Next month I'm going to be partially resheathing and residing an
exterior wall, so it will be open to the studs from the outside. The
wall currently has cotton batt insulation, and I understand batt
insulation can be leaky where it meets the framing. So I'm wondering
what I can do from the exterior to seal up the wall cavity better, and
I'm thinking foam. I'm considering a few options:
1) Discard the batts and foam the entire wall cavity. Probably too
expensive given my climate (Berkeley, CA).
2) Spray the perimeter of each batt to seal it to the framing. Will
3) Temporarily remove the batts and spray a thin layer of foam against
the wallboard. The interior was recently remodeled, so the
wallboard won't need replacing any time soon.
Which of these is the way to go? In some areas I won't be removing
the existing 1x4 plank sheathing, in which case option (3) isn't
available. The plank sheathing has wide gaps everywhere, so I
should still be able to access the wall cavity through it.
4) Staple black aluminum window screen into the south wall cavities to
compress 3.5" R11 batts to 1.5" R6.2 batts and screw clear corrugated
polycarbonate Dynaglas or Sun Tuf "solar siding" over the wall to make
a thermosyphoning solar air heater which gains 0.9x1050 = 945 Btu and
loses 6h(70-48.7)1ft^2/R1 = 128 Btu by day and 18h(70-48.7)1ft^2/R6.2
= 62 by night, for a net gain of 945-128-62 = 755 Btu/ft^2-day, vs the
original wall which loses 24h(70-48.7)1ft^2/R11 = 46 Btu/ft^2 on an
average January day in Berkeley.
On 2005-10-24, firstname.lastname@example.org wrote:
Well, this is a nice idea, but I'm afraid that aesthetic concerns
preclude it. Unless there is some way to make this work with cedar
shingles on top? :-)
As to the computations, let's see. 1050 Btu is the daily insolation
per ft^2 in Berkeley on an average January day, and 0.9 is a
correction factor for vertical walls given the angle of the sun in
January, yes? My interior design temperature is 68F, not 70F, but
that's a small difference. Why is the heat loss resistivity during
the "day" R1 instead of R6.2? 48.7F must be the average January
temperature (average low temperature?), seems like it would be more
accurate to use one temperature during the 6 hour "day" and another
during the 18 hour "night". Not that it really matters, as the heat
loss is an order of magnitude less than the solar heat gain.
Did I get all that right? Thanks for the education. Oh, and why does
the thermosiphon require a 2" air space? I assume the 1050 Btus/day
is an average of sunny and cloudy days, how much insolation can I
expect when it is raining? It does that alot in Berkeley in January,
and so I imagine this heat source would be very uneven day to day.
I would just leave the bats and add tyvek and, if your remodel would
allow for it, insulated foam board to the outside.
Actually the foam boad goes under the tyvek. Some may say no need for
tyvek if you use foam board.
Anyway, the advice is free.
Yes, the insulation is cotton batts, I put them in when I remodeled
the interior side. They are _so_ much nicer to work with than
fiberglass batts. Speaking of cellulose fill (with a binder), does it
create an effective air barrier? Should I consider pulling out the
batts and spraying in cellulose? It's more economical than foam.
Umm, these cotton batts are a "modern conventional insulation", they
are treated with borates to provide fire and vermin resistance. See,
e.g., <http://www.bondedlogic.com/ . Cotton batts are a much nicer
alternative to fiberglass batts, but more expensive.
Anyway, the original question was about batts insulation in general,
and sealing the batt/framing interface.
Maybe I need to get up to speed. I have not seen any for many years and
it was old then. I did not even know they were selling the stuff. It sounds
like they have done the same kind of work on it as the cellulose stuff.
Blue jeans have a good R value, I have cedar, I removed it, ran 2" of R
7.2" foamboard for R 14.4, plus foamed the cavity for R 32. I put 3/8
Osb then tyvek, only to protect the Osb. But Im zone 5 to - 20f. I dont
know your extremes, zone, but im sure cooling is the issue. 1/4"
foamboard of blue or pink is apx R 1.25, foifaced foamboard apx R 1.8R.
Not very much for all that extra work, then you need Osb. If you want to
foam add more the labor is the same, tyvek alone will help alot without
more insulation. There is R 7 foam blown in but it outgasses, and R5
that does not outgas. 1" of 7.2R per inch foamboard would help alot, but
your windows and trim will be recessed, I put in new windows and doors
while doing this, upped attic insulation and new Hvac, my utility bills
are down 75%
Well, I fall on the felt side of the tyvek vs felt argument, so I'll
be using 30 lb felt. The siding is cedar shingles, and at least some
cedar shingle manufacturers recommend felt instead of tyvek. I may
use cedar breather (1/4" thick mesh) under the shingles as a mini rain
As to the foam board, is there any value to 1/4" foam board? Would it
be an effective air barrier, and what would I use to tape the seams?
I don't want to make the wall any thicker than it is, as I don't want
to mess with the frieze board and molding details at the top of the
wall. So I'll be leaving 2-3' of the 7/8" plank sheathing at the top
of the wall, and there I'll just have to seal the gaps in the
sheathing with spray foam.
But the new sheathing will be 1/2" or 3/8" CDX, and I'll have 3/8" or
1/2" of thickness to make up. So I could use 1/4" foam board and 1/4"
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