August 27, 2020

Cup of Joe Flow-Through Assemblies

Once we understood the need for air control and vapor control layers and started putting them in to our buildings to develop an enclosure, we saw that, once they became wet, they could only dry in one direction.

The climate zone the building is in makes a difference in the failure that occurs. Example: Buildings in a cold climate with vapor BARRIERS on the inside, could only dry out. In buildings in a hot-humid climate, vapor BARRIERS could only dry in. This seems obvious now, but it didn't become obvious until we did it.

We then saw the need to slow down the vapor transmission rather than attempt to stop it altogether. This vapor throttle helped to control condensation in these enclosures. We still needed to control air flow to keep moisture-laden air out, and found it easiest place to design and install an air control layer was on the outside of the wall. We found these airtight but more vapor-open assemblies to be more forgiving in that they helped increase durability of the wall.

Then we found that installing rigid insulation outside of the air control layer helped keep the interior wall cavity from condensing on the back of the sheathing as vapor could now flow through the assembly.

This wall works even better if between the cladding and the rigid insulation, a small amount of airflow can occur and if the rain that gets behind the cladding can dry out. So we needed a gap.

Oh, and don't try to undo all the progressive steps we have made by now installing a vapor BARRIER on the inside finish of the wall. Vinyl wallcoverings are just such a BARRIER. Don't use them. No matter who tries to convince you that you could use perforated vinyl without trapping the vapor. You can't. Not oil-based or epoxy paints, either.

The same technology works for a roof but not for a slab. Slabs are always touching the wet ground and you absolutely need a vapor BARRIER under your slab. But 2 out of 3 ain't bad.

If you want to see more like this, consider coming to one of our seminars!

https://www.buildingscience.com/upcoming-events-and-training