What is the difference between a vapour control layer and an air tightness layer?
Honestly speaking, nothing in the most cases. A vapour control layer does always act as an air tightness layer, because vapour is conveyed through the air.
Why do I require a vapour control layer for spray foam insulation?
Because the open cell spray foam, as used for insulation in dwellings, can absorb vapour and could lead to condensation issues in the building structure.
Why is sprayfoam mostly claimed as air tight?
In my oppinion for marketing purposes only. It puts saving into the clients mind, by having no need for an additional air tightness strategy.
As coming from my own air tightness testing experience, spray foam insulation as the sole air tightness layer is never as air tight as insulation strategies (even spray foam) with an additional
air tightness layer.
Overall, please bear in mind that spray foam insulation requires a vapour control strategy.
Another interesting aspect of spray foam insulation was highlighted recently in an article of the Passive House Plus magazine: