What new windows and doors change, and what they don't
New windows stop the draught and the cold pane at the sill. They are also the upgrade people want first and the one SEAI fund last.
What it is
The old windows come out and new ones go in, one opening at a time, usually a room a day. Doors are the same job on a smaller scale. The measuring happens weeks before anything is fitted, because the units are made to the hole: SEAI put the wait between order and installation at six to ten weeks for windows and four to six for doors.
What you are buying is a number. Every window is rated for how fast heat gets through the whole unit — glass, frame and all — and SEAI won't grant one unless that number is 1.4 or better. Frames are a fifth to a third of the area of a window, so the frame matters nearly as much as the glazing.
The rest is detail that changes the number: two panes or three, how wide the gap between them is, a low-emissivity coating on the glass, argon in the gap instead of air.
Compared with what's in the openings now
| Old single glazing | Old double glazing | New, to the grant standard | |
|---|---|---|---|
| Panes | one | two, often a narrow gap | two or three, wide gaps, coated glass |
| The pane in winter | cold to stand beside, runs with water | cool | close to room temperature |
| Draughts | round the frame and the sash | seals hardening with age | new seals, and trickle vents if specified |
| Noise | little help | some | noticeably better |
| Grant | €3,000 for a semi, €800 a door | same | same |
SEAI's own share of the loss puts windows, doors and uninsulated floors together at up to 10% of a home's heat, against up to 30% for a poorly insulated roof and walls. That is the whole argument for the order of work, and it's why the windows grant has a condition the others don't: the attic and the walls have to be good already, or the house has to come in under the heat-loss limit after the job.
Grant €1,500 to €4,000 by house type for a whole-window upgrade, €3,000 semi-detached, plus €800 per external door to a maximum of two. Whole-window U ≤ 1.4 W/m²K required. Eligibility: HLI ≤ 2.3 W/(K·m²) after works, or attic and walls rated Good or Very Good in the BER advisory report. Restrictors required where the drop below an opening exceeds 1.4 m. Amounts verified 2026-09-07.
Is it right for a house like yours?
Your cert may say B2 — that's a B on the new scale.
A heat pump runs on electricity. Pick "Heat pump" if you have one.
A 1970s–80s detached house on oil, rated DEnergy rating D
- Grants you’d get
- €19,000
- It costs about
- €36,400
- You pay
- €9,000 – €16,300
- Saves you each year
- about €1,790
- Pays for itself in
- about 9 years
The commonest home on file — 23,707 like it. Change an answer above for yours.
Modelled, not observed payments. Middle case €16,329; a quarter of these homes are over €23,200. Repaid in 9.1–13.6 years. Confidence 0.9. One job in this list has no SEAI median price, so its cost is an estimate.
Show what’s included
This is the job list that suits most homes like yours (2 in 10). The money above is the middle of every route we priced, so it will not add up to this list exactly.
Attic insulation — a thicker blanket in the roof space. Half a day, nobody moves out, you lose some storage height.
Roof U-value → 0.16 W/m²K (≈300 mm). Cost €2,000 · grant €1,500 · you pay €500.
Costs about €2,500 · grant €2,000 · you pay €520
Wall insulation — the walls get a warm coat: pumped into the cavity (a morning), fitted inside (rooms shrink about 10 cm, redecorate, one to two weeks) or wrapped outside (scaffolding, new render, two to four weeks). We pick what your walls allow.
Cavity → 0.55, €1,550 · grant €1,300 · pay €250. Internal → 0.27, €10,420 · €3,500 · €6,920. External → 0.21, €23,247 · €6,000 · €17,247. Pre-1970 solid walls can't take cavity fill.
Costs about €9,000 · grant €4,500 · you pay €4,500
New windows and doors — double or triple glazed, fitted a room at a time over two to three days. You need not do every window, but the grant is paid once.
Whole-window U → 1.4 W/m²K. Estimate: €490/m² of window (no SEAI median), about €9,085 on a semi-detached house. Grant €3,000 + €800/door (max 2).
Estimated at €13,700 · grant €4,000 · you pay €9,700
Is the house warm enough to hold the heat? — SEAI's test before they'll fund a heat pump. The insulation steps are usually what gets you through.
Heat Loss Indicator ≤ 2.3 W/K·m² after fabric works. 1% of homes like yours already pass. Assessment about €276, grant €350 through a One-Stop-Shop.
Costs about €280 · grant €350 · the grant covers it
Heat pump — replaces the boiler, runs on electricity, pulls heat from the outside air. An outdoor unit the size of a large suitcase, bigger radiators in some rooms, three to five days. Runs cooler for longer — the house feels even, not blasted.
Air-to-water, SPF 3.5 heating / 2.5 hot water. Cost €16,832 · grant €6,500 + €4,000 (replacing oil/gas/coal) + €2,000 pipework + €700 controls = up to €13,200 · you pay from €3,632. 9.8c per kWh of heat vs gas 14.4c / oil 18.0c.
Costs about €16,500 · grant €6,500 · you pay €10,000
The paperwork — assessment first, grant approval, then the work, then a new energy cert, then the grant is paid. A One-Stop-Shop does all of it and takes the grant off the bill.
Order is mandatory — work started before approval is not eligible. Energy cert €276, assessment grant €350 through a One-Stop-Shop. Typically 3–6 months end to end (estimate).
Costs about €280 · grant €350 · the grant covers it
Where the saving comes from
Two different things are going on, and only one of them is glazing.
The first is conduction: heat crossing the glass and the frame. A modern unit slows that down a lot, and the number quoted on the quote is exactly that. The second is air — the old sash that never closed right, the gap under the door on a windy night. Air leakage is a real cost and a bigger part of what people feel than the glazing figure suggests, which is why a new window can transform a room that the numbers say should only have improved a little.
What new windows can't do is make a cold house warm. There is far more wall than there is window, and if the wall is losing heat the windows will not save you.
Modelled whole-window heat loss for that group of homes: U 2.80 → 2.01 W/m²K, an average over all the glazing in the cell rather than a single new unit. Cost €9,085 for a semi is an estimate at €490/m² — SEAI publish no median cost for windows. detached|1970-1989|D|oil: cost p50 €36,393, grant €19,000, net p25–p75 €9,019–€23,200, saving €1,791/yr, payback 5.8–13.6 yrs, n=23,707, confidence 0.9
Common questions
- Do I have to replace every window to get the grant?
- No. SEAI say you do not have to do them all, but the grant is paid once per property, so a job in two halves gets paid once. The house also has to come in under the heat-loss limit after the work, which is easier when the whole lot is done together.
- Double or triple glazing?
- Either, once it comes in at 1.4 or better — that is the number SEAI put in the grant conditions, and it is a whole-window figure covering the frame as well as the glass. Triple glazing performs better and costs more. It also lets in a little less sun, which is worth a thought on a north-facing room.
- Will new windows make the house stuffy?
- They can. Old windows leak air constantly, and when you replace them that leak stops. SEAI require a room with an open-flued appliance to keep a permanent vent, and they point to trickle vents in the frames to keep the air moving. Ask what ventilation is in the quote before you sign it.
Where to next
Where this comes from
Last checked 2026-09-08.