Does the thermal resistance of thin insulation matter?

Where the figure comes from

Thermal resistance R is calculated by dividing the thickness in metres by the conductivity of the material. A product twenty centimetres thick at 0.040 gives an R of 5. The formula is simple, it is correct, and it was written for materials that work by conduction through their mass.

What it does not take into account

It takes into account neither reflection of radiation, nor airtightness, nor summer behaviour. Those are exactly the three points a reflector works on. The figure obtained is therefore not wrong, it is beside the point.

Why you will be asked for it anyway

Thermal resistance serves two administrative purposes: qualifying for grants and drawing up the energy performance certificate. Both uses are real and have to be answered. That does not turn the figure into a technical selection criterion.

What to ask for instead

Emissivity and the method used to measure it, airtightness and vapour tightness, fire behaviour, and the installation conditions that decide the result, starting with the air gaps. Those four answers say far more about what you will get in the room.

The demonstration in pictures is on the page about the thermal resistance of thin insulation, and the full set of criteria on the one about what really makes it effective.

How to answer when someone asks for the R value

We give the equivalent thermal resistance of R'BULL Pro 13, that is 3.19 m².h°C/Kcal with two still air gaps, along with a conductivity of 0.022. Those values can be used for an administrative file. They come with the installation conditions they correspond to, because without the air gaps they no longer mean anything.