Ventilation when replacing windows: do you need air inlets?
New windows close better. That is why Portaria 138-I/2021 deals with ventilation when windows are replaced in bedrooms and living rooms. Answer three questions and see what the text says. Free, and your values stay in your browser.
Your case
Result
The text asks for an air-inlet solution in the facade.
When windows are replaced in main spaces and it is not a major renovation, the Portaria asks for one of two solutions:
- Fixed air inlet
- free area of at least 60 cm² (3 spaces: 180 cm² in total)
- Self-regulating vent
- at least 25 m³/h at 2 Pa, or 75 m³/h at 20 Pa (NP 13141-1)
Ask the installer to write in the quote which of the two solutions each window gets, and the make and model of the vent. A new window with no air inlet can leave the home stuffy and damper.
What 0.50 air changes per hour means
- Air volume of the home
- 200 m³
- Flow that 0.50 per hour stands for
- 100 m³/h (27.8 l/s)
This is only an order of magnitude: the SCE Manual calculates the home with more data. It does not prove the home meets the Portaria.
Work out the price of the windows · See the condensation risk
What the Portaria says
What the Portaria says
Portaria 138-I/2021 sets the ventilation requirements for new and renovated buildings. For housing, Annex II, point 1.2.1, asks for a minimum air change rate of 0.50 per hour, calculated with the SCE Manual.
The same point covers windows. When the work replaces windows in main spaces and is not a major renovation, there must be air admission in the facade of those spaces: either inlets with at least 60 cm² of free area, or self-regulating vents with at least 25 m³/h at 2 Pa (or 75 m³/h at 20 Pa), according to NP 13141-1.
The text has two ways out: if the home already reaches 0.50 air changes per hour or more, calculated with the SCE Manual, it is exempt. And if there is a technical or functional constraint, the project technician picks an alternative solution that does not put air quality at risk.
What we could not confirm
- The Portaria says it applies to «renovated» buildings. We found no official source explaining how a simple window replacement in a private home is enforced. Treat this page as a guide, not legal advice.
- The text does not say how many air inlets each window gets. It speaks of a solution «in the facade of those spaces». Ask the installer or the designer how it will work in your home.
- In buildings with a residents' association, the look of the facade may have its own rules. Check the building rules before drilling the frame or the wall.
Sources
- Portaria n.º 138-I/2021, de 1 de julho (Diário da República). Annex II, point 1.2.1 (housing), Table 10 and items a) to d). Text read in October 2026.
- Decreto-Lei 101-D/2020 of 7 December, article 3(q): a «major renovation» is when the cost of the work exceeds 25% of the building's value. The Portaria defines «main spaces» (bedrooms and living rooms) in article 2.
Frequently asked questions
That is not what the text says. It asks for an air-admission solution in the facade of the main spaces (bedrooms and living rooms) when windows are replaced, and it allows exemptions. Kitchen and bathroom are not main spaces.
0.50 air changes per hour, in Table 10 of Annex II, calculated with the SCE Manual. If you have heard a lower figure, such as 0.40, we did not find it in this text.
If the home gets tighter with no ventilation, the air humidity tends to rise and condensation to appear. See the condensation guide. The Portaria does not explain the reason for the air-inlet rule.
The calculation follows the SCE Manual method and is not a sum you do at home. The Portaria does not say who does it: ask an SCE expert or a designer.
A self-regulating vent limits the flow in strong wind. For noise, ask the installer for the acoustic value of the vent. This page does not calculate it.
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