Insulation Calculator
The first hundred millimetres does most of the work.
Work out Insulation. The first hundred millimetres does most of the work. States the assumption instead of hiding it.
Check your local building code for the current minimum
Mineral wool about 0.025, PIR board about 0.045
For your location and base temperature
A heat pump can exceed 100% — enter its seasonal performance factor as a percentage
Insulation thickness to add
200 mm
Payback in 0.4 years · 1,556 a year
Insulation has sharply diminishing returns, because loss is proportional to 1/R rather than to R. Going from R-1 to R-2 removes half the loss; R-2 to R-4 removes half of what remains; R-4 to R-8 half again. The first hundred millimetres does most of the work and every hundred after it does less, which is why very high targets stop paying back even when the material is cheap. Note that this models the insulation alone — air-sealing frequently saves more than adding insulation to an already-insulated but leaky building, and it costs far less.
How the Insulation Calculator works
Thickness needed to reach a target R-value, with the energy saved and a simple payback. Insulation has sharply diminishing returns, because loss is proportional to 1/R rather than to R.
Also known as: how thick should insulation be · loft insulation depth calculator · insulation payback calculator · how much insulation do i need
Diminishing returns, and where they bite
Heat loss is proportional to 1/R, not to R. Going from R-1 to R-2 removes half the loss. R-2 to R-4 removes half of what remains — a quarter of the original. R-4 to R-8 removes half again, an eighth of the original.
Each doubling costs roughly the same in material and saves half as much as the one before. That is why the first hundred millimetres does most of the work and why very high targets stop paying back even when the material is cheap.
It also explains why insulating an uninsulated element is transformative and topping up an already-insulated one is marginal. The best available return is almost always the worst-insulated element, not the one that is easiest to reach.
Air-sealing frequently beats adding insulation
Fibrous insulation does nothing about air moving through gaps. Heat carried out by air leaving the building bypasses the insulation entirely, and in a leaky building that path can dominate the fabric loss.
Air-sealing costs far less than insulation and is frequently the higher-return measure in an older building. Loft hatches, service penetrations, floor perimeters and around window frames are where most of it goes.
The corollary is that measuring air-tightness tells you which measure to do first. Without that number, insulation is the visible option and air-sealing is the invisible one, which is why it is so often skipped.
Moisture, compression and the vapour question
Insulation works by trapping still air, so anything that displaces that air destroys the performance. Compressing mineral wool into a smaller depth loses more R-value than the thickness reduction alone suggests, because the trapped air is squeezed out.
Water is worse. Water conducts heat far better than air, so wet insulation can lose most of its performance and does not recover until it fully dries — which in a wall cavity may be never.
Which is why the vapour control layer's position matters more than its presence. On the wrong side of the insulation it traps moisture inside the construction, and that is worse than having none. It is climate-specific and assembly-specific, and it is worth checking locally rather than generalising.
Where to go next
The Insulation question rarely arrives on its own. These are the ones that usually come with it:
- R-Value Calculator — R adds in series, U does not.
- Heat Loss Calculator — Floor area alone cannot tell you the answer.
- Attic Ventilation Calculator — Intake must exceed exhaust, not the reverse.
- Ring Size Calculator — Finger measurement to US, UK, EU and Japanese sizes.
Frequently asked questions
How thick should insulation be?
Divide the R-value you need to add by the material's R per millimetre. Mineral wool is around 0.025 and PIR board around 0.045, so the same performance takes roughly half the thickness in board.
Why does more insulation save less?
Because heat loss is proportional to 1/R. Going from R-1 to R-2 removes half the loss; R-2 to R-4 removes half of what remains; R-4 to R-8 half again. Each doubling costs the same and saves less.
What R-value should I aim for?
Whatever your local building code requires as a minimum, which differs by climate and by element and is revised periodically. Going beyond it is a payback calculation rather than a compliance one.
Is air-sealing more important than insulation?
In a leaky building, frequently yes — and it costs far less. Insulation slows conduction; it does nothing about air moving straight through gaps carrying heat with it.
Does compressing insulation reduce its performance?
Yes, substantially. Mineral wool works by trapping still air, and compressing it into a smaller depth reduces the R-value by more than the thickness reduction alone would suggest.
Should insulation have a vapour barrier?
It depends on the climate and the assembly. Getting it on the wrong side traps moisture inside the construction, which is worse than having none — this is a detail worth checking locally rather than generalising.
Which insulation material should I use?
It depends on the space, the moisture conditions and whether thickness is constrained. Mineral wool is cheap and vapour-open; PIR board gives more R per millimetre; cellulose fills irregular cavities well.
Can I insulate a solid wall?
Internally or externally, and both change the wall's moisture behaviour significantly. Internal insulation makes the wall colder and wetter behind the insulation, which is why the build-up needs care rather than just thickness.
How do I insulate a loft hatch?
It is a small area with a large effect, since an uninsulated hatch is a hole in the insulation layer. Insulating and draught-sealing it is one of the cheapest measures available.
Should loft insulation go under the water tank?
No — leaving the area under a tank uninsulated lets warmth from below keep it from freezing. Insulating over and around the tank while leaving beneath it clear is the usual approach.
Does insulation stop draughts?
Fibrous insulation does not — air moves through it. Air-sealing is a separate job and frequently saves more than adding insulation to an already-insulated but leaky building.
Can insulation cause condensation?
It changes where surfaces are cold, which changes where condensation forms. Insulating part of an assembly without considering the vapour path can move the dew point into the construction, which is worse than leaving it alone.
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