Windows and doors can either help or hinder your plans to make a home energy efficient and heat pump ready. Older windows lose heat through the glazing itself and, if poorly sealed, around the frames — so they are an insulation problem and a draught-proofing problem at the same time.
Resolve both and they significantly improve energy efficiency, and can add kerb appeal and value along the way. This article covers:
In many cases you can make real progress without spending excessive amounts of money.
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The heat pump ready series A nine-part guide to preparing your home, produced with low-carbon homes architect Colin Usher. |
There are two things to check: whether the units are draughty, and whether the double glazing has blown.
On a windy day, with the internal door closed and the window or external door shut, hold a lit incense stick or smoke pen beside the unit and watch where the smoke goes. Older windows often let smoke escape under and around the frame. On doors, check the edges and any openings such as a cat flap or letterbox.
Under the sill is a very common finding. As Jenny Usher asks of any existing installation:
"When the installers put double glazing in, did they seal underneath the windowsill or leave a great big hole?"
Jenny Usher
The full method is in our guide to draught proofing your home.
If you see condensation between the panes, the seal has failed and the insulating gas has escaped, letting air and water in. Because air conducts energy more readily than the gases used in sealed units, the window now loses heat far more easily than it should.
The sealed unit normally needs replacing, but the frame can often be kept if it is still in good condition.
Colin Usher, the low-carbon homes architect advising on this series, points out that older units usually have both problems at once:
"First of all, they aren't the high performance that you get out of modern double glazing. Many of the draught seals may have failed by this time. It's quite probable that the double glazing units themselves were starting to fail, and you get a bit of condensation in between the panes."
Colin Usher, low-carbon homes architect
A U-value measures how effective something is at insulating. The lower the number, the better.
| Glazing type | Typical U-value (W/m²K) |
|---|---|
| Single glazing | Around 5.0 to 6.0, or worse |
| Double glazing | 1.2 to 3.0, higher in older units |
| Triple glazing | Below 1.0, but at least 20% more expensive |
Three different U-value figures appear in Part L, and they are routinely mixed up. Here is what each one means.
| Situation | Requirement |
|---|---|
| Replacement window in an existing home | Maximum 1.4, or WER Band B or better |
| New dwelling — limiting value | Maximum 1.6 |
| New dwelling — target specification | 1.2 |
The 1.6 figure for new builds looks weaker than the 1.4 required for a replacement, which confuses people. It is a backstop — the worst any single window may be — while the home as a whole must hit an overall energy target. In practice new builds are specified at around 1.2 to meet it.
For a homeowner replacing windows, 1.4 or WER Band B is the number that matters. This full standard has applied since 15 June 2023.
Figures apply to England under Approved Document L. Scotland and Wales have their own standards. Correct as of August 2026.
A G-value measures how much solar radiation the window lets in. The ideal figure depends on which way the window faces and whether your priority is reducing summer overheating or maximising winter solar gain.
The Window Energy Rating (WER) combines the U-value and the G-value into a single band, running from A++ at the best down to G at the worst. Most modern windows are sold as A-rated, which comfortably clears the Band B requirement.
Doors follow the same principle:
An older house with wooden doors and single glazing can have its efficiency significantly improved by addressing windows and doors alone. The same applies to a home with ageing double glazing.
You do not need to sit through a hard sell from a double glazing salesman because your windows are blown or draughty.
A local builder can seal frames from underneath. Specialist companies can replace blown sealed units while keeping the existing frames. Some frames will have reached the end of their useful life, and a quality repair company will tell you if that is the case.
As with any such work, get at least three quotes and check online reviews before contracting anyone.
If full replacement is necessary, cost need not be a barrier to starting. Colin's approach is to phase it:
"At £300 to £400 per window, a phased approach avoids spending it all in one hit — perhaps doing one side of the house one year and the other the next."
Colin Usher
There are several reasons you might not want to replace windows at all:
In these cases secondary glazing may be the answer. There are three options.
| Type | What to know |
|---|---|
| Film | Cheapest and DIY-installable, but the weakest thermally. Degrades quickly and is not a good long-term solution |
| Magnetic | Glass or polycarbonate fixed with magnetic strips around the frame. More thermally efficient than film |
| Permanent | Usually custom-made in wood or uPVC. The most expensive but by far the most efficient, and the least disruptive to a building's character — making it the usual choice for heritage and listed properties |
One thing to be clear about: most secondary glazing has ordinary air between it and the existing pane, rather than the inert gas used in a sealed unit. It will therefore be less thermally efficient than a new double- or triple-glazed window. It is a good solution where replacement is not possible, not a superior one.
If neither repair nor secondary glazing suits, it may be worth replacing the windows altogether.
You do not need to sit through a hard sell from a salesman who will not leave until the sale is made. It can occasionally be cheaper to endure that rigmarole, but a company that lets you consider a quote for a week is usually the better one to work with.
Choose a contractor on:
That last point tends to put the pushy salesman offering a large discount valid only today somewhere near the bottom of the list.
Triple glazing reaches U-values below 1.0 but costs at least 20% more than double. Modern A-rated double glazing already performs well and comfortably meets the regulations. The answer depends on your budget, the condition of the rest of the home, and the ratings of the specific products you are comparing.
In a home where the walls and loft are still poorly insulated, that money almost always does more work elsewhere first.
You may want to keep existing doors, whether for budget reasons or to maintain the character of the house. A smoke test will show you where the problem is.
Around the edges, the seals on older uPVC doors weather and perish. The same repair team visiting for your windows can often replace them. On older wooden doors you can buy sealing tape and fit it yourself, though a professional will make a neater job of it.
These usually produce the strongest air movement during a smoke test.
Given how few paper letters most people now receive, it may make sense to do without the letterbox altogether and fit a secure parcel box outside the front door. Colin notes these "can have the added advantage of a parcel shelf for your online shopping parcels."
For cat flaps, the fix is a better flap rather than no flap:
"You can get electronic Passivhaus quality cat flaps. If not, I think the most important thing is to have one that is related to a microchip. These hold themselves closed and won't blow open."
Colin Usher
A microchip-triggered flap costs more, but it also stops the neighbourhood cats dropping in for dinner.
Bifold doors and conservatories are attractive additions and an energy efficiency headache. Glazing is less efficient than the wall it replaces, bifold seals weather and decay, and polycarbonate conservatory roofs typically have worse U-values and G-values than the glass in the sides. Like everything else, they degrade with age.
If you have inherited older bifold doors or a conservatory, look for:
As with the rest of the house, these can often be repaired rather than replaced. If replacement is necessary, choose on Window Energy Rating and value for money.
One point specific to heat pumps: a conservatory is usually outside the heated envelope, and heating one properly is expensive whatever the heat source. Keep the door between it and the house closed and well sealed, and treat that door as an external one.
Often yes. Draught-proofing, replacing failed sealed units while keeping the frames, and adding secondary glazing all improve thermal performance without a full replacement. A local builder can seal frames from underneath, which addresses one of the most common sources of draughts.
Not necessarily. Triple glazing insulates better but costs at least 20% more, and modern double glazing already performs well. The right choice depends on your budget, the condition of the rest of your home, and the Window Energy Ratings of the products being compared.
Usually not. If the frame is still in good condition, many installers can replace only the sealed glazing unit, which reduces both cost and waste. A quality repair company will tell you if the frame itself has reached the end of its life.
In England, a replacement window must achieve a maximum U-value of 1.4 W/m²K, or a Window Energy Rating of Band B or better. This full standard has applied since 15 June 2023. Scotland and Wales set their own requirements.
A U-value measures how much heat passes through the window, where lower is better. A Window Energy Rating combines that with solar heat gain and air leakage to give an overall band from A++ down to G.
Often yes, because it usually leaves the original windows in place. Permanent custom-made secondary glazing affects a building's character least and performs best. Always check with your local planning authority if your property is listed or in a conservation area.
No. Most secondary glazing has ordinary air between the panes rather than the inert gas used in a sealed unit, so it is less thermally efficient than a new double- or triple-glazed window. It is the right answer where replacement is not possible, not a better one.
Windows and doors are among the most important parts of your home's thermal envelope. By repairing draughts, replacing failed glazing, or upgrading where it makes sense, you reduce heat loss and improve comfort all year round.
Not every home needs expensive replacement windows. Targeted repairs or secondary glazing often deliver worthwhile improvements at a fraction of the cost, and a phased replacement spreads the expense over several years.
Choose the solution that suits your property and budget, and you take another practical step towards a home that is ready to get the best from a heat pump — while reducing your energy bills for years to come.
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Next in this series 6. Wall Insulation Options for Heat Pump Homes — cavity, external and internal insulation compared, and how to tell which your home needs. Previously: 4. Floor and loft insulation |
Produced with Colin Usher, low-carbon homes architect. Last updated: 3 August 2026.
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