New window section. How on earth do I vapor seal this?

[Olf Oggler]

I have replaced a window section in a house from the '70s. Since the entire facade is re-paneled on the outside, it is well windproofed with a wind barrier sealed with diffusion-open tape against the frames.

But what do I do on the inside? This feels like an impossible task. Even if I spent a workweek cutting small pieces and trying to patch together with tape for some kind of vapor barrier, it still wouldn't feel right. The old vapor barrier is cut flush with the gypsum. You can't splice onto that.


How would you have done it? How critical is it? Can I skip the vapor barrier around these windows?

 

[Dsl]

Tape! A good tape solves a lot.

If the vapor barrier is cut so that it can't be taped, place a sealant first and tape over it to make it tight.

 

[AXS]

Or you tear down the little plaster left on the wall and re-plaster everything.

Take the opportunity to check the insulation, wiring, and what else is in there.

 

[Stefan1972]

You probably don't need to do anything special. An older house is not super airtight anyway, and a small change right there probably doesn't change much.

 

[Ursidae]

One has to be a bit pragmatic in an old house. It's wise to reduce moisture migration through the packing, but I guess you didn't have a barrier on the inside before and it probably worked fine? A small complication is that you have (diffusion-open) tape on the outside, which probably still has a certain vapor-retarding effect.
There are a couple of options: Try taping between the casing and the stud. There are plasticized packings that can be used on the inside. Backer rod and sealant between the stud and casing might be the most effective option.