Not every old window has to go. Some only need new rubber seals or an adjustment. Others are rotten, warped or fogged between the panes, and repairing costs about as much as replacing. This guide helps you decide window by window, before you ask for quotes.
According to DECO PROteste (an interview from December 2020), most Portuguese homes, more than three million, still have old single-glazed windows. Before replacing anything, it helps to know what is really wrong and what a new window will fix.
Signs: repair or replace
Look at each window on its own.
| What you notice | Likely cause | Repair or replace |
|---|---|---|
| Draught with the window closed | Worn seals, hardware out of adjustment, warped sash | Often repairable. Replace if the sash or frame is deformed |
| Window hard to close | Loose or dry hardware, sash rubbing, swollen wood | Adjust or replace the hardware. Replace if the structure has given way |
| Soft, crumbling wood | Damp and rot | Small areas can be repaired. If it reached the frame, replacing is usual |
| Mist or drops between the two panes | The seal of the double glazing has failed | Replace the glass unit, not the whole window |
| Street noise | Thin single glass, gaps | Glass and seals change a lot. See sound insulation |
| Water on the glass, indoor side | Too much moisture in the air | Rarely a window defect. See condensation |
Photograph each problem and note when it shows up. It helps whoever comes to look at the window.
What a new window improves, and what it does not fix
A new window usually improves sealing against air and water, the glass, and comfort next to the window. In noisy streets, it improves the noise too.
Some things it does not fix on its own:
- Cold or damp walls. If mould grows in a corner because of the wall, the window will not stop it.
- Thermal bridges. These are spots where heat escapes faster, for example around the opening. They depend on how the wall was built and how the window is set in it.
- The roller shutter box. If it is poorly insulated, it lets air and noise through above the new window. See the guide to the roller shutter box.
- Ventilation. Below, we explain why it can get worse.
Energy efficiency depends on the whole house.
Repair: three options
Ask for a repair price too. An English local council (Cotswold) advises, for old buildings, to repair first and to check whether the frame can stay and only the glass be replaced. The guide is for historic English houses, but the reasoning applies.
- Seals. Replacing worn rubber and re-sealing the joints often ends the draught.
- Hardware. Hinges and catches out of adjustment make a window close badly. Adjusting or replacing them usually costs less than a new window.
- Glass only. If the window is sound but the glass is fogged between the panes, the glass unit can be replaced. In single-glazed frames, slim double glazing sometimes exists. Ask whoever does the work to confirm that the frame can take the thickness.
Repair stops paying off when the frame is rotten or twisted, when many windows have the same fault, or when parts are no longer available. Compare the price of repairing with the price of replacing. If the difference is small, replacing is usually the sensible choice.
Keep the old frame or remove it
Installers generally work in one of two ways. Neither is always the best. It depends on the state of the frame and the wall.
| Old frame stays | Old frame comes out | |
|---|---|---|
| What it means | The new window sits on or fits into the old frame | Everything is taken out to the opening and the new window goes in |
| Mess | Less dust and less wall finishing | More dust. Plaster and paint around the window |
| Opening | You lose some glass area | The whole opening is used |
| Condition needed | Frame sound, straight and dry | Works with a rotten, twisted or damp frame |
| Ask | Was the frame checked from the inside? | Are finishing and painting in the price? |
Either way, ask for the quote to say whether it includes removing the old window, taking away the rubble and finishing the wall. Also ask how the installer checked the frame. Installation matters as much as the window.
After the change: ventilation and condensation
Old windows let air in, which is cold but renews the air. With a new window that seals well, the house can keep the moisture of people cooking, showering and sleeping in it.
A University of Porto project on ventilation in homes describes the same thing: more airtight window frames reduced air permeability and made condensation appear more and more often. It proposes general, permanent ventilation.
In practice, before you sign:
- ask the installer how ventilation will work (vents in the window, openings, extraction in the kitchen and bathroom);
- open the windows every day, even in winter;
- if you have mould today, fix the cause first. A new window does not make it go away.
Condominium and historic areas
Building with a condominium. The Civil Code (article 1422) forbids owners from harming, with new works, the architectural line or the aesthetic arrangement of the building. Works that change them need prior authorisation from the owners’ meeting, approved by two thirds of the total value of the building. If the new window has another colour, material or design, it may be the case. Read the condominium rules and talk to the administrator before you order.
Town hall (câmara municipal). Decree-Law 108/2026 lists, among works of minor urban relevance that need no licence or prior notice, replacing frames with ones that improve energy efficiency, keeping the geometry and the relation between the components and with an outer finish identical to the original. Classified buildings, and buildings in classified groups or sites, are left out. The town hall must be told up to five days before work starts. According to Decree-Law 155-B/2026, the decree comes into force on 1 October 2026, so it already applies. Some news pieces give another date, so check with your town hall that the rule applies to your case.
If the outside look changes, or the building is in a historic area, ask the town hall.
Step by step
- List the windows and photograph the problems.
- Decide, window by window, between repair and replacement.
- With a condominium or in a historic area, ask in writing what is allowed.
- Measure the openings.
- Ask for three quotes with the same information (how to ask).
- Compare them in the quote checker and read how to read a window quote.
- Choose and agree a date. Ask how long manufacturing and installation take, and get the answer in writing. We did not find a reliable timeframe to give here.
- On the day, go through the list in how to check the installation.
If you are also painting the facade or insulating the walls, agree the order of the works with the installer.
Sources
- PGDL: Código Civil, artigo 1422.º — limits on owners’ works and the owners’ meeting authorisation (in Portuguese).
- Decreto-Lei n.º 108/2026 (copy of the Diário da República by the Ordem dos Arquitectos) and Decreto-Lei n.º 155-B/2026, 31 July — articles 6, 6-A and 80-A of the urban-planning regime in the new wording, and the entry into force on 1 October 2026 (in Portuguese).
- DECO PROteste: Gastamos muito na casa, mas sabemos pouco sobre as janelas — single-glazed windows (December 2020, in Portuguese).
- Universidade do Porto: Ventilation in Residential Buildings — airtight frames and condensation.
- Cotswold District Council: Windows — repair first and check whether only the glass can be replaced.
What to do next
- See what support exists today and what the label asks for: CLASSE+. Always confirm with Fundo Ambiental and ADENE.
- Get an idea of the price with the calculator.
- Choose the material: PVC vs aluminium.
- See the questions for the installer.
- Replacing windows in bedrooms or living rooms? See whether you need air inlets.