How can you insulate wooden windows for energy efficiency?
The Wood Guide · Do it yourself
TechniqueInsulating wooden windows
Timeless charm, but without insulation a source of energy loss. Six methods, and the order in which they deliver the most.
- Draughts first, glass second
- Draught-proofing is the cheapest
- The frame itself counts too
The order that delivers the most
Most people start with the glass. Start with the gaps — that costs a fraction and is often the biggest share of the loss.
Heat loss through a window takes three routes: air leaks through gaps, conduction through the glass, and conduction through the frame itself. In an old wooden window with single glazing, that first route often accounts for a third to half of the total, and it can be sealed with draught strips for very little money.
Only once the gaps are sealed does it pay to look at the glass. And then the question turns out to be not so much whether you insulate as what the frame can still take: thicker glass is heavier and deeper, and not every old frame has the rebate or the hinges for it.
A wooden frame conducts heat much less than aluminium and slightly less than PVC. So with wood the frame itself is rarely the weak point — unlike old aluminium windows without a thermal break, where the frame is a cold bridge you can't get rid of.
- Draught strips
- cheap, big effect, DIY
- Sealing between frame and wall
- sealant or expanding foam; often forgotten
- Secondary glazing
- effective, reversible, good for heritage
- Insulating film
- temporary, limited effect
- Double glazing in the existing frame
- provided the rebate and hinges allow it
- New frame with high-performance glass
- biggest effect, biggest cost
To be fair: not every old frame can carry double glazing. The rebate must be deep enough, the hinges must cope with the extra weight, and for listed buildings it is often simply not allowed. Have that checked before you order glass — a window that sags or no longer closes costs more than you save.
Gaps, glass and frame
Heat loss through a window takes three routes: air leaks through gaps, conduction through the glass and conduction through the frame. In an old wooden window with single glazing, the first route often accounts for a third to half of the total.
Click a chapter or use the arrows · heat flows schematic.
From cheap to thorough
| Method | Effect | Difficulty | Cost |
|---|---|---|---|
| Draught strips in the rebate | Large on old windows | Low | Very low |
| Sealing the frame-to-wall gap | Large and often forgotten | Low | Very low |
| Insulating film on the glass | Limited | Low | Low |
| Secondary glazing | Large | Medium | Medium |
| Double glazing in the existing frame | Very large | Medium to high | Medium to high |
| New frame with high-performance double or triple glazing | Largest | High | High |
A second window on the inside gives almost the insulation value of double glazing, leaves the original window completely intact and is reversible. For listed buildings and façades where the outside appearance must be kept, it is often the only permitted solution — and a good one even without that restriction.
Where to start
- 1 · Find the leaksOn a windy day hold a lighter or a piece of toilet paper along the joints. Where it moves, it leaks. Don't forget the joint between frame and brickwork — it is often hidden behind the trim.
- 2 · Seal the gapsDraught strips in the rebate of opening parts, and sealant or expanding foam in the joint between frame and wall. This is the cheapest intervention with the biggest effect.
- 3 · Repair the woodDeal with rot and loose paint first. Insulating over a decayed frame is work you will be doing again within ten years.
- 4 · Assess the frameCan it take double glazing? Measure the rebate depth and assess the hinges. If in doubt, have a joiner take a look.
- 5 · Choose glass or secondary glazingIf the frame can take it, double glazing is the most durable solution. If it can't, or it is protected heritage, secondary glazing is the way.
- 6 · Finish the woodA well-maintained frame doesn't insulate better, but it does last decades longer. See wood for exterior joinery.
Frequently asked questions about insulating wooden windows
What do I insulate first on a wooden window?
The gaps. In an old window with single glazing, a third to half of the loss goes through air leaks, and you seal those with draught strips for a fraction of the cost of new glass.
Can double glazing go into my old frame?
Sometimes. The rebate must be deep enough and the hinges must cope with the extra weight. Have that checked before you order glass.
What is secondary glazing?
A second window on the inside. It gives almost the insulation value of double glazing, leaves the original window intact and is reversible — ideal for heritage buildings.
Does a wooden frame insulate well?
Better than aluminium and slightly better than PVC. With wooden windows the frame is rarely the weak point; the gaps and the glass are.
Insulation in perspective
Start with the gaps. That is a tenth of the cost and often half of the loss.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Thal (Sulzberg), Vorarlberg — Friedrich Böhringer, CC BY-SA 2.5 · Corner of a double-glazed insulating unit with a spacer bar — HTL-Mödchen23, CC BY-SA 4.0
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