Insulating a roof means adding a layer that slows the passage of heat. It keeps warmth in the house in winter and lets less heat in during summer. There are three places the insulation can go, and the choice depends on what sits under the tiles. This guide explains each one, the care needed over damp and ventilation, the prices we read and the official rule. For state aid, see our guide to aid for insulating the roof.
Updated on 3 November 2026. General information. Every roof is different: only someone who sees it can say what suits it.
The essentials
- There are three places to insulate: on the floor of an unused loft, under the tiles (in the plane of the roof), or inside the roof structure when the loft is lived in.
- The APICER and CTCV guide prefers insulation on the loft floor when the space is not habitable and well ventilated, and on the slope when it is habitable.
- Do not lay the tile straight on the insulation. The same guide says the tile needs ventilation underneath and, without it, suffers thermal shock.
- Fix leaks first. Insulating over a water entry traps the damp inside. See our guide to leak causes.
- Prices: we only found ranges from private portals, with no VAT stated. They are an order of magnitude, not a quote.
- Old asbestos-cement roof: do not drill or touch it before reading the asbestos guide.
The three places to insulate
| Where | When it makes sense | What the APICER and CTCV guide says |
|---|---|---|
| On the loft floor (the ceiling slab) | Loft that is not used and is ventilated | Preferred in traditional roofs with a non-habitable, freely ventilated loft |
| In the plane of the roof, under the tile | Loft you want to use, or a roof with no loft | Can go between the rafters or continuous over them, always with the tile ventilated underneath |
| Inside the structure, from the inside | Lived-in loft, without lifting the tile | The guide asks for insulation on the slope, keeping the loft and the underside of the tile ventilated |
On the loft floor leaves the tile alone and means working on a flat surface. We have no source comparing costs, so we do not say it is the cheapest. Under the tile means lifting the tiles, and an air gap stays between tile and insulation. From the inside avoids touching the tiles, but air must still move beneath them.
If you are replacing the roof, this is the moment
When the roof is rebuilt, the tiles are already up. Insulating then avoids paying twice for taking them off and putting them back. This is our own conclusion, with no source giving figures. Ask for the quote to show the insulation on a separate line, with material, thickness and area. Our quote checker helps spot what is missing, and the area calculator gives the real roof area.
Damp, condensation and ventilation
The APICER and CTCV guide (2018) explains that tiled roofs can suffer condensation, with damp stains or even dripping water. It depends on how the roof is built and on conditions inside and outside. Ventilation under the tile reduces the problem, and ventilated ridges and eaves help.
For anyone insulating, four cautions follow:
- Do not block the ventilation. Insulation stuffed into the eaves or against the ridge cuts off the air that dries the tile. This is our conclusion from the guide’s rule.
- A vapour barrier only if needed, and on the warm side. The guide says to use it with care and always on the warm side of the insulation, meaning the house side.
- Membranes under the tile must let vapour through. If they do not, they create a strong risk of condensation.
- On a sloping concrete slab roof, the guide prefers insulation on the outside of the slab.
Which material
Price portals list rock wool, mineral wool, EPS, XPS, PIR, cork and sandwich panels. There is no single best choice. The official rule calls a material “thermal insulation” when its conductivity is below 0.065 W/(m.°C) and its thermal resistance above 0.30 (m².°C)/W. Ask the contractor for the data sheet with these values, the CE marking and the thickness proposed.
What it costs: what the portals say
We found no official price. There are two private portals, both built on quote requests, and neither says whether VAT is included.
- Zaask, page read on 3 November 2026, no update date: “15€ - 100€ /m2” as the national average. By insulation type, in one table: EPS 15 to 36, rock wool 15 to 32, cork 20 to 80, sandwich panels 30 to 100 euros per m². The same page has a second table with lower values (EPS at 2 to 13 euros per m², for example) and does not explain the gap. The opening text gives 12 euros as the minimum and the heading 15.
- Habitissimo, page updated on 10 June 2026: an overall range of 350 to 35,000 euros, plus sample quotes. Six centimetres of XPS in a loft, 1,400 euros (area not given); an 80 m² roof with 8 cm of XPS, 6,000 euros; 50 m² from the inside, 3,500 euros. Divided out, that is about 75 and 70 euros per m² (our sum). The portal itself calls these estimates.
Always ask for a written quote. On VAT, see our guide to VAT on roof works.
What the official rule requires
Portaria 138-I/2021 of 1 July sets maximum heat-transfer values (U) for the building envelope in new or renovated homes. For mainland Portugal, horizontal elements that are exterior, or interior with a coefficient btu above 0.7 (the rule uses it for unheated spaces), have a maximum of 0.40, 0.35 and 0.30 W/(m².°C) in climate zones I1, I2 and I3. With technical or functional constraints on record, its Table 5 applies (1.25, 1.00 and 0.90). Conservation works are excluded.
This is a rule for design and certification. We did not find an official source saying when a stand-alone insulation job on an older home must meet these values.
Permit and paperwork
See the roof works permit guide. For insulation alone we found no clear rule: ask the town hall. Before hiring, use the contractor check.
What we could not confirm
- An official price per m² for roof insulation in Portugal. There are only private portals, with no VAT stated.
- The ADENE roof insulation guide. A 2016 government news item says it exists, but we could not open it.
- Whether a pitched roof counts as a “horizontal” element in the Portaria’s table, and which climate zone your municipality falls in.
- Whether a stand-alone insulation job needs a permit or notice.
- Recommended thicknesses in centimetres: none of the sources we read gives them.
Sources (read on 3 November 2026)
- APICER and CTCV, Guia para Projeto e Aplicação de Telhas Cerâmicas, 2nd edition, 2018 (chapter 4.2; section 5.2.5, pages 62 and 63; section 5.3, guidelines 5, 6, 11 and 12).
- Portaria n.º 138-I/2021, Diário da República n.º 126, 1 July 2021, annex I, point 1.2, tables 1 and 5.
- Zaask, Quanto custa isolamento térmico no telhado, no update date on the page.
- Habitissimo, Isolamento térmico de sótão, updated 10 June 2026.
- Portuguese Government, news item of 6 May 2016 on the ADENE guides.
- Our guide to aid for insulating the roof.