Comparing thick and thin insulation

Two families, two mechanisms

Heat moves in three ways: by conduction through matter, by convection with moving air, and by infrared radiation from one surface to another. Thick insulation works mostly on the first two, by trapping still air within a thickness. Reflective thin insulation works mostly on the third, by sending radiation back instead of absorbing it.

Why a single figure cannot separate them

Thermal resistance is calculated from thickness and conductivity. It describes resistance to conduction and says nothing about radiation. Comparing the two families on that figure alone is like ranking a car and a bicycle on top speed: the answer is correct and the question means nothing.

The case where the question no longer arises

When the depth available is counted in millimetres, a converted basement, a low loft, a narrow corridor, the choice is no longer between thick and thin. It is between thin and nothing. That is the situation most renovations are in, and it explains the place these products have taken.

What happens when you combine them

The two families do not compete, they complete each other. A wool deals with conduction, a reflector deals with radiation, and together they cover all three modes of transfer. That is the configuration we recommend as soon as the thickness allows it.

The criteria worth examining beyond thickness are set out on the page about what really makes insulation effective, and the full range on the thin thermal insulation category.