Glass for the Spanish sun: the g-value explained
In a UK home the key number is the U-value. On the Costa Blanca the heat comes in as sunshine through the glass, so the number that decides your summer is the g-value.
We speak English. Eduardo answers on WhatsApp, 9:00–20:00 Spain time (8:00–19:00 UK time), and replies within 24 hours.
Last reviewed: 26 September 2026
On the Costa Blanca, the glass value that matters most in summer is the g-value, not the U-value. The g-value is the share of solar energy that passes through the glass, from 0 to 1. Lower means less heat. Choose solar control glass for south and west openings, low-E glass everywhere, and skip triple glazing.
Three ways heat is controlled
| Value | What it measures | When it matters most |
|---|---|---|
| U-value (Uw, Ug) | Heat that passes because of a temperature difference between inside and outside. | Cold winters, and cold surfaces where condensation forms. |
| g-value (solar factor) | The share of solar energy that passes through the glass, from 0 to 1. | Long summers, strong sun, and south- and west-facing openings. |
| External shading | Sun that never reaches the glass: roller shutter, awning or louvres. | Always, where it can be fitted. It is the most effective measure. |
The exact U-value and g-value depend on the exact glass make-up. Ask for both in writing: they come from the glass data sheet, not from a sales leaflet.
U-value and g-value measure different things
The U-value measures heat that moves because it is warmer on one side than the other. It is the main number in a cold climate, and the one behind UK window ratings.
The g-value measures sunshine that passes straight through the glass. It heats the room even when the air outside is no warmer than inside.
A glass can be excellent on one value and average on the other. In a UK window rating, a high g-value earns points. Here, on a west-facing window, it heats your living room.
Why the sun decides on this coast
The Spanish building code (CTE) places the low-lying coast of Alicante province in climate zone B4. That means a mild winter and a demanding summer.
The AEMET weather station in Alicante records about 2,850 hours of sunshine a year (1981–2010 normals). Frost on the coast is rare.
With so little winter, most of the energy you pay for goes on cooling. The heat you want to stop enters as sunshine through the glass, not through the frame.
Which glass for each orientation
| Facing | What happens | What to fit |
|---|---|---|
| South | High midday sun. A balcony, eave or awning cuts it easily. | Low-E glass plus solar control glass, especially on large openings. |
| West | Low afternoon sun straight into the room, when the home is already warm. The hardest orientation. | Solar control glass, and a shutter or awning as well. |
| East | Morning sun, while the home is still cool. | Low-E glass. Solar control only on large glazed areas. |
| North | Almost no direct sun. | Low-E glass only. Solar control would just remove light. |
Low-E glass and solar control glass: two coatings, one unit
Low-E glass has an invisible metal coating that reflects long-wave heat. In winter it keeps heat in. In summer, with the air conditioning on, it keeps the cool air in.
Solar control glass has a coating that reflects part of the sun's energy before it enters. That is what lowers the g-value.
They do different jobs and can share the same double-glazed unit. On this coast, south and west openings usually deserve both.
Solar control glass removes a little light and gives a slight tint. On a north-facing room that is a loss with no gain, so we do not fit it there.
Shade outside first, then choose the glass
Sun stopped outside never enters. A roller shutter at half height, an awning or louvres beat any glass, because they stop the sun before it reaches the pane.
An inside curtain is not solar protection. When it acts, the heat is already in the room.
Solar control glass is essential where shading is not possible: fixed panes, floor-to-ceiling sliding doors and sea views nobody wants to cover.
Laminated safety glass: when you need it
Laminated safety glass is two panes bonded by a plastic interlayer. If it breaks, the pieces stay stuck to the layer instead of falling.
The CTE sets rules for glass in areas with a risk of impact. In doors, that area runs from the floor up to 1.50 m. In fixed panes, it runs from the floor up to 0.90 m.
Glass in those areas must meet an impact classification under UNE-EN 12600, unless a protective barrier is fitted. Laminated or toughened glass is the usual answer.
We also suggest laminated glass on ground floors, first floors, and openings reachable from a terrace. It helps in a holiday home that stays empty for months. See holiday home windows.
Triple glazing rarely pays here
Triple glazing lowers the U-value, which solves a hard winter. It does nothing special against sunshine, which is the real problem here in summer.
It is also much heavier, which demands stronger hardware and makes a large sliding door harder to move.
On most homes on this coast, the same money works harder in solar control glass, external shading and an insulated shutter box.
How to read the glass on a quote
Glass is written as a code in millimetres, from the outside in. A common one is 4/16/4: a 4 mm pane, a 16 mm cavity and a 4 mm pane.
- A plus sign, as in 6+6, means that pane is laminated: two 6 mm panes bonded together.
- Five numbers, as in 4/12/4/12/4, mean triple glazing.
- The code does not say whether there is a low-E coating, solar control or argon. Ask for each one in writing.
- Ask for the g-value on every opening with solar control glass.
- Ask for a warm-edge spacer. The spacer is the strip that separates the panes. An aluminium one cools the edge of the glass and causes a band of condensation there.
Will new windows help a hot apartment?
New windows help a hot apartment if the glass is chosen for the sun. Tu Ventana Costa Blanca, the Benidorm office of Tu Ventana Madrid, fits Kömmerling uPVC windows for English-speaking owners in 14 Costa Blanca towns, including Albir and L'Alfàs del Pi. It fits solar control glass on south and west openings, and insulated shutter boxes.
Read about our uPVC windows for this coast.
Sources
- CTE, Documento Básico HE: energy saving, Annex B climate zones and table 3.1.2 (solar control) (codigotecnico.org)
- CTE, Documento Básico SUA: safety in use, section SUA 2, 1.3 impact with fragile elements (codigotecnico.org)
- AEMET: standard climate values, Alicante/Alacant station, 1981–2010
- BFRC: how UK window energy ratings treat solar gain
Glass chosen opening by opening
We check which way each window faces and what shade it has, then quote the right glass for each one. Ask for a free quote.
Eduardo answers in English on WhatsApp, 9:00–20:00 Spain time (8:00–19:00 UK time).
Questions about glass and the sun
Last reviewed: 26 September 2026
What is the g-value of glass?
It is the share of the sun's energy that passes through the glass into the room, from 0 to 1. The lower it is, the less heat enters. It comes from the data sheet of the exact glass.
Does solar control glass make the room darker?
A little. It removes some light and gives a slight tint. On a sunny south or west façade it is worth it. On a north-facing room, we do not recommend it.
Is triple glazing worth it on the Costa Blanca?
Rarely. It targets cold winters, which this coast does not have. It is heavy and does not solve the summer sun. Solar control glass and external shading work better here.
My apartment is very hot in summer. Will new windows help?
Yes, if the glass suits each orientation and the shutter box is insulated. Solar control glass on south and west openings, plus external shade, makes the biggest difference.
Do I need laminated safety glass?
The CTE requires impact-rated glass in doors up to 1.50 m, and in fixed panes up to 0.90 m, unless there is a barrier. We also suggest it on ground floors and in homes left empty.
Why does condensation appear only along the edge of the glass?
Usually because the spacer between the panes is aluminium and cools the edge. A warm-edge spacer prevents it. See our guide to mould on windows in a holiday home.