U-values · Part L · Real savings
Energy Efficient Timber Windows
Timber is a natural insulator, and our standard double-glazed timber windows achieve a whole-window U-value of 1.4 W/m²K — the Part L requirement for replacement windows. With triple glazing they go lower still. Here’s how U-values work, what actually improves them, and what you can expect to save — across our range of bespoke timber windows.
U-value 1.4 W/m²K as standard
Meets Part L
Low-E, argon & warm-edge as standard
Triple & acoustic glazing available
U-value calcs for Building Control
5-year warranty
The short answer
Are Timber Windows Energy Efficient?
Yes. Wood is a natural insulator: softwood conducts heat at around 0.13 W/m·K and hardwood around 0.18 W/m·K, compared with roughly 160 W/m·K for aluminium. That’s why a timber frame loses far less heat than a metal one, and matches or beats uPVC for the same glazing.
Strictly speaking, wood itself doesn’t have a U-value — it has a thermal conductivity. A U-value describes a whole building element, such as a window frame, a glazing unit or the complete window. A typical timber window frame achieves 1.2–1.5 W/m²K, and our standard double-glazed timber windows reach 1.4 W/m²K for the whole window.
U-values
Timber Window U-Values Explained in Plain English
U-value measures how fast heat passes through a building element, in watts per square metre per degree of temperature difference (W/m²K). Lower is better. An uninsulated solid brick wall is about 2.0; a well-insulated cavity wall 0.25–0.30. Windows sit somewhere in between, and they’re usually the weakest thermal link in the building envelope by a long way.
Building Regulations Part L (England) currently requires replacement windows to achieve a whole-window U-value of 1.4 W/m²K or better. Our standard double-glazed timber windows meet this, and with triple glazing we’re well below it.
Glazing
What Affects Energy Efficiency: Glazing, Gas Fill, Spacers and Coatings
Most of a window’s performance comes from the sealed unit. Here’s how a spec builds from basic to high-performance, one upgrade at a time (typical centre-pane values):
| Glazing specification | Typical centre-pane U-value | What it adds |
|---|---|---|
| Double glazing, air-filled, uncoated | ≈2.7–2.8 W/m²K | The cheapest possible sealed unit — barely legal for new installations |
| + argon gas fill | ≈2.6 W/m²K | Argon conducts about a third less heat than air; a small gain on its own |
| + low-E coating | ≈1.4 W/m²K with air | Reflects long-wave heat back into the room — the single biggest jump for the money |
| Low-E + argon | ≈1.1–1.2 W/m²K | Our standard specification — the sweet spot for UK double glazing |
| + warm-edge spacer bar | −0.1 to −0.2 on the whole window | Reduces the thermal bridge at the glass edge |
| Triple glazing (two argon cavities, two low-E coatings) | ≈0.8–1.0 W/m²K | Best available — about 50% heavier and 25–35% more expensive |
The low-E coating does the heavy lifting: a soft-coat silver layer on the inner pane lets light through while reflecting heat back into the room. Argon then adds a smaller gain on top, and there’s no rational reason to accept an air-filled unit in 2026. More on what goes inside the unit in our guide to double glazed wooden windows.
Note that these are centre-pane figures. The whole-window U-value — the number Part L cares about — includes the frame and the glass edge, so it’s slightly higher. Timber frames create less thermal bridging than aluminium or steel, so the penalty is smaller.
Frame materials
Timber vs uPVC vs Aluminium: Which Frame Is Most Energy Efficient?
This is where the physics works in your favour. Typical frame U-values:
| Frame material | Typical frame U-value | Why |
|---|---|---|
| Timber | 1.2–1.5 W/m²K | The wood itself insulates |
| uPVC | 1.3–1.5 W/m²K | Multiple air chambers in the profile provide the insulation |
| Aluminium with thermal break | 2.5–4.0 W/m²K | Even with an insulating strip, aluminium bleeds heat |
For the same glazing unit, a timber window will match or beat uPVC and significantly outperform aluminium. The frame is a small share of the window area, but it has the highest U-value of any part, so its contribution to heat loss is disproportionately large.
Wood-effect uPVC is ordinary uPVC with a printed foil laminate. The foil changes the appearance, not the thermal performance, so a wood-effect window performs exactly like any other uPVC window with the same glazing — and unlike timber, it can’t be sanded and refinished when the surface wears.
Running costs
How Much Can Energy Efficient Timber Windows Save on Heating Bills?
We get asked this constantly, and the honest answer is: it depends — on your heating system, your other insulation, your thermostat habits and local energy prices. But here are realistic ranges based on typical UK scenarios.
Replacing single glazing with our standard double-glazed timber windows reduces heat loss through those openings by 50–65%. For a typical 3-bed semi with 12–15m² of window area, that’s roughly £150–250 a year off your heating bill at current gas prices, paying back the glazing upgrade alone within 5–8 years.
Upgrading from 1990s or early-2000s double glazing to current spec is more modest — maybe £80–150 a year. Old sealed units often have failed seals (condensation between the panes is the giveaway), no low-E coating and air rather than argon, so they’re not performing anywhere near their original spec.
The factor people forget is comfort. Cold glass cools the room air next to it, which drops and creates a chill near the window that your heating fights endlessly. Better-insulated windows eliminate that downdraught, so the room feels warmer at the same thermostat setting — and people often turn the heating down a degree or two without noticing, which saves more than the numbers suggest.
Triple glazing
When Triple Glazing Makes Sense (and When It Doesn’t)
Triple glazing saves more heat per window, but it costs 25–35% more per unit and weighs roughly 50% more — which matters for sash balance calculations and for the structural load on your walls.
Our honest recommendation: for standard UK retrofit projects, double glazing with low-E glass and argon is the sweet spot. The jump from basic to enhanced double glazing is enormous; the jump from enhanced double to triple is incremental. Our standard units already include low-E glass, argon and warm-edge spacers, so you start from the sweet spot — triple glazing is an upgrade for specific situations rather than a default.
Wooden triple glazed windows earn their keep in Passivhaus or near-Passivhaus new builds, very exposed highland and coastal locations, north-facing bedrooms where condensation persists, and properties that have already maximised insulation everywhere else. For exposed sites, pair them with a weather-resistant finish.
Sash windows
Sash Window U-Values
A timber sash window with a standard 24mm double-glazed unit achieves the same 1.4 W/m²K as our casements, comfortably inside Part L. Each sash is individually balanced for the weight of its glazing, which matters because double-glazed units are much heavier than the single glass these windows originally carried.
Georgian sash windows with solid through-bars are the exception: each pane needs its own slim 14–20mm sealed unit, which typically achieves around 1.6–1.8 W/m²K. That’s still dramatically better than the single glazing it replaces and is accepted by conservation officers; duplex or applied bars over a full 24mm unit keep the 1.4 W/m²K figure.
Sustainability
Sustainable, Eco-Friendly Timber Windows
Energy efficiency is only half the environmental story; the other half is what the window is made from and how long it lasts. Timber is a renewable material that stores carbon for as long as the window is in use, and it takes far less energy to process than PVC or aluminium.
Longevity matters just as much. With proper maintenance, engineered pine lasts 25–35 years, meranti 35–45 years and solid oak around 50 years — and a timber window can be sanded, refinished and repaired rather than thrown away when the surface wears. Our pine is slow-grown Scandinavian timber and our Dark Red meranti comes from managed forests in South East Asia; see softwood windows and hardwood windows for how the species compare.
Building Regulations
Building Regulations Part L: Do Timber Windows Comply?
All our windows can be specified to meet or exceed Part L requirements (1.4 W/m²K whole-window). We provide U-value calculations and Window Energy Ratings where needed for Building Control sign-off.
For listed building windows, where Part L exemptions may apply, we’ll work with your conservation officer on an appropriate specification. Softwood and hardwood achieve the same glazing U-values — the timber species doesn’t meaningfully affect thermal performance when painted. The difference between species is durability and maintenance, not insulation.
Prices
Energy Efficient Timber Window Prices
Our standard double-glazed windows, already Part L compliant, start from £449+VAT for a sash on weights, a sash on hidden springs or a flush single casement. Low-E glass, argon and warm-edge spacers are included as standard; acoustic glass adds about £100–150 per window and triple glazing typically 25–35%. Tell us your window sizes and we’ll quote for the spec that makes sense — standard double or triple glazing. Full breakdown in our timber window prices guide.
Our Best-Selling Timber Windows:
Energy Efficient Timber Windows — Frequently Asked Questions
Are timber windows energy efficient?
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Yes. Wood is a natural insulator, so a timber frame loses far less heat than aluminium and matches or beats uPVC with the same glazing. Our standard double-glazed timber windows achieve a whole-window U-value of 1.4 W/m²K, meeting Part L for replacement windows, and triple glazing goes lower.
What U-value do timber windows achieve?
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Our standard double-glazed timber windows achieve 1.4 W/m²K for the whole window. With low-E glass, argon and a warm-edge spacer the glazing reaches around 1.1–1.2 W/m²K centre-pane, and triple glazing around 0.8–1.0. Georgian sashes with solid bars and slim units achieve around 1.6–1.8 W/m²K.
Is timber more energy efficient than uPVC?
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For the same glazing, a timber window matches or slightly beats uPVC: typical timber frames achieve 1.2–1.5 W/m²K against 1.3–1.5 for uPVC. Most heat loss is through the glass, so the glazing specification matters more than the frame — but timber also lasts longer and can be refinished.
Are wood-effect uPVC windows as energy efficient as timber?
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They perform like any other uPVC window. The wood effect is a printed foil laminate that changes the look, not the insulation. With the same glazing, a real timber window performs at least as well, and its surface can be sanded and refinished when it wears, which a foil can’t.
Is triple glazing worth it in timber windows?
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For most UK retrofits, no — our standard double glazing with low-E glass and argon is the sweet spot. Triple glazing costs 25–35% more and weighs about 50% more. It earns its keep in Passivhaus-standard builds, very exposed sites, north-facing rooms with persistent condensation, or homes already insulated everywhere else.
Do timber windows meet Part L building regulations?
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Yes. Part L requires replacement windows in England to achieve 1.4 W/m²K or better, and our standard double-glazed timber windows meet it. We provide U-value calculations and Window Energy Ratings for Building Control, and for listed buildings we’ll agree a suitable spec with your conservation officer.
Get a Quote for Energy Efficient Timber Windows
Send your window sizes and we’ll quote for the spec that makes sense — standard double or triple glazing — within 48 hours, with U-value calculations for Building Control if you need them.
