[Greenbuilding] insulating beneath an existing floor.

[Alan Abrams]

I am haunted by some crawlspace photos (courtesy of Joe Lstiburek, IIRC) of
the bare underside of some floor joists, with insulation in the spaces
between, and glistening green tendrils of some unspeakable growth dripping
from the bottom edges of the joists.

this image could of course be a figment of the tormented imagination of a
builder who did a whole lot of this kind of stupid shit, before he became
so "enlightened," but assuming this recollection is valid, the condition
was due to the thermal bridging of the joist itself, providing surfaces at
the dewpoint, and air in the crawlspace humid enough to condense on and
saturate the exposed bottom of the joists.

suggesting, in such a case--regardless of other measures--a layer of
insulation applied to underside of the joists might be a good idea. One
approach would be to insulate and air seal the perimeter of the floor
cavity, by any means necessary, and apply a permeable air barrier like
tyvek, directly under the joists, and then a layer of Roxul to that,
supported by furring strips. insulating the floor cavity itself would be
optional. Air tight, fire resistant, mold resistant, and able to dry in
either direction, after the bathtub overflows, or the washer hose craps
out...

-aa

Alan Abrams

*certified professional building designer, AIBDcertified passive house
consultant, PHIUS*
*certified passive house builder, PHIUS*
cell     202-437-8583
alan at abramsdesignbuild.com
HELICON WORKS *Achitecture and Education*
<http://www.heliconworks.com/index2.html>
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