Thursday, November 12, 2009

Vapor Retarder Two-Thirds Rule

When placing insulation in Portland, Oregon, I rarely use material with a bonded-on vapor retarder (kraft face). I am aware of propaganda favoring cellulose, and raising fear of fiberglass batts, in The Big Burn Video. Proponents of cellulose have dealt with criticism of organic cellulose as inherently less safe than inorganic mineral insulation, and tilt the table too far in the other direction. The video presents ideal cellulose, and fiberglass with no description. I imagine the fiberglass was a least-favorable setting, with air channels (chimney path) the length of flammable kraft facing. A fair comparison would have fiberglass totally filling joist bays, with no kraft facing.

I will never use kraft facing in a wall or in a new-construction ceiling. There I use complete, taped sheathing with polyethylene sheeting designed for the purpose, Swedish Tenoarm. Aside from fire safety concern, I save time and get better fit, in custom-cutting unfaced batts. I also follow the advice of the USA importer of Tenoarm:

I will use kraft batts in an attic floor, to isolate new material from found loose-fill insulation. I use original reasoning in study of dew point data for my location, as reviewed in this posting, with a current update. The commonly-understood two-thirds rule states that a vapor retarder should be to the warm side of two thirds of the applied insulation.

1 comment:

viji said...

Great thoughts you got there, believe I may possibly try just some of it throughout my daily life.