Home insulation
A Socratic walk-through of home insulation — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does insulating a house save more than simply turning the heating down?
Two households want a smaller heating bill. One turns the thermostat down two degrees and wears a jumper. The other insulates the loft and keeps the house as warm as before. Both are doing the same sensible thing, surely — using less heat.
But they are not doing the same thing at all, and the difference is not one of effort or virtue. They are acting on two different terms in the same equation, and only one of those terms keeps paying. Which one, and why?
Reasoning it through
REASONING #Start with a question that sounds too simple. Why does a heated house need heating at all, once it is warm? Because heat leaks out continuously through walls, roof, floor, windows and gaps, and the boiler is not warming the house so much as replacing what escapes. In steady state, the energy you buy equals the energy you lose.
So the real object of study is the leak. What sets its rate? Two things, and they multiply. First, how hard the heat is pushed — the temperature difference between inside and outside. Second, how easily the building lets it through — the conductance of the envelope, which is the reciprocal of its insulation. Loss rate equals conductance times temperature difference. That is the whole of it, and everything else follows from reading it carefully.
Now look at the two households. Turning the thermostat down attacks the temperature difference. Insulating attacks the conductance.
Ask what each can achieve. On a day when it is 5 outside and you keep the house at 20, the difference is 15 degrees. Drop the thermostat to 18 and the difference becomes 13 — about a thirteen percent cut, and you feel it. Can you do better? Only by being colder still, and the term you are shrinking has a floor: at the outdoor temperature it reaches zero, and long before that you are no longer heating a home. The saving is real, but it is bounded by how much discomfort you will accept, and it is paid for in discomfort every hour.
Insulation shrinks the other factor. Halve the conductance and you halve the loss at every temperature difference — on mild days and on the coldest night of the year, at whatever thermostat setting you choose, without anyone noticing anything except that the house is easier to keep warm. It is a change to the multiplier rather than to one of its inputs, and it does not need to be re-earned each morning.
That is the answer in its simplest form. But two complications matter, and leaving them out would make the picture too flattering.
The analogy
THE ANALOGY #Think of a bucket with holes, kept full by a running tap. Raising the water level makes it drain faster, so you can reduce the flow you need by settling for a shallower bucket — that is the thermostat. Or you can plug some of the holes, and then the same water level needs less flow, at every level. Only one of those lets you keep the bucket full.
Water drains out of holes and nothing enters, whereas a house both loses heat and gains it — from sunlight, appliances and bodies — so a well-insulated house can overheat in summer in a way the bucket has no equivalent of.
Clarifying the model
THE MODEL #The first complication is diminishing returns, and it is arithmetic rather than opinion. Insulation is usually described by thermal resistance, and loss falls as one over that resistance. Take a bare wall as 100 units of loss. Doubling its resistance drops it to 50 — a saving of 50. Doubling again drops it to 25, a further saving of only 25. The next doubling saves 12.5. Each layer is as effective as the last at what it does, and yet buys progressively less, because it is working on a flow that the previous layers already reduced. This is why loft insulation is spectacular and the fifth centimetre of wall insulation is not, and why an uninsulated house is always the better place to spend the money.
The second follows directly from the same equation, and is the one most often missed. The envelope's paths are in parallel, so the total is dominated by whichever path is worst. A wall improved to an excellent standard next to a single-glazed window, an uninsulated floor, or an unsealed gap around a door has not fixed the house — it has fixed the part that was already doing least of the leaking.
That points at air leakage, which is not the same mechanism at all. Conduction through the fabric is what insulation addresses. Infiltration is warm air physically leaving and cold air replacing it — through gaps at floor edges, around openings, at service penetrations, up chimneys. It bypasses the insulation entirely. In older houses this uncontrolled exchange frequently accounts for more loss than any wall does, which is why draught-proofing is routinely the cheapest useful measure and why insulating a leaky house disappoints people.
Two honest cautions. Sealing a house without providing deliberate ventilation moves moisture problems rather than solving them, and condensation and mould after a retrofit are a well-documented failure. And measured savings after insulation regularly fall short of the modelled figures, partly because people take some of the benefit as extra warmth rather than as money — the rebound effect — which is a real outcome, not an accounting error.
A picture of it
THE PICTURE #How to readMove rightward one step at a time and read the drop in bar height as what that step of insulation saves. The first step, from a bare wall to twice its resistance, removes half of all the loss on its own. The step from 4 to 5 removes five percent, and the step from 7 to 8 removes one. The curve is the shape of diminishing returns — the whole chart is drawn at a fixed temperature difference, and turning the thermostat down would scale every bar by the same fraction without changing the shape at all.
What became clearer
WHAT CLEARED #The heating bill is a rate multiplied by a difference. Turning the thermostat down shrinks the difference, which works, but is capped by comfort and must be endured continuously. Insulation shrinks the rate itself, so it applies at every difference, on every day, once. The qualifications are that each additional layer buys less than the one before it, and that in an older house the largest leak is often not through the fabric at all but around it.
Where to go next
ONWARD #- Why a heat pump changes the arithmetic again, by altering the cost of each unit of delivered heat rather than the loss.
- How thermal mass shifts when a house loses heat without changing how much.
Key terms
TERMS #| Term | What it means |
|---|---|
| Thermal conductance | how readily a building element passes heat, per unit area and per degree of temperature difference. |
| Thermal resistance | the reciprocal of conductance, the quantity that insulation adds and that loss falls in proportion to. |
| Infiltration | uncontrolled air exchange through gaps in the building envelope, distinct from conduction through the fabric. |
| Rebound effect | the tendency to consume some of an efficiency gain as extra comfort, so measured savings fall short of predicted ones. |
Every term the collection defines is gathered in the glossary.