Insulating cold dormer cheeks and window reveals
If your upstairs dormer bedrooms are freezing in winter even with the heating on, the culprit is almost always uninsulated timber dormer cheeks (the triangular side walls of the dormer box) combined with an older double-glazed window. Insulating and re-cladding the dormer cheeks while the window frame is out costs €650 to €1,450 per dormer.
Why replacing the glass alone only solves half the heat loss on a 1980s–2005 Irish dormer bungalow, and how to insulate the dormer box properly.
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Calculate your dormer window & insulation cost
Select whether you are replacing existing dormer windows, upgrading a whole dormer bungalow, insulating cold dormer cheeks, or building a new attic dormer.
Estimated supply & fit guide
€3,900 to €10,800
Windows: €1,950–€5,000 · Cheek & roof insulation: €1,950–€5,800
Replacing 3 to 4 dormers in colour-foiled triple-glazed uPVC or flush sash while simultaneously stripping the side cheeks and window reveals, fitting rigid PIR insulation, breathable membrane, new maintenance-free uPVC/composite cheek cladding and checking the lead flashing (€650 to €1,450 per dormer).
Ask a local specialist for a quoteWhy dormer bedrooms lose heat four times faster
On hundreds of thousands of Irish dormer bungalows built during the 1980s, 1990s and early 2000s, the dormer projection was framed in 100mm × 50mm softwood timber studs clad externally in timber shiplap, slate hanging or thin fibre-cement board. When opened up during a window change, fitters routinely find:
- Empty or slumped fibreglass in the cheeks: Old 100mm glass-wool roll sagged down the triangular cheek cavity years ago, leaving the top half of the dormer side wall with only plasterboard and outer cladding between your bedroom and a gale.
- Unsealed 20mm gap around the window frame: Cold air from the ventilated roof eaves blows straight into the hollow cavity behind the dormer plasterboard and leaks out around the window architrave.
- Cold thermal bridge at the dormer flat roof or pitched soffit: Only 50mm to 100mm of mineral wool sits above the dormer ceiling head.
How dormer cheeks are insulated while the window is out
| Upgrade layer | Specification | What it does |
|---|---|---|
| 1. Strip outer cheek cladding or inner reveal | Done while the old dormer window frame is removed | Exposes the full triangular timber stud cheek and head cavity |
| 2. Rigid PIR insulation between & over studs | 75mm–100mm foil-faced PIR board cut tight & foil-taped | Brings cheek U-value from ~1.8 W/m²K down to 0.18–0.22 W/m²K |
| 3. Airtight foam & breather membrane | Low-expansion airtight acoustic foam + breathable roof membrane | Stops eaves wind blowing around the new triple-glazed window reveal |
| 4. Maintenance-free exterior cheek cladding | Cellular uPVC shiplap, slate-hung tiles, fibre-cement or zinc | Eliminates painting rotten timber dormer sides up on a ladder |
Typical cost when done alongside your dormer window replacement: €650 to €1,450 per dormer depending on cheek size and whether you choose uPVC shiplap, slate hanging or composite cladding.
What dormer windows cost in Ireland
Our supply-and-fit price guide for single dormer windows, full dormer bungalows, cheek insulation and new attic dormers.
| Job | Typical cost | What moves the price |
|---|---|---|
| Replace single dormer window (existing dormer, white uPVC) | €450 to €950 per window | Includes roof access, lead apron check & Part B egress stays |
| Replace dormer window in coloured triple uPVC or flush sash | €650 to €1,250 per window | 0.8 W/m²K triple glazing · stops upstairs cold draughts |
| Replace dormer window in aluminium or alu-clad | €850 to €1,750 per window | Ideal for high-wind coastal and upland dormer roofs |
| Full dormer bungalow replacement (8–12 windows), uPVC | €5,200 to €10,500 | After €4,000 SEAI detached grant: €1,200 to €6,500 |
| Full dormer bungalow replacement (8–12 windows), aluminium / alu-clad | €9,500 to €17,500 | After €4,000 SEAI detached grant: €5,500 to €13,500 |
| Insulate & re-clad dormer cheeks & reveals (while window is out) | €650 to €1,450 per dormer | PIR insulation board, breathable membrane, uPVC/zinc cladding & lead check |
| Build new structural attic dormer (carpentry, roof, lead & window) | €3,500 to €7,500 per dormer | Structural rafters, Code 4/5 lead flashing, slate or GRP roof & window |
Real examples and installation detail
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Questions dormer homeowners ask
Should I fit triple-glazed windows when insulating dormer cheeks?
Yes. Because dormer windows sit high on the roof slope exposed to wind chill, pairing 0.8 W/m²K triple-glazed dormer frames (€650 to €1,250) with insulated cheeks eliminates both cold wall draughts and cold glass downdraughts.
Can dormer cheeks be insulated from the inside or the outside?
Both. If your exterior slate hanging or uPVC cheek cladding needs replacing, stripping from the outside while the window is out avoids disturbing bedroom wallpaper and skimming. If the exterior slates are perfect, insulated PIR plasterboard (37.5mm to 62.5mm) can be fitted to the internal cheeks and reveals.
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Replacing cold, draughty or rotten dormer windows?
One vetted window company covering your county inspects the dormer frames, lead apron and side cheeks, and gives you a written price.