Heat loss through a surface is proportional to its U-value, and U-value falls roughly as the reciprocal of insulation thickness. That makes insulation a diminishing-returns problem: going from none to 100 mm might cut the loss through that element by three quarters, and going from 100 mm to 200 mm cuts what remains by about half again — which is a much smaller absolute saving.
The consequence is that the first intervention on an uninsulated element is almost always the best-value one available, and the fourth is frequently not worth doing at all.
What does a U-value mean?
Watts lost per square metre per degree of temperature difference. A wall with a U-value of 2.0 loses twice as much as one at 1.0 for the same area and the same conditions, and the room’s heat requirement scales directly with it.
| Element | Typical uninsulated | Typical improved |
|---|---|---|
| Solid brick wall | ~2.0 | ~0.5 with internal insulation |
| Cavity wall, unfilled | ~1.5 | ~0.5 filled |
| Loft, uninsulated | ~2.3 | ~0.15 at 270 mm |
| Single glazing | ~5.0 | ~1.4 double, ~0.8 triple |
Glazing is the outlier and the reason it dominates so many heat-loss calculations. Even good triple glazing loses several times as much per square metre as an insulated wall, which is why a room with a large window has a heat requirement out of proportion to its size.
Why does the loft come first?
Because it is the cheapest per unit of U-value improvement and the largest single change available in most houses. Going from 2.3 to 0.15 is a reduction of more than 90 per cent through that element, for a material cost that is trivial next to wall or window work.
It is also the element where the diminishing return arrives most visibly. Moving from 100 mm to 270 mm is worthwhile; moving from 270 mm to 400 mm produces a change small enough to be lost in the noise of a single mild winter.
The radiator sizing article covers what happens on the other side of the equation once the loss falls — a well-insulated room needs less output, and the equipment sized for the old fabric is then oversized.
What breaks the arithmetic?
Air movement, which none of these figures include. A U-value describes conduction through a material; a draught is convection around it, and an otherwise well-insulated house with substantial air leakage loses heat in a way no U-value predicts.
That is why draught-proofing is consistently the best value intervention per unit spent, and why it is the one most often skipped in favour of something more substantial-looking.
Thermal bridging is the other gap. A cold bridge — a lintel, a joist end, an uninsulated reveal — conducts far more than the wall around it, and it is where condensation forms first because it is the coldest internal surface.
Does more insulation cause damp?
It can, and the mechanism is worth understanding rather than fearing. Insulation makes internal surfaces warmer, which reduces condensation on them — but it can move the dew point into the construction, and it reduces the incidental ventilation a leaky house had.
A house that was dry because it was draughty becomes damp when it is sealed without ventilation being addressed. That is not an argument against insulating; it is an argument for treating insulation and ventilation as one decision rather than two.
Extract ventilation in kitchens and bathrooms, and background ventilation elsewhere, is what keeps the moisture that people generate moving out of the building. The volume figures the room area calculator produces are what ventilation rates are specified against.
How should the order be decided?
By cost per unit of loss removed, which usually produces the same list.
- Draught-proofing — cheapest, fastest, and it improves comfort disproportionately.
- Loft insulation to a modern depth, if it is not already there.
- Cavity fill, where the construction allows it.
- Glazing, which is expensive per square metre and addresses the worst element.
- Solid wall insulation, which is the largest job and usually the last justified.
Heating controls sit alongside all of them and are frequently the cheapest change of the lot: a system that heats an empty house well is losing heat that no fabric improvement recovers.
Questions people ask
Does insulation work in summer? It slows heat moving in as well as out, and the effect is smaller because the temperature difference is smaller. Shading and ventilation do more for summer comfort than insulation does.
Is a lower U-value always better? For the element, yes. For the money, only until the marginal saving stops covering the marginal cost, which arrives sooner than most product marketing suggests.
What about reflective foils? They work against radiant transfer across an air gap, which is a real mechanism and a small share of total loss in most constructions. They are not equivalent to their claimed "thickness" of conventional insulation.
Does it change what heating I need? Substantially — and it is the reason to insulate before sizing a heat pump rather than after. Sizing to the old fabric produces an oversized and less efficient system.
The first hundred millimetres does most of the work; the fourth does very little. The radiator size calculator and BTU calculator show what a change in loss does to the equipment, and the room area calculator gives the volume both are sized against.