Attic & roof insulation glossary

A to Z

Every term explained

0.16 W/m²K Ceiling Joist U-Value

SEAI's mandatory thermal transmittance target for horizontal loft insulation laid at ceiling level; achieved by cross-laying mineral wool, glass wool or sheep's wool quilt to a total depth of 300 mm.

0.20 W/m²K Pitched Rafter U-Value

SEAI's thermal target where insulation is fitted along the sloping roof rafters (for example in a dormer bungalow or converted roof space) rather than flat across the horizontal ceiling joists.

300 mm Cross-Laid Quilt Build-Up

Standard Irish grant-compliant loft specification: a first layer of 100 mm mineral or glass wool rolled tightly between the ceiling joists, topped at 90 degrees by a 200 mm layer across the joists to eliminate timber thermal bridging.

50 mm Eaves Ventilation Tray (Rafter Tray)

Corrugated plastic air baffle stapled between every pair of rafters at the roof eaves before rolling out the quilt, holding open a continuous 50 mm ventilation airway from the soffit vents into the cold roof space.

Cold Water Storage Tank & Pipe Lagging

Mandatory SEAI grant requirement: because insulating the ceiling floor makes the attic space above much colder in winter, all cold water tanks, feed/expansion tanks and attic pipework must be fully insulated with certified lagging jackets and foam sleeves, with no insulation placed directly underneath the tank base.

Dormer Cheek & Dwarf Stud Wall Insulation

In T-shaped and dormer houses, the vertical timber stud walls ('dwarf walls') and sloping dormer cheeks separating heated bedrooms from cold roof voids must be insulated to 0.20–0.27 W/m²K with rigid board or backed quilt.

Draught-Proofed & Insulated Attic Hatch

SEAI specification requiring the loft trapdoor lid to be insulated to match the surrounding ceiling, fitted with a continuous perimeter draught-strip seal, and held tight against the seal by mechanical catch latches.

Higher Attic Grant Rate (€1,000 to €2,000)

Increased SEAI Better Energy Homes grant tier from 6 October 2026 for homeowners on qualifying welfare payments and landlords with HAP or RAS tenants: €2,000 detached, €1,750 semi-D or end of terrace, €1,500 mid-terrace and €1,000 apartment.

Insulated Loft Storage Walkway / Raised Deck

Raised timber or high-density PIR/OSD walkway platform built on stilts above the 300 mm quilt between the attic hatch and the cold water tank so homeowners and plumbers can inspect the tank without squashing the mineral wool.

One Stop Shop Rafter Grant (€1,500 to €3,000)

Separate higher SEAI grant available only inside a full One Stop Shop B2 home upgrade when pitched roof rafters are insulated with rigid PIR board and insulated plasterboard: €3,000 detached or semi-D, €2,000 mid-terrace and €1,500 apartment.

Part F Roof Space Ventilation

Irish Building Regulations requirement that cold pitched roofs maintain cross-ventilation through soffit eaves vents and ridge/tile vents so moisture vapour escaping from the house below cannot condense into timber rot on the rafters.

Permanent Safety & Depth Notice

Mandatory laminated specification card fixed permanently beside the attic hatch or cold water tank by the SEAI contractor, recording the insulation product, installed thickness in mm, achieved U-value and date of completion.

Pre-2011 Dwelling Eligibility Rule

SEAI Better Energy Homes rule stating that the house or top-floor apartment must have been built and occupied before 1 January 2011 (31 December 2010 or earlier) and must not have received an SEAI attic insulation grant on that MPRN before.

Recessed Downlighter Fire Hood / Clearance

Electrical fire-safety requirement under SEAI standards: 300 mm loft insulation must never be laid straight over unsealed bathroom or bedroom ceiling spotlights; each fitting must either sit inside a certified fire-rated loft cap or have a fixed clearance guard.

Standard Attic Grant (€800 to €1,500)

Fixed SEAI Better Energy Homes grant for horizontal ceiling-level attic insulation: €1,500 for a detached house, €1,300 for a semi-D or end of terrace, €1,200 for a mid-terrace and €800 for a top-floor apartment.