When people talk about windows, almost everyone asks about the cold. But in Portugal the summer sun counts too, especially in a living room that faces west or south. The number that measures how much sun comes through the glass is called the solar factor, or g-value. And Portaria 138-I/2021 sets a limit on it for windows in new or renovated homes.
This guide explains what the solar factor is, what the Portaria says and what to ask the installer. For the cold, see the guide to double glazing, Low-E and argon and the Uw calculator.
What the solar factor is
The Portaria defines the solar factor as the ratio between the solar energy that passes indoors through the glazed opening and the solar radiation that falls on it. It is a number between 0 and 1: at 0.50, half the sun’s energy comes in. The lower it is, the less heat comes in.
A low g helps in summer. In winter it does the opposite: the sun that comes in heats the home for free. So the right value depends on the window, the orientation and the shade. There is no single «best» g for every window.
What the Portaria asks
Annex I of Portaria 138-I/2021 asks, for the glazed openings of new or renovated buildings, that this condition is met:
gtot × Fo × Ff ≤ gtot,máx
- gtot is the solar factor of the opening with the protection devices fully activated, for example the shutter closed.
- Fo is the shading factor of horizontal elements above the window: overhangs, balconies, other parts of the building.
- Ff is the shading factor of vertical elements at the side.
- gtot,máx is the maximum value from Table 8.
In Table 8, for housing, the maximum depends on the summer climate zone (V1, V2, V3) and the inertia of the space:
| Space inertia | V1 | V2 | V3 |
|---|---|---|---|
| Weak | 0.15 | 0.10 | 0.10 |
| Medium or strong | 0.56 | 0.56 | 0.50 |
A space has weak inertia when the ceiling, the floor and the walls all have insulation on the inside, or are lightweight construction, or have a lining with a thermal resistance of at least 0.14 m²·°C/W. All three conditions must hold. If not, the space has medium or strong inertia. A home with no insulation on the inside tends to fall in this second case, but the classification needs knowledge of the construction: ask a designer or an SCE expert for help.
The summer zone for your town is on the location pages. The zone depends on the altitude of the building.
When it does not apply
The text of the Portaria has several exemptions:
- North-facing windows. Openings facing the north quadrant, inclusive, are exempt.
- Little glass. In housing, in a space where the windows (not counting north-facing ones) add up to 5% or less of the floor area, it is exempt.
- Thermal comfort ensured. In housing, it is exempt if the thermal comfort requirements of Decreto-Lei 101-D/2020 are met.
- Maintenance works. Components subject to maintenance works are excluded.
- Technical or functional constraint. The project technician provides solutions that reduce solar gains.
There is also a second condition, for spaces where the glass area exceeds 15% of the floor area. We do not detail it here.
The 0.10 is not the glass g
The limit of 0.10 or 0.15 looks very low, but it is for gtot, with the solar protection activated. With the shutter closed, hardly any sun comes in. The g of the glass alone is a different number, which tends to be higher because it does not count the protection. So when you read a g-value on a sheet, check whether it is for the glass only or for the opening with protection.
The documents the Portaria asks for
For new or renovated windows, the Portaria also asks:
- that they are described by documentation or a technical sheet, or by an energy label (such as CLASSE+);
- that they carry the CE marking and a declaration of conformity;
- that fitting is done by qualified people or companies, following the standards, the project, the manufacturer’s fitting instructions and good practice.
What to ask the installer
- What the solar factor (g) is, for the glass and for the opening, written in the quote.
- Whether the value is for the glass only or with the shutter closed.
- Which orientation each window has. West and south ones weigh more in summer.
- Whether there is shade (balcony, overhang, tree) that counts in the calculation.
- Who checks the Portaria in your case: the designer, the SCE expert or the installer? Ask before ordering.
Use the quote checker to see what is missing.
What we could not confirm
- How this limit is enforced on a simple window replacement in a private home. The Portaria says it applies to new and renovated buildings, and we found no official source that explains it for this case.
- Typical g-values for each glass type in Portugal: we do not give them, because they depend on the product. Ask for the technical sheet.
- The formula of the second condition (spaces with more than 15% glass).
Sources
- Portaria 138-I/2021 of 1 July (Diário da República): article 2 (definitions), Annex I, point 2 (glazed envelope), Table 8, items g) to o).
What to do next
- See the guide to double glazing, Low-E and argon.
- See the thermal insulation of windows.
- Get an idea of the price with the calculator.