In an old house, the wooden window is often part of the design of the facade. It also lets air through and needs repainting. This guide explains what a sash (guillotine) window is, what you can do without replacing it, when replacing makes sense and what to ask the câmara before you touch anything.
What a sash window is
The janela de guilhotina (also called corrediça) opens by sliding vertically: the sashes run inside a fixed timber frame that acts as a track. According to a dissertation from the Universidade Lusófona do Porto (2015), it spread in Portugal from the 18th century and probably came with the English merchants settled in Porto after the Methuen treaty of 1703.
Another dissertation, from the Faculty of Engineering of the University of Porto (2020), shows sash breast windows on the Porto “casa burguesa”, next to side-hung windows with timber shutters inside. I could not confirm that the sash window is as typical in Lisbon as in Porto.
First: are you allowed to touch the window?
Decreto-Lei 108/2026 (Diário da República, 29 May 2026) says:
- Listed building, or one in the process of being listed: conservation and alteration works need a licence (article 4(2)(d)). The exemption for replacing window frames does not apply (article 6-A(2)).
- Protection zone of a listed building: exterior alteration needs a licence, except the frame replacement described in article 6-A(1)(j).
- Exempt replacement: frames that improve energy efficiency, with the same geometry and an exterior finish identical to the original.
The same decree defines “alteration works” as works that change the physical or aesthetic characteristics of the building, including “the nature and colour of the exterior cladding materials, which include doors and windows” (article 2(d)). A timber sash window replaced by a side-hung PVC window is therefore hardly an “identical” replacement. See the guide on replacing windows. For historic zones in Lisbon and Porto I could not read each municipality’s rules: ask the câmara.
Restoring: what the conservation literature says
The two most useful documents I read are foreign or academic. They are not Portuguese rules.
Historic England (England, guidance of June 2018) says that historic windows of traditional design usually contribute to the significance of the building, that they should be retained and repaired where possible and that, if beyond repair, they should be replaced with accurate copies. It also says that complete replacement is rarely cheaper than repair and upgrading, and that even very neglected windows can usually be brought back into condition fairly easily.
The Porto engineering thesis separates “restoration” (repairing with old techniques and materials) from “restoration with improvement” (old techniques, modern materials), and reminds us that new timber needs regular maintenance, at a considerable cost.
Improving without replacing
| Option | What it does | Care (source) |
|---|---|---|
| Repair and repaint | Replaces rotten parts, glass and putty | Use the same type of timber, properly dried and treated (FEUP, 2020) |
| Draught-proofing | Fewer draughts | Cheap and discreet. Do the necessary repairs first (Historic England) |
| Secondary window inside | Less air and less heat loss, also through the frame | It must open easily to ventilate. Do not draught-proof the main window: the gap between the two is then vented by outside air, with less risk of condensation (Historic England) |
| Slim double glazing in the old frame | Improves the glass | It does not reduce draughts or loss through the frame. Hand-made old glass is worth keeping. Thin double-glazed units use a lot of energy to make and last less, and vacuum units are expensive (Historic England) |
| Shutters and curtains | Internal shutters reduce heat loss | They are part of the design of the Porto house (FEUP; Historic England) |
For condensation, see the guide and ventilation. If you are thinking of a second pane, the double glazing guide explains the terms.
When replacing makes sense
- The rot has reached the frame, or the frame and sash are warped.
- Repair costs as much as a new window (ask for both prices).
If you replace, the Porto thesis says it is essential to keep the original design, in timber or another material. It suggests, as a possibility, aluminium or PVC lacquered to imitate wood, “without forgetting to check the thickness of the profiles”. That is a study proposal, not a rule. For sash windows made in PVC, I could not find any maker data sheet. For aluminium, see the guide to thermal-break aluminium windows.
What to ask for
- Two quotes: repair and replace. Get the design, material and colour in writing.
- The carpenter: which timber, which paint, which draught seal, and whether the glass can be changed without losing the original parts.
- The câmara: is the building listed, or in a protection zone? Are there local rules for window frames? Keep the answer.
- The condominium, if it is a flat: colour and design may need an assembly. See the guide.
What I could not confirm
- Each câmara’s rules for window frames in historic zones (Lisbon, Porto, others).
- Whether the sash window is as typical in Lisbon as in Porto.
- Historic England’s “Traditional Windows” guide: I read only the 2018 note “How to Improve Energy Efficiency”.
- Whether certified PVC sash windows are on sale in Portugal.
Sources
- Diário da República, Decreto-Lei 108/2026 of 29 May (copy from the Ordem dos Arquitectos): articles 2(d), 4(2), 6-A. Read on 23 October 2026.
- Historic England, “Energy Efficiency and Historic Buildings: How to Improve Energy Efficiency”, June 2018 (foreign reference, in English). Read on 23 October 2026.
- Diana Sousa, “Reabilitação de caixilharias de madeira”, Universidade Lusófona do Porto, 2015: origin of the sash window and how the frame is built. Read on 23 October 2026.
- Ricardo Lopes, “Metodologia de reabilitação de fachadas de edifícios antigos”, FEUP, 2020: types of intervention and window frames on the Porto casa burguesa. Read on 23 October 2026.
What to do next
- Read Can I replace my windows? for the câmara and the condominium.
- See replacing old windows to decide window by window.
- Compare the age of buildings in the Lisbon parishes and Porto.