Flat roof insulation
Most flat roofs are insulated as a warm roof: insulation boards on the deck over a vapour control layer, with the new membrane on top. It costs €130 to €190 per m² with the new covering, about €3,300 to €4,800 for a 25 m² extension. A cold roof, insulated between the joists, needs a ventilated gap above the insulation.
Warm, cold and inverted flat roofs: which one an extension, dormer or garage needs, and what it costs.
Free and no obligation. Your plan goes to one contractor, not a list.
- €130–€190per m², warm roof and membrane
- 0.20W/m²K to aim for
- 2%of BER roofs are flat
- 50 mmvent gap in a cold roof
Warm roof: insulation on the deck
The boards sit above the timber, so the deck and joists stay warm and dry and there is no void to ventilate. It is the usual choice when a flat roof is re-covered, and the extra thickness is taken up by deeper trims and a raised upstand at the wall.
Cold roof: insulation between the joists
Insulation goes between the joists from inside, with a 50 mm ventilated gap above it and vents at both ends, and a vapour control layer on the warm side. It suits a roof whose covering is still good, but cross-ventilation is hard on a roof that meets a wall, and condensation is the usual failure.
An inverted roof puts the insulation above the waterproofing, held down with ballast or paving: common on roof terraces. Replacing the covering itself: flatroofreplacement.ie.
Plan your roof insulation
A few quick taps, and your plan fills in as you answer: the right method for your roof, what it costs, the grant and the standard to ask for. Contact details come last, and only if you want a contractor to price it.
A warm flat roof, 25 m²: €3,300 to €4,800.
How the estimate is worked out
Attic floor insulation uses what SEAI-registered contractors were paid for each kind of home in 2026 and what an Irish installer charges, with the SEAI grant taken off. A room in the roof and a flat roof are priced per m² of the area you enter: rafter boards with a plasterboard finish, or a warm roof with a new membrane. VAT is included. Use it to plan with. It isn't a quote: a contractor prices your roof after a survey.
Where the insulation goes
Each roof in our drawings, with its own page: the method, the cost, the grant and what to watch for.
Attic insulationThe usual job: wool over the joists, a cold attic above, an SEAI grant towards it.
Rafter insulationBoards between and under the rafters, so the whole attic stays warm.
Room in the roofSloping ceilings, knee walls and a small top ceiling, wrapped all round.
Insulating from aboveBoards over the rafters when the slates come off anyway.Garage and extension roofsFlat or pitched, heated or not: what each needs.Questions before you insulate
How should a flat roof be insulated?
As a warm roof, with boards on the deck under the new membrane, usually done when the roof is re-covered.
How much does flat roof insulation cost?
€130 to €190 per m² as a warm roof with a new membrane.
Can a flat roof be insulated from inside?
Yes, as a cold roof between the joists, but it needs a ventilated gap and a vapour control layer, and condensation is the usual failure.
Is there a grant for flat roof insulation?
Not an individual one. It can be part of a One Stop Shop whole-home upgrade.
Every guide and county page
Methods
Costs, grants and rules
Keep the heat under your roof
Pick the roof, what's there now and the home to see the method and the cost. When you want a firm price, your plan goes to a single vetted insulation contractor for your area.