. For the rest of the house, our internal doors, door choosing guide and internal door buyers guide are the fastest way in. Finish the opening with internal frames, architrave sets and skirting profiles.
Prices are correct at the time of writing and are subject to change. The regulatory information in this article is a general overview and is not a compliance document. Building regulation and building standards requirements vary by property type and by nation within the UK and are revised periodically, so confirm the current requirements for your project with your local building control body, your competent person scheme installer, or another competent professional before ordering or installing any door.