ᐅ Remove the glass panes and insulate the opening.

Hello. I want to close one side of my conservatory. (I already have the building permit / planning permission). It is made of wood, and the frames contain double-glazed insulating glass units from 1985. I now want to remove the glass and close the opening with OSB boards on both the outside and the inside. Between them, I plan to install insulation and a vapor barrier. I would like to use rigid PUR (polyurethane) insulation boards.

My question is: How thick does the insulation need to be to provide equivalent or better performance than the glass units?

Equivalent would be about 5 mm (0.2 inches)... thickness and PUR, however, don’t make sense here.
Use MW 035 according to the maximum possible thickness and that’s it.

Thank you for the quick reply.
I thought PUR had better insulation performance than wool. 5mm or 50mm (2 inches)?

Yes, it does, but 5mm (0.2 inches) is somewhat impractical.
PUR is also intended for other applications. For these, mineral wool is used.

Yes, that was the question because you wrote 5mm. So would 50mm (2 inches) of glass wool with a thermal conductivity of 0.032 or 0.035 be sufficient? Would that then achieve the insulation value of the pane I am removing, or would it even provide better insulation?

A window from the 1980s usually has a U-value of 3 W/m²K, while 50mm (2 inches) of mineral wool has about 0.6, which is roughly five times better.
Glass also provides solar gains. For a north-facing window, this is mostly irrelevant, but for a south-facing one, closing the windows could actually worsen the overall energy balance.

By the way, OSB boards cannot be used on the exterior.