Thanks for this excellent informational piece. I understand and agree with the requirement to seal penetrations to prevent air movement. While you mention that a ceiling vapor barrier should not be installed under insulation in lower zones, I have increasingly noticed that in new homes in my area of the South (Zone 3 in my case), insulation is merely blown-in on top of the drywall ceiling. I continue to recommend that the installer is consulted to verify the requirement for a vapor retarder installed to minimize moist airflow from the attic, especially in our humid summer climate, but also from the conditioned space in the winter, to prevent condensation formation in the attic timber members. The example I usually give to homeowners is the Kraft facing on roll insulation which is installed face towards the conditioned area. Clearly, after the fact, builders are advising that no vapor retarder is required (probably due to the cost of removal of insulation, retrospective installation of a retarder then re-insulating) but I have found mixed recommendations through review of insulation manufacturer’s websites and would ask your, and obviously all other members, opinion.
Often no vapor barrier is required for attics with Cellulose Insulation
Thanks, I had seen this article in the past and understand the rationale however a greater majority of homes around me are insulated with blown in fiberglass and the same arguments would not apply. With the fiberglass I have read numerous articles that require vapor retarders under the insulation hence the reason for my recommending same and drawing comparison with the kraft faced roll material, but I am more than willing to review any other literature that would provide information to the contrary.
In counties identified by footnote
(a) in Table N1101.2 of the IRC
standard where the local environmental
conditions do not support
a recommendation for vapor
retarders. These locations, in
general, have high outside temperature
and humidity levels during
a significant portion of the
year. These locations may be
found along the Southern Coast
(SC, GA, FL, AL, MS, LA, TX),
Puerto Rico, and Hawaii. See
Figure 2 for the locations speci-
fied for Exception 3.
Check local practices and code
regulations for the specific area.