Ask anyone where their house loses heat and they will point at a window. It is the visible cold surface, it is where the draught is felt, and it is almost never where most of the money goes.
Heat leaves a home through whatever offers the least resistance and the largest area. Windows are a small fraction of the surface. The roof is enormous, the walls are larger still, and the gaps — which have no insulating value at all — are collectively bigger than people imagine.
The order that matters
Broadly, and in most older homes, heat loss ranks like this: the roof first, then the walls, then uncontrolled air leakage through gaps, then the floor, and only then windows and doors as surfaces.
That ordering explains why the cheapest measures are so often the most effective ones. Loft insulation is inexpensive, can usually be done without trades, and addresses the largest single loss. Replacing windows is one of the most expensive things you can do to a house and addresses one of the smaller ones.
The US Department of Energy’s Energy Saver guidance is a good neutral reference here, and it consistently puts air sealing and insulation ahead of window replacement on cost-effectiveness.
In the UK, the Energy Saving Trust publishes comparable guidance aimed at the older, draughtier housing stock most of the country lives in.
Find the gaps before you buy anything
Air leakage is the loss people most underestimate, partly because it is invisible and partly because it is spread across dozens of small openings rather than one obvious hole.
The usual suspects, roughly in order of how much they leak:
- The gap under and around an external door, including the letterbox and keyhole
- Loft hatches, which are often uninsulated and rarely sealed
- Where pipes and cables pass through external walls
- Unused chimneys and flues
- Floorboard gaps and skirting board joints in older houses
- Extractor fans and vents with failed or missing flaps
A cold, windy day is the best time to find them. Walk the perimeter of each room with the back of a damp hand held a few centimetres from the junction between wall and floor, around frames, and at service entries. Wet skin registers moving air far better than dry skin does.
The cheap fixes that genuinely work
Draught-proofing strip on door and window frames, a brush strip on the door bottom, a chimney balloon in an unused flue, and expanding foam or sealant around pipe penetrations. Total cost is usually less than a single window, and the effect is felt the same evening.
The one caution: a house needs ventilation. Sealing every gap in a home with no extraction, no trickle vents and no airflow moves the problem from heat loss to condensation and mould. Seal the uncontrolled leaks; keep the deliberate ventilation.
The window question, answered properly
None of this means windows do not matter. Single glazing is genuinely poor, and a room with a large single-glazed area will feel cold no matter how well the loft is insulated, because the cold glass pulls radiant heat off your body directly.
But the honest comparison is against what else the same money could do. If the loft is under-insulated and the doors leak, spending on windows first buys the smallest improvement per pound of anything available.
Cheaper intermediate steps exist and are underrated: heavy lined curtains that finish below the sill rather than above it, closed at dusk; secondary glazing film for a season; and simply moving furniture off the cold wall, which does nothing for the house but a great deal for how the room feels.
The floor is the forgotten one
In houses with suspended timber floors over a ventilated void, the floor can account for a surprising share of the loss, and it is the surface people are in constant contact with. You do not necessarily need to lift the boards: sealing the gaps between them and around the skirting removes most of the draught, and a dense underlay under a carpet or rug does more than the rug alone.
If the boards are already up for other work, insulating between the joists at that moment costs very little extra and is close to impossible to justify as a standalone job later.
Heating the person, not the building
The last idea is the one people resist and then adopt permanently. A house is expensive to heat because it is large. A person is cheap to heat because they are small.
A warmer room is not the same as a warmer you, and for the hours you spend sitting still, a heated throw draws a tiny fraction of what raising the whole house by two degrees costs. It is not a substitute for insulation. It is what makes the winter evenings comfortable while you save up for the insulation.
What to do this month
- Insulate the loft properly if it is not already, including the hatch
- Draught-proof external doors, the loft hatch and service penetrations
- Block unused chimneys, keeping deliberate ventilation intact
- Line the curtains and close them at dusk
- Only then price up glazing
That order is uncomfortable because it starts with the least interesting work. It is also the order that produces the largest reduction in bills for the smallest spend.
More around the house in Home & Living, including when the cheaper option isn’t actually cheaper.
