Roof insulation glossary
20 terms from quotes and surveys, in plain English.
Free and no obligation. Your plan goes to one contractor, not a list.
- 20terms explained
- 0.16W/m²K pitched roof
- 0.20W/m²K flat roof
- 50 mmrafter air gap
Roof insulation terms, A to Z
BER
Building Energy Rating: the home's energy label. Each certificate records the roof's U-value.
Breathable membrane
A roof underlay that lets water vapour out but keeps rain in check. Older bitumen felt doesn't breathe.
Ceiling joist
The horizontal timbers the ceiling below is fixed to: the attic floor.
Cold roof
A pitched roof insulated at the ceiling, leaving a cold, ventilated attic above; or a flat roof insulated between the joists with a vented gap.
Collar ceiling
The small flat ceiling at the top of a room in the roof.
DEAP
SEAI's method for working out a BER.
Eaves
Where the roof meets the top of the wall. Air comes in here to ventilate a cold attic.
Heating degree days
A measure of how cold a place is over a year: the more degree days, the more heating a home needs.
Inverted roof
A flat roof with the insulation above the waterproof layer, held down by ballast or paving.
Knee wall
The short wall at the side of a room in the roof, with the eaves space behind it.
Mineral wool
Glass or stone wool insulation in rolls or slabs, laid on attic floors.
One Stop Shop
An SEAI-registered company that manages a whole-home upgrade and claims every grant together.
PIR board
Rigid polyisocyanurate insulation board: thin for its performance, used between and over rafters and on flat roof decks.
Rafter
The sloping timbers that carry the roof covering.
Spray foam
Polyurethane foam sprayed onto the underside of a roof. Open-cell foam is softer and lets vapour through; closed-cell is rigid.
Thermal bridge
A path where heat escapes faster, like a rafter or joist not covered by insulation.
U-value
How much heat passes through a roof, in W/m²K. Lower is better: 0.16 for pitched roofs, 0.20 for flat roofs in new work.
Vapour control layer
A sheet or taped layer on the warm side of insulation that stops moist air getting into the roof.
Vent card
A plastic tray fixed between rafters to hold a ventilation gap open under the felt.
Warm roof
A roof insulated at or above the rafters or deck, so the roof structure is inside the warm part of the house.
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.
Questions about the terms
Should I insulate the attic floor or the rafters?
Insulate where the warmth should stop. If the attic is only for storage, insulate the attic floor and keep the attic cold and ventilated. If people use the space under the roof, insulate at the rafters, all round the room. A flat roof is usually insulated on top of the deck as a warm roof, ideally when it is re-covered.
Is there a grant for roof insulation?
SEAI's individual grant for roof insulation is for the attic floor: €2,000 for a detached house, €1,500 for a semi-D or end of terrace, €1,400 for a mid-terrace and €1,100 for a top-floor apartment, with higher amounts for first-time buyers of an older home and some people on welfare payments. Rafter and room-in-roof insulation are grant-aided only as part of a One Stop Shop whole-home upgrade.
What U-value should a roof have?
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.
How much does it cost to insulate a flat roof?
€130 to €190 per m² as a warm roof with a new membrane.
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.