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Old 06-18-2015, 11:34 PM   #16
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Since you said you can feel air moving into that space, not insulating the abutting knee wall will give major R-value reductions. This is air permeable fiberglass, same as used in furnace filters for its non-restrictive (of air) qualities. It won't mold if there is air movement when it can dry out if ever wet.
Where is the house located- State? Is a vapor barrier required? Will you be installing rigid foam board on the attic side of knee wall (may or may not require an ignition barrier, more on that later) or at very least- housewrap to protect FG from attic wind-washing?


If the insulation is not in FULL CONTACT with the drywall (air barrier), you will have a lacking design. Inset stapling alone is only a 2-3% reduction in R-value, but "


"These observations provide strong evidence of convective airflow due
to inset stapling,
resulting in moisture transport within the stud
bay. Outward vapor diffusion through the painted
drywall introduces interior moisture to the vert
ical channels formed between the facer and the
drywall. These channels provi
de a clear airflow path; a conv
ective loop can form, bypassing the
vapor retarder at the top of the wall and depos
iting condensation on the top portion of the stud
bay sheathing, as seen in field results. Note th
at only a small gap is required at the top and
bottom of the insulation to allow
convective airflow: Brown et al
. (1993) reported that gaps less
than 1 mm (1/32”) wide allow these loops to form
between air spaces on both sides of insulation.
It is unlikely that these results are due
to one-dimensional vapor diffusion through the
gaps at the edges of the batt facers, as show
n by the spatial moisture content patterns.
Specifically, no moisture deposition
was seen at the bottom of any
of the inset-stapled stud bays.
It is worth noting that two vapor retarder materials that were detailed as air barriers
(polyethylene and PA-6 film) resulted in a
ssemblies with excellent performance (very low
sheathing moisture contents). These vapor reta
rder materials were in
stalled overlapping the
foam gaskets on the face of the st
ud, so they would have been ve
ry airtight. It is uncertain
whether their performance is solely
due to their vapor re
tarding characteristics, or also due to this
detailing.
Note that in this research, we did not see ev
idence of convective l
ooping within the cavity
insulation in bays that did not have inset stapling.
As noted in the literature (Powell et al. 1989),
convective loops are more likely with these air gaps
, and less likely if an air barrier is placed in
direct contact with the insulation. If significan
t convective looping were occurring within the
insulation, there would have been greater depositio
n of moisture at the top portion of the latex
paint cavity to due air movement. Instead, MC leve
ls were, if anything, slightly lower at the top
of the wall (edges of cavity). On
e might conjecture that these su
rfaces were slightly warmer, due
to thermal bridging of the framing.
Furthermore, given (a) the mild climate (maximum observed
Δ
T of 47 F/26.3 C), (b)
careful installation of the insulation (cut to
cavity size, not compressed), and (c) air barrier/
1-
318
2008 ACEEE Summer Study on Energy Efficiency in Buildings
compartmentalization efforts, convective loopi
ng within the insulation
(as opposed to around the
insulation, in the air channels) wo
uld be relatively unlikely. It
appears that under this set of
conditions, the density of this fiberglass batt
is sufficient to prevent significant convection.' Read it better here; http://aceee.org/files/proceedings/2...papers/1_8.pdf

Gary
PS. Barry, note the "1/32"?
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Last edited by Gary in WA; 06-18-2015 at 11:38 PM.
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