Water tightness in reflective thin insulation

Watertight and vapour permeable are not the same

Liquid water and water vapour do not cross materials in the same way. A product can stop the first and let the second through, or stop both. That choice is not a detail: it decides where condensation will form within the wall build-up, and therefore what will be damaged.

The case of an aluminium reflector

R'BULL Pro 13 stops both. Its aluminium faces make it a vapour barrier, which is an advantage and a constraint. The advantage: the product never takes up moisture, so it does not lose its properties over time, unlike a wool that loads with water. The constraint: its position in the build-up has to be chosen, not accepted by default.

The positioning rule

A vapour barrier goes on the warm side of the insulation, which means the inside. Fitted on the wrong side it traps moisture within the wall instead of keeping it out. That is the one genuinely serious design error with these products, and it is visible on a section drawing.

What that gives over time

Insulation that never takes up water keeps the performance measured on the first day. It does not settle, it does not go mouldy, and it does not attract what lives in damp. That is what allows us to speak of a service life beyond fifty years.

Fitting and the treatment of junctions are set out on the page about airtightness, and the product on the R'BULL Pro airtight page.