# Heat in the attic and on the roof: why the top floor overheats, cool roofs and what works

> Heat numbers according to IPMA, what the official calculation counts (colour, ventilation, insulation), what the evidence says about reflective paints and radiant barriers, safety in hot weather and what we could not confirm.

- Page: https://telhadoseguro.pt/en/guides/summer-heat-attic-cool-roof
- Language: English
- Updated: 2026-11-05

## Summary

- Heat varies a lot: according to IPMA, Évora averages 86.5 days a year with a maximum of 30 °C or more, and Porto 13.0. In 2026 there were 9 heat waves by mid-October, a record.
- The official home calculation (SCE Manual) counts the colour of the finish (absorptance 0.4 to 0.8), the ventilation of the void and the insulation. It is a calculation convention, not a measurement of your roof.
- Reflective paints and radiant barriers: the evidence we read comes from the USA and is weak for Portugal. A radiant barrier yields little with a lot of insulation and loses effect with dust.
- In hot weather, do not climb onto the roof: the DGS says to avoid the sun between 11:00 and 17:00. Ask for work early in the morning and postpone what is not urgent during a heat wave.

The top floor is the first to feel the summer: the sun heats the roof all day and the stored heat is slow to leave. This guide shows the heat numbers, what the official home calculation counts, what the evidence says about reflective paints and radiant barriers, and what to ask a contractor. Insulation has its [own guide](https://telhadoseguro.pt/en/guides/roof-insulation).

*Updated on 4 November 2026. General information. Every house is different: only someone who sees it can say what suits it.*

## The essentials

- **Heat is not the same across the country.** According to IPMA, Évora averages 86.5 days a year with a maximum of 30 °C or more; Porto, 13.0.
- **2026 broke the record for heat waves:** 9 by mid-October.
- **The official calculation counts the roof colour, the ventilation of the void and the insulation.**
- **Reflective paints and radiant barriers:** the evidence we read comes from the USA and is weak for Portugal.
- **In hot weather, do not climb onto the roof.** The DGS says to avoid the sun between 11:00 and 17:00.

## Heat in Portugal

IPMA averages for 1991-2020 (days per year):

| Station | Maximum ≥ 30 °C | Maximum ≥ 35 °C | Minimum ≥ 20 °C (tropical nights) |
|---|---|---|---|
| Porto / Pedras Rubras | 13.0 | 1.7 | 4.9 |
| Bragança | 45.5 | 7.3 | 1.9 |
| Lisbon / Gago Coutinho | 36.3 | 6.9 | 16.8 |
| Évora | 86.5 | 31.9 | 4.2 |
| Faro | 35.5 | 3.1 | 44.7 |

IPMA says there is a heat wave when, for at least 6 days in a row, the daily maximum is 5 °C above the average of that month's maximums (reference 1991-2020). In 2026 there were 9 by mid-October, with 98 days in a heat wave, the highest value since 1942; the longest lasted 13 days. The summer was the 3rd hottest since 1931, and Mora reached 44.3 °C on 3 July. IPMA says heat waves have been increasing since the late 1990s.

## Why the top floor heats up

The LBNL laboratory measured, on a summer afternoon, a black roof 30 °C hotter than a white one. A roof that reflects 35% of the sun stays about 12 °C cooler than one that reflects 10%.

According to the ORNL laboratory (USA), the ceiling accounts for about 15 to 25% of a house's cooling load. On a top floor, it should account for more (our reading, no source).

## What the official calculation counts

Houses fall under the REH; the RECS is for commerce and services. Both use the SCE Manual (Despacho n.º 6476-H/2021). The summer zone (V1, V2 or V3) depends on the mean outdoor temperature in the cooling season: up to 20 °C, 20 to 22 °C, above 22 °C. For houses, cooling is calculated with 25 °C indoors.

The Manual gives the solar absorptance (the share of sun absorbed) by colour:

| Colour of the outer finish | Absorptance |
|---|---|
| Light: white, cream, yellow, orange, light red | 0.4 |
| Medium: dark red, light green, light blue, light grey | 0.5 |
| Dark: brown, dark green, bright blue, dark blue, dark grey | 0.8 |

In a roof void, the value is multiplied by a factor: 0.8 if the void is strongly ventilated (more than 1500 mm² of openings per m² of roof), 1.0 if it is weakly ventilated (500 to 1500) and 1 if it is not ventilated. With a low-emissivity face (ε ≤ 0.20), the factors are 0.7 and 0.9. Our sum: a dark tile with an unventilated void counts 0.80; a light tile with a strongly ventilated void, 0.32. **This is a certificate convention, not a measurement of your roof.**

Portaria n.º 138-I/2021 does not ask for a roof colour and excludes conservation works.

## What helps and what is not proven

- **Insulation and ventilation.** APICER and CTCV say unwanted solar gains are reduced by roof insulation or natural ventilation of the underside of the tile. That air also prevents [condensation](https://telhadoseguro.pt/en/guides/attic-condensation-roof-void).
- **Light colour.** APICER says lighter tiles tend to reflect more (see [tiles by climate](https://telhadoseguro.pt/en/guides/roof-tiles-by-climate)). LBNL says the extra winter cost is generally small next to the summer saving, but it speaks of the USA: for Portugal we have no number.
- **Radiant barrier** (reflective foil in the void). In American attics with R-19 insulation, ORNL measured 16 to 42% less heat through the ceiling, but says the cooling bill falls 2 to 10%. It yields less with a lot of insulation, and dust on the attic floor can remove half the effect in 1 to 10 years. In a test in Tulsa, neither the barrier nor the insulation gave measurable savings.
- **Reflective paints.** The physics is the LBNL one, but we found no independent measurement of the saving on Portuguese roofs. On old fibre-cement, read the [asbestos guide](https://telhadoseguro.pt/en/guides/asbestos-roof-law-removal) before touching it.

## Roof windows, panels and green roofs

**Roof windows.** Portaria n.º 138-I/2021 limits the solar factor of openings (Table 8): in houses, 0.56 (V1, V2) or 0.50 (V3) with medium or strong inertia, and 0.15 (V1) or 0.10 (V2, V3) if the room is lightweight. Exempt are north-facing openings, those covering up to 5% of the floor and houses that meet the thermal comfort of article 6, n.º 9, of Decreto-Lei n.º 101-D/2020. In our reading, it also applies to roof windows in the works it covers. VELUX Portugal says exterior awnings and blinds stop the sun before the glass, without numbers. APICER mentions light tubes. See the [roof windows guide](https://telhadoseguro.pt/en/guides/skylights-roof-windows).

**Sandwich panel and green roof.** In our reading, the colour table also applies to sheet: a dark colour counts 0.8 (see the [panel guide](https://telhadoseguro.pt/en/guides/sandwich-panel-roof)). The summer benefits of a [green roof](https://telhadoseguro.pt/en/guides/green-roof-portugal) come from an association, with no independent study.

## Health and safety

The DGS says to close shutters in the hot hours, let air circulate at dusk, avoid the oven, spend 2 to 3 hours a day somewhere cool and drink water without waiting to feel thirsty. More vulnerable are children, older people, people with chronic illness and people living in poor housing. Emergency: 112; advice: SNS 24 (808 24 24 24). According to RTP, INSA counted 680 deaths above expected in three heat periods of 2026, most of them aged 75 or more.

The ACT says heat increases fatigue and accident risk and asks for acclimatisation. On a roof there is also the fall: see [work at height](https://telhadoseguro.pt/en/guides/work-at-height-roof). As common sense, not an official rule: ask for work early in the morning, with water and breaks, and postpone what is not urgent during a heat wave.

## What to ask the contractor

1. What colour and data sheet does the tile or sheet have?
2. What insulation, what thickness and where?
3. How will the void be ventilated (eaves, ridge)?
4. Does changing the colour need council approval or a permit? See the [permit](https://telhadoseguro.pt/en/guides/roof-works-permit-2026) and the [historic zone](https://telhadoseguro.pt/en/guides/roof-historic-zone-coast-islands).
5. Is there a test report, with standard and climate, for «reflective» products?
6. What hours and breaks does the contractor plan for the hot hours?
7. Everything in writing: [choosing a contractor](https://telhadoseguro.pt/en/guides/choosing-a-roofing-contractor), [warranty](https://telhadoseguro.pt/en/guides/roof-works-warranty) and [costs](https://telhadoseguro.pt/en/guides/roof-cost-per-m2).

**See also:** On the layers under the tile and on ventilation, see [underlay, membrane and ventilation](https://telhadoseguro.pt/en/guides/roof-underlay-and-ventilation).

## What we could not confirm

- **Savings in Portugal** from reflective paints, light tiles and radiant barriers: we only read US sources.
- **Article 6, n.º 9, of Decreto-Lei n.º 101-D/2020:** we did not read it, nor the comfort it demands.
- **The summer zone of your municipality:** the Manual refers to an annex we did not consult.
- **The official 2026 mortality report:** we only read the news item.
- **Whether the council accepts a change of roof colour:** it varies. Aid for insulation: see the [aid guide](https://telhadoseguro.pt/en/guides/roof-insulation-aid).

## Sources (read on 4 November 2026)

- IPMA, Climate normals 1991-2020 (2024 version): [Lisbon](https://www.ipma.pt/bin/file.data/climate-normal/cn_91-20_LISBOA_GAGO_COUTINHO.pdf), [Évora](https://www.ipma.pt/bin/file.data/climate-normal/cn_91-20_EVORA.pdf), [Faro](https://www.ipma.pt/bin/file.data/climate-normal/cn_91-20_FARO.pdf), [Porto](https://www.ipma.pt/bin/file.data/climate-normal/cn_91-20_PORTO_PEDRAS_RUBRAS.pdf), [Bragança](https://www.ipma.pt/bin/file.data/climate-normal/cn_91-20_BRAGANCA.pdf).
- IPMA, [Heat waves](https://www.ipma.pt/pt/oclima/ondascalor/), [Heat waves in 2026](https://www.ipma.pt/pt/media/noticias/documentos/2026/Onda_calor_16out2026.pdf) (17 October 2026) and [Summer 2026 seasonal bulletin](https://www.ipma.pt/resources.www/docs/im.publicacoes/edicoes.online/20260930/zVaqHhFbvHTxnvueUuzO/cli_20260601_20260831_pcl_sz_co_pt.pdf).
- DGEG, [SCE Manual, with amendments up to 2024](https://www.sce.pt/wp-content/uploads/2024/02/Manual-SCE-05022023.pdf) (tables 8, 20 and 21).
- [Portaria n.º 138-I/2021, of 1 July](https://files.dre.pt/1s/2021/07/12602/0001200053.pdf), Diário da República (table 8).
- DGS, [Recommendations](https://www.dgs.pt/saude-ambiental-calor/recomendacoes.aspx) and [Questions and answers](https://www.dgs.pt/paginas-de-sistema/saude-de-a-a-z/calor/perguntas-e-respostas.aspx) on heat.
- ACT, [Exposure to heat](https://portal.act.gov.pt/Pages/exposicao-condicoes-climaticas-extremas-calor.aspx) and [worker leaflet](https://portal.act.gov.pt/AnexosPDF/Dossiers%20tem%C3%A1ticos/Calor/Flyer_Calor_Trabalhador.pdf).
- LBNL, [Cool Roofs](https://heatisland.lbl.gov/coolscience/cool-roofs), no date on the page.
- ORNL, [Radiant Barrier Attic Fact Sheet](https://web.archive.org/web/20231206115334/http://web.ornl.gov/sci/buildings/tools/radiant/rb2/), Internet Archive copy of 6 December 2023 (the original page no longer opens).
- APICER and CTCV, [Guide for the Design and Application of Ceramic Roof Tiles](https://www.apicer.pt/apicer/media/5c06bd0e8d436.pdf) (Portuguese), 2018, section 4.3.9.
- VELUX Portugal, [Proteção exterior VELUX 2021](https://afd-mv2.velux.com/-/media/brochures/pt/velux-produtos-de-proteccao-exterior.pdf) (manufacturer leaflet).
- Unofficial reference: RTP, [680 excess deaths in summer 2026](https://www.rtp.pt/noticias/pais/portugal-registou-680-obitos-em-excesso-em-tres-periodos-do-verao-de-2026_n1758105).

## FAQ

### Does painting the roof white lower the heat in the house?

The physics is known: the LBNL laboratory measured a black roof 30 °C hotter than a white one on a summer afternoon. But we found no independent measurement of the saving on Portuguese roofs. Before painting, ask the council and, if the roof is old fibre-cement, read the asbestos guide.

### Does a light tile make a difference in summer?

APICER says lighter tiles tend to reflect more and heat up less. The SCE Manual uses an absorptance of 0.4 for light colours and 0.8 for dark ones, but it is a calculation convention of the certificate. We have no saving measured in Portugal.

### Is reflective foil (a radiant barrier) in the attic worth it?

ORNL (USA) measured 16 to 42% less heat through the ceiling with R-19 insulation, but says the cooling bill falls 2 to 10%. It yields less with a lot of insulation, dust on the attic floor can remove half the effect in 1 to 10 years and in a test in Tulsa there were no measurable savings.

### What is a heat wave for IPMA?

At least 6 days in a row when the daily maximum temperature is 5 °C above the average of that month's maximums, with a 1991-2020 reference. It does not depend on reaching a fixed temperature. In 2026 there were 9 waves by mid-October, the longest lasting 13 days.

### Can I have roof work done during a heat wave?

The DGS says to avoid the sun between 11:00 and 17:00 and the ACT says heat increases fatigue and accident risk. On a roof there is also the risk of falling. As common sense, ask for work early in the morning, with water and breaks, and postpone what is not urgent.

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Source: https://telhadoseguro.pt/en/guides/summer-heat-attic-cool-roof. Independent information, not legal or tax advice.