Every winter the same advice circulates: turn the thermostat down a degree, bleed the radiators, close the curtains at dusk. All sensible, and all minor. The reason most homes are expensive to heat is simpler and less convenient. The heat you pay for is leaving through the fabric of the building, and until that is fixed, everything else is tinkering.
The useful question is where it leaves, because that tells you what to insulate first. Spend the money in the wrong order and you can halve the return on it.
In short: attic first, then walls, then draughts and floors, with ventilation planned alongside. Windows usually come later than people expect.
Where an uninsulated house loses heat
The exact split depends on the house, but for a typical older semi-detached or detached home with little insulation the pattern is remarkably consistent.
| Part of the house | Typical share of heat loss | How hard it is to fix |
| Walls | 30 to 35% | Easy if cavity, a bigger job if solid |
| Roof and attic | 25 to 30% | Easiest and cheapest of all |
| Draughts and ventilation losses | 10 to 15% | Cheap, fiddly, often DIY |
| Windows and doors | 10 to 20% | Expensive per unit of heat saved |
| Floors | 10 to 15% | Easy for suspended timber, disruptive for solid |
Two things stand out. Walls and roof together account for well over half, and windows, which get most of the attention, are rarely the main culprit.
The order that gives the best return
- Attic insulation. Cheapest measure, fastest payback, a day’s work. Current guidance is around 300 mm of mineral wool or equivalent. If yours has 100 mm from the 1990s, topping it up is the best money you will spend on the house.
- Wall insulation. The biggest single saving. The right method depends entirely on how the wall is built, covered in the next section.
- Draught-proofing. Attic hatch, letterbox, chimney balloons for unused flues, gaps around pipes and at skirting level. Low cost, noticeable comfort gain.
- Floors. Suspended timber floors can be insulated from below with little disruption. Solid floors are best left until you are renovating anyway.
- Windows and doors. Worth doing when they are failing or as part of a deep retrofit, but seldom first.
- Heating system. A heat pump or new boiler should be sized after the insulation is done. Insulate first and you need a smaller, cheaper system.
Which wall insulation suits which house
| Wall type | How to recognise it | Best option | Things to know |
| Cavity wall | Built after about 1930; bricks all laid lengthways; wall around 270 to 300 mm thick | Cavity fill (bead or foam) | Done in a day from outside through small drilled holes. Cheapest wall measure by far. |
| Solid brick or block | Pre-1930, or alternating long and short brick faces; wall around 225 mm | External wall insulation | Wraps the house in insulation and new render. No loss of room size, removes cold bridges, renews the facade. |
| Solid wall, protected facade | Period brick or stone front you cannot cover | Internal dry-lining | Loses 60 to 100 mm per external wall; needs careful detailing around joists and windows. |
| Hollow block | Common in 1960s to 1980s estates in parts of Ireland | External wall insulation | Cavity fill is not suitable; the hollow cores do not form a continuous cavity. |
| Timber frame | Lightweight inner leaf, often 1990s onwards | Usually already insulated | Check the attic and airtightness before touching the walls. |
External wall insulation is the most misunderstood of these. It is more expensive than cavity fill, but for solid and hollow-block walls it is the only option that treats the whole envelope without shrinking rooms. Contractors who specialise in it, such as Osber in Dublin, will survey the wall construction first, because the wrong system on the wrong wall is how damp problems start.
The mistake that undoes good insulation: forgetting ventilation
An old, leaky house ventilates itself by accident. Insulate and seal it and that accidental air change disappears. Without a plan you swap heat loss for condensation and mould.
- Minimum: working extractor fans in kitchens and bathrooms, and trickle vents or wall vents in habitable rooms.
- Better: demand-controlled extract ventilation, which ramps up when humidity rises.
- Best for deep retrofits: mechanical ventilation with heat recovery, which supplies fresh air while keeping most of the heat.
Any contractor quoting for wall insulation should raise ventilation before you do. If they do not, ask why.
What it costs, and the help available
| Measure | Relative cost | Typical payback |
| Attic top-up | Low | 2 to 4 years |
| Cavity wall fill | Low | 3 to 5 years |
| Draught-proofing | Very low | 1 to 3 years |
| Internal dry-lining | Medium | 8 to 15 years |
| External wall insulation | High | 10 to 20 years before grants, much shorter with them |
Grants change the arithmetic considerably, particularly for external wall insulation. In Ireland the SEAI publishes current grant amounts for attic, cavity, internal and external wall insulation; in the UK, schemes vary by nation and by household circumstances. Always check what is available before you accept a quote, and use a contractor registered with the scheme, since unregistered work is not eligible.
A checklist before you ask for quotes
- Measure what is in the attic now. Depth in millimetres, and whether it covers the whole area including over the eaves.
- Find out how your walls are built. Thickness at a window reveal, brick pattern, and the decade the house was built.
- Note every damp or mould patch. Cold corners and black spotting tell an installer where the cold bridges are.
- List what ventilation you already have. Extractor fans, wall vents, trickle vents, and whether they work.
- Get your current energy rating, or order an assessment. It sets the baseline for any grant and shows which measure moves the rating most.
Frequently asked questions
Should I replace my windows before insulating? Usually not. Unless they are failing, the attic and walls will save far more per pound or euro spent.
Will external insulation cause damp? Done correctly it reduces damp, because the wall stays warm and dry. Problems come from poor detailing at sills, eaves and ground level, which is why the installer matters.
How do I find out what my walls are made of? Measure the wall thickness at a door or window, look at the brick pattern, and check the age of the house. A surveyor or installer can confirm with a borescope through a small drilled hole.
Can I do it in stages? Yes, and the order above is designed for that. Just plan the whole sequence first so that early work, such as new windows, leaves room for later insulation.
Start with the attic this autumn, get the walls surveyed, and let the heating system be the last decision rather than the first. That order is what separates a warm house from an expensive one.
