Roof insulation U-values and the regulations
For new work, the Building Regulations ask for 0.16 W/m²K in a pitched roof, whether it is insulated at the ceiling or along the slope, and 0.20 W/m²K in a flat roof. About 300 mm of mineral wool on an attic floor gets there.
The numbers to ask for on a quote, and what they mean in mm of insulation.
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- 0.16W/m²K, pitched roof
- 0.20W/m²K, flat roof
- 0.27middle roof U-value on Irish BERs
- 2.3W/m²K, a roof with none
The levels for new work
| Roof | U-value, W/m²K | Usually means |
|---|---|---|
| Pitched roof, insulated at the ceiling | 0.16 | About 300 mm of mineral wool |
| Pitched roof, insulated on the slope | 0.16 | Boards between and under the rafters |
| Flat roof | 0.20 | Boards on the deck in a warm roof |
The lower the U-value, the less heat escapes. These are the levels Technical Guidance Document L sets for new roofs and new work on existing ones, and the level SEAI's standard asks for in grant work at the attic floor.
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.
Attic floor insulation, semi-d or end of terrace: €1,800 to €2,101 before the €1,500 SEAI grant, so about €300 to €601 to pay.
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 Irish roofs are now
On BER certificates for houses built before 2011, the middle roof U-value is 0.27 W/m²K. 24% reach 0.16 or better, 29% are between 0.17 and 0.30, 40% are worse than that, and 7% are recorded with no insulation. A roof with none loses about fourteen times the heat of one at 0.16.
Questions before you insulate
What U-value should a roof have in Ireland?
For new work, the Building Regulations ask for 0.16 W/m²K in a pitched roof, whether it is insulated at the ceiling or along the slope, and 0.20 W/m²K in a flat roof. About 300 mm of mineral wool on an attic floor gets there.
What is a U-value?
The rate heat passes through a roof, wall or window, in watts per m² for each degree of difference. Lower is better.
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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.