To Martin Holladay:

Last week you gave this reply to a question about putting cellulose
insulation on top of fiberglass batts in new construction. You have answered
that in full and I thank you for it. However, you made a further comment as
follows: ” I hope, however, that your builder remembered to do a careful air
sealing job at the ceiling plane before any insulation was installed.”

Please let me know more particularly what should have been done to the
ceiling to seal it properly.

The only air sealing that I know about is that the fiberglass batts have
paper vapor barrier on their bottoms and the ceiling was covered with 1/2″
sheetrock, Behr primer-sealer, & two coats of Behr ceiling paint.

The fiberglass batts were pushed up into place from below through the
stapping. The sheetrock was applied afterwards. (The attic is full of
trusses & girders.)

The Devil is always in the details.

Here is a copy of your complete reply last week:

“Robert,

The only mystery here is why your builder ever installed the fiberglass
batts. If he is smart enough to suggest cellulose insulation, he should have
just installed 100% cellulose, without the fiberglass, as A.J. noted.

Once the fiberglass batts were installed — whether deliberately or
mistakenly — the decision to cover the fiberglass batts with cellulose was a
good one. Fiberglass batts have several disadvantages, and covering such
attic batts with cellulose is a standard solution.

I hope, however, that your builder remembered to do a careful air sealing job
at the ceiling plane before any insulation was installed.

The cellulose may compress the fiberglass batts a little, but the potential
downsides to this compression are far outweighed by the advantages of the
added cellulose. When a fiberglass batt is compressed, the R-value of the
batt decreases slightly, but the R-value per inch increases. I will attach a
chart that shows the relationship between compression and R-value for
fiberglass batts. [Click on the image to enlarge it.]” [Chart omitted here]