ᐅ Is spray foam harmful to vapor barriers?

Hello everyone,

I have a question regarding whether polyurethane (PU) foam is harmful to a vapor control layer. In our case, the vapor control layer (see green marking in the attachment) was glued at the bottom to the ring beam with a bead of adhesive.

The cavity between the sill and the interior was insulated with polystyrene, and the gaps (between the polystyrene and the foil, and between the polystyrene and the ring beam – see red marking) were filled with PU foam.

My question is whether this was a good approach or if the PU foam could cause problems when in contact with the vapor control layer?

Best regards and thank you
Hannes

Mineral wool, stone wool, wood wool, or similar materials should be used there, not EPS...
Aside from that, this does not affect the vapor retarder.

Hi @Lumpi_LE,

why not EPS? What are the disadvantages in this case?

Well, as you probably noticed yourself, it was a rather awkward patchwork and fiddling, which resulted in gaps that had to be foam-filled, and so on. It should be clear that this is not the ideal approach. For this purpose, there are flexible insulation materials available, preferably with moisture-regulating properties, such as wood, hemp, or similar.

Fortunately, it wasn’t assembled in pieces. We were able to cut it directly at the correct angle using a hot wire. In my opinion, PU foam wouldn’t have been necessary, but this way it should be 100% secure now (it’s already airtight anyway, as there is a vapor retarder behind it). If it had been mineral wool, I would have wondered how to achieve a clean plaster finish. The structural ring beam already had Styrodur applied by the shell constructor, and with the EPS on top, we can now easily apply a mesh and then plaster it.

There are render carriers made from different materials. But if there is already EPS underneath and a mesh is embedded and then rendered, that should be fine.