How do you insulate an old roof?

Find out which of three roofs you have, because each one puts the insulation in a different place, and only the loft floor is a straightforward job for your own hands. An old roof with an accessible loft is insulated on the loft floor, with the eaves left open to ventilate. A sloping ceiling with no loft is lined under the rafters from inside, or insulated over the structure when the tiles come off anyway. A flat roof or terrace is insulated together with its waterproofing.

Which of the three roofs is yours?

An older Portuguese house may have a roof of canal tiles (telha de canudo), laid on timber battens and rafters or on a concrete slab. Under a pitched roof there may be a loft, the desvão, above a flat ceiling; or the ceiling may follow the slope of the roof with no space above it at all. An Algarve house may also have a flat roof or a roof terrace over part of it.

Your roof	Where the insulation goes	Who does it
Pitched roof with an accessible loft (desvão)	On the loft floor, over the ceiling of the rooms below	A careful DIY job from inside, with the precautions below
Sloping ceiling, no loft	Under or between the rafters from inside, or over the structure when the tiles come off	The lining from inside: skilled DIY or a professional. Over the structure: a professional roof job
Flat roof or roof terrace	Above the slab, as part of the waterproofing	A professional, in the same job as the waterproofing

Before you go into the roof space

The roof itself is not a DIY place. Anything done from the outside, on the tiles or with them off, is work at height on a fragile surface with an open edge, and it needs scaffold or edge protection and people who work on roofs. What follows is about the inside.

• The ladder to the hatch. Stand it on firm, level ground at the angle its maker gives, long enough to reach above the edge of the hatch, with someone at the foot holding it who knows you are up there. Climb with both hands free and have tools and rolls passed up to you rather than carried.
• Loft access. Kneel and move on boards laid across the joists, never between them. Between the joists there is only the ceiling of the room below, which may be plaster on lath or a thin board, and a foot through it is a fall and a ruined ceiling. A structural concrete slab is built to carry people; a ceiling hung between timber joists is not, and if you cannot tell which you have, treat it as the second.
• Light and heat. Work with a lamp that leaves both hands free, not a torch in your hand, and in the cool of the morning: an Algarve roof space in summer gets hot enough to cause heat exhaustion, so take water, keep the hatch open for air and come down at the first sign of dizziness. Mineral wool calls for a dust mask, gloves, long sleeves and eye protection.
• Recessed downlights and old cables. A downlight set into the ceiling gives off heat into the roof space. Buried under insulation, a fitting that is not rated for cover cannot shed that heat and overheats. A cable buried in insulation runs hotter too, so it safely carries less than it did, and old cables with brittle sheathing are a risk of their own. Before the insulation goes down, an electrician should say which cables can stay where they are and what may go over each light.
• Fibre-cement sheets. Corrugated fibre-cement (fibrocimento) sheets in an old roof or an outbuilding may contain asbestos. They are not cut, drilled or broken, because damaging them can release fibres, and they are never walked on, because a fragile sheet gives way under a person’s weight. Nothing is fixed to them, and removing them is specialist work under its own rules.
• Damp. A roof space with stained timber, a musty smell or drip marks under the tiles is repaired and ventilated first. Insulation laid over wet timber holds the moisture against it.

How do you insulate a loft floor?

1. Clear the floor and look at it: joists, ceiling, cables, any fittings coming up through it. Mark the downlights and have the electrical side settled first.
2. Lay boards across the joists to kneel and move on while you work, and move them along as you go. A permanent walkway, to the hatch or to anything you will need to reach later, such as a fan or a junction box, sits on supports raised above the full depth of the insulation, so it does not squash it.
3. Lay the first layer between the joists, cut to fit snugly without being squashed. If the roll has a paper or foil facing, that face goes downwards, towards the rooms, and the second layer goes in unfaced. Insulation that is compressed loses part of its performance.
4. Lay a second layer across the joists, at right angles to the first, so the timbers are covered too.
5. Stop short of the eaves. The air that ventilates the loft comes in along the eaves and leaves near the ridge, and insulation pushed into that path closes it.
6. Insulate and draught-strip the hatch itself, and seal the gaps where cables and pipes pass through the ceiling, so the warm, moist air of the house does not rise straight into a loft that is now colder.

On a concrete ceiling slab the sequence is the same. Where you need to walk or store things, the boards sit on supports raised above the insulation, or on a rigid insulation board made to be walked on, so the insulation is not squashed.

Can you insulate a pitched roof between the rafters?

Yes, from inside, where the ceiling follows the slope. The insulation goes between the rafters and, because their depth limits how much fits, a second layer may go across their undersides before the lining. Two details decide whether it works. A ventilated gap, open to outside air low and high, stays between the insulation and the tiles, or a membrane under them that does not breathe; a breathable membrane made to be in contact with insulation follows its maker’s instructions. And a vapour control layer goes on the room side, sealed at its joints, before the ceiling goes back. Light points and cables in that ceiling are moved or refitted by an electrician, because the new lining and the insulation change where they sit and how they shed heat.

In an old canal-tile roof laid straight on the battens, with no membrane, you can see the underside of the tiles from inside, and sometimes daylight at their joints. The insulation must never press against them.

The other route is over the structure: when the tiles have to come off anyway, rigid boards go over the rafters or the slab, then a membrane, new battens and the tiles back on. It leaves the ceiling untouched and the structure on the warm side, and it is a professional roof job from start to finish.

Do you need an air gap between roof and insulation?

The gap is there because of condensation. Warm indoor air carries water vapour, and some of it gets past any ceiling. When it meets a surface colder than its dew point, it turns back into water: on a winter night, that surface is the cold side of the insulation, the timbers and the underside of the tiles. A vapour control layer on the room side limits how much gets in, and a ventilated gap carries away what does, before it wets the wood.

With insulation on a loft floor, the whole loft is the ventilated space, which is why the eaves stay open. With insulation under the tiles, the gap is built in, unless the membrane under the tiles is a breathable one made to touch the insulation. That membrane, a roof designed as a system over the structure and a flat roof built around its waterproofing handle moisture differently, and there the maker’s detail is followed exactly.

What about a flat roof or terrace?

Here the insulation and the waterproofing are one job. On a flat roof the insulation goes above the slab, either under the waterproof membrane or, in an inverted roof, as boards laid over the membrane, which then have to be a type that tolerates water, held down and protected by gravel or paving, whose weight the slab has to be able to carry. Insulation itself is not waterproofing: the membrane keeps the water out, and the insulation only does its job if it stays dry or is a type made to tolerate water.

That makes a flat roof a job for whoever does the waterproofing, done at the moment the old surface comes off. Adding insulation to a terrace that already leaks means doing the waterproofing again anyway, so the two are quoted together as one job.

What type of insulation is best for a roof?

The one that suits its position. Each of these can do the job when it is fitted in the right place.

Type	Where it suits an old roof	What to watch
Mineral wool, in rolls or batts	Loft floors, between joists and rafters	Loses performance when squashed or wet; wear protection when handling it
Rigid foam boards, such as extruded polystyrene	Sloping ceilings where depth is short, over the structure, flat roofs	Joints sealed; inside the house always covered by the ceiling lining; extruded polystyrene is the type used over a membrane
Expanded cork board	Over the structure, ceiling linings	A natural Portuguese material; compare its conductivity with the depth available

Whatever the type, the figures on the product sheet do the choosing: the conductivity of the material and the thermal resistance of each thickness.

How thick should roof insulation be?

There is no single right figure. For roofs that are new or being renovated, Portugal’s building energy rules (Portaria n.º 138-I/2021) set a maximum thermal transmittance of 0.40, 0.35 or 0.30 W/(m².°C) in winter climate zones I1, I2 and I3. They are national values that bind a roof being built or renovated, not one left as it is, and components that only undergo conservation works are excluded. A building’s zone follows its region and its own altitude, and the technician who signs the project or the energy certificate states it.

The thickness is whatever the chosen material needs to reach that value, and because mineral wool, foam board and cork conduct heat differently, the same target means a different depth in each. Where the job has a project, the thickness is set there.

Why does the summer sun matter for an Algarve roof?

Over the cooling season, June to September, a horizontal surface in the Algarve receives 865 kWh/m² of solar energy, against 405 kWh/m² on a south-facing wall and 225 kWh/m² on a north-facing one. Source: Manual SCE, Despacho n.º 6476-H/2021, Tabela 10, reference values for the Algarve region, in force as of 2026. Read against the same manual’s summer-zone criteria (Tabela 8), those reference values put every building in the Algarve in summer climate zone V3, at every altitude.

A flat roof is that horizontal surface, and a pitched roof faces the sky far more than any wall does. Insulation on an Algarve roof is doing a summer job as much as a winter one: it slows the heat soaking down through the ceiling into the rooms below, and the ventilated loft or gap above it carries some of that heat away.

Where the DIY stops

A loft floor you can reach through a hatch, with the electrical side checked first, is a job you can do yourself. A sloping ceiling lined from inside is skilled work. Insulation over the structure, anything on a flat roof, a roof with fibre-cement sheets, or a roof space that is already damp belongs to people with the equipment and the roofing work around it.

If that is the job in front of you, roof insulation in the Algarve sets out what the work includes and how it is quoted free, in English or Portuguese, from photos of the roof space through the hatch, the eaves, the ceiling below and the roof from the ground. As at September 2026 the Fundo Ambiental has no open grant for insulation in a private home; what exists is a loan instrument created by Portaria n.º 442-A/2025/1, with no rates or window published yet. If the walls are also on your mind, whether external wall insulation is a good idea is a separate decision with its own answer.