Insulation Calculator

About the Insulation Calculator

Insulation is specified by R-value — its resistance to heat flowing through it. Building regulations, energy guides and product labels all speak in R-values, or in their inverse, U-values. What you actually buy, though, is a thickness of a particular material, and different materials give very different R-values for the same thickness.

The link between the two is a single property: the material's thermal conductivity, written λ (lambda). With it, working out the thickness needed for any target R-value is one multiplication.

This insulation calculator takes a target R-value — in US or SI units — and a material, and gives the thickness required, the R-value per inch, the U-value of the layer, the quantity to buy, and how thick the same R-value would be in other common materials.

How to Use the Insulation Calculator

Choose imperial or metric. In imperial, R-values are US R-values such as R-19 or R-38; in metric they are SI values in m²K/W, such as 3.35 or 6.3.

Enter the area to insulate and the target R-value.

Pick a material. Each has a typical conductivity, which you can override with the figure from the product's data sheet.

Optionally, enter the coverage per pack to get a pack count, and adjust the waste allowance.

How the Thickness Is Worked Out

  thickness (m)   =  R (SI) × λ
  R (US)          =  R (SI) × 5.678
  R per inch (US) =  0.0254 ÷ λ × 5.678
  U-value         =  1 ÷ R (SI)

Step-by-Step Example

R-19 over 500 ft² of wall, in glass wool (λ 0.040).

  To SI:       19 ÷ 5.678            =  3.35 m²K/W
  Thickness:   3.35 × 0.040          =  0.134 m  =  5.3 in
  Per inch:    0.0254 ÷ 0.040 × 5.678  =  R-3.6
  U-value:     1 ÷ 3.35              =  0.30 W/m²K

The same R-19 in PIR board would need about 2.9 inches.

Thermal Conductivity Explained

Thermal conductivity is how readily heat passes through a material. The lower it is, the better the insulator, and the thinner the layer needed.

These are typical design values; individual products vary, and the λ-value on the product data sheet is the one to use.

Two R-Value Scales

The US measures R-values in ft²·°F·h/BTU; most of the world uses m²·K/W. The two differ by a fixed factor of 5.678, so:

  R-13  =  2.29 SI      R-19  =  3.35 SI
  R-30  =  5.28 SI      R-38  =  6.69 SI

Mixing the two is a common source of confusion. An "R-6" product is far better insulation in SI units than R-6 in US units — by a factor of nearly six.

R-Value, U-Value and the Whole Wall

R-value is resistance: higher is better. U-value is transmittance: lower is better. For a single layer, U is simply 1 ÷ R.

A whole wall, roof or floor has several layers — boards, air cavities, framing — and heat also bypasses the insulation through studs and joists, which conduct heat far better. So the U-value of the complete construction is worse than the insulation layer alone suggests. Building regulations are usually written as whole-element U-values, which designers calculate with all the layers included.

Depth, Compression and Space

Fibrous insulation — glass wool, mineral wool — works by trapping still air. Squashing it into a shallower space reduces its R-value, so a batt rated for 6 inches forced into a 4-inch stud bay performs well below its label. Where depth is limited, a higher-performance board or spray foam gets more R-value into the space.

Lofts are the easy case: plenty of depth, so thick mineral or glass wool laid in two layers — the second across the joists to cover them — is usually cheapest.

Where Insulation Pays Back Most

Not every part of a building repays insulation equally, and the order matters when the budget is limited.

Lofts and roofs usually come first. Warm air rises, loft insulation is cheap and easy to lay, and the depth is available, so the saving per pound spent is often the best of any measure.

Walls come next. Cavity walls can often be filled from outside; solid walls need insulation on the inside or outside, which costs more but makes a large difference in older houses.

Floors matter less for heat loss, but insulating a suspended timber floor also cuts draughts, which people notice immediately.

Air sealing is the cheap step people skip. Gaps around pipes, loft hatches and skirting boards let warm air escape straight past the insulation, so sealing them first makes every layer that follows work harder.

Understanding Your Result

Thickness needed is the depth of your chosen material that gives the target.

R- and U-values shows the target in both scales and the layer's U-value.

Per inch gives the R per inch and millimetres per unit of SI R.

Quantity gives the volume or number of packs to buy.

Other materials compares the thickness in alternatives.

Worth knowing covers depth and compression.

When Should You Use This Calculator?

Choosing between materials for a space with limited depth.

Converting between US and SI R-values.

Planning loft, wall or floor insulation to meet a target.

Checking a product will reach the required value.

Common Mistakes

Mixing US and SI R-values. They differ by 5.678.

Compressing batts. It lowers their R-value.

Ignoring thermal bridging. Studs and joists leak heat past the insulation.

Leaving gaps. Small gaps cut performance far more than their area suggests.

Forgetting ventilation. Lofts and some walls need air gaps to stay dry.