What is the insulation in the rest of the house? Why would spray foam be an advantage in a knee wall?

No spray foam insulation. They wanted to install polyiso over the batt insulation.

I charge a hefty price to use foam board to seal kneewalls.
just bid (& got) a job where materials are $900 and install of
materials is another $1500
my bid includes insulation in walls not already
insulated, 1" R-7 foam/foil sheathing on walls
from top plate to bottom plate.
foam to continue from bottom plate to between
floor joists to attic floor.
all air sealed.

this is very labor intensive work.
while you can pre-cut some of the foam board,
other areas have to be fitted in that cramped attic
space.

I'd make sure that they were using a button cap
or roofing nail to attach the foam board.
that the foamboard is caulked to top & bottom plates.
all seams taped & sealed.

to get an idea of what is involved take a look at
page five of this document:

http://www.southface.org/default-interior/Documents/airsealingkeypoints.pdf

note that #17, (the between floor joist air sealing)
is included. and that this area also is air sealed.

I usually use a combo of caulk & paint on mastic.
don't let them seal the foam board with great stuff
foam. caulk or mastic. and be generous with both.

best of luck

Thanks for the reply, that helps a lot. How many sq feet was your bid for?

110 sq ft. small attic space to work in, wall isn't straight.
has bump out for upstairs bathroom, which will need extra
time to do because of the way the shower unit was installed.
there are also two storage spaces assessable from the
upstairs bedroom that have to be sealed. one side is included
in the big area of install, the other side & continuing to the
front of the house is smaller, more cramped area to work.

I see 20 hours of work if all goes as planned.
4 hours to clean the walls of any thing that will keep
the foam board from installing easily, and making working
space in the attic. the heating system & ductwork also make
for extra time as these have to be worked around, but the
air seal has to be continuous.

open cell foam would be an option for her, but she can't
afford it all at once. thus the reason for breaking the
job in to more affordable bits.
this is a 100+ year old balloon framed house with
several renovations.

in addition to this house, I have another house, 5 years old
mcmansion (7000 sq ft) where at every change of ceiling height
the kneewalls need to be air sealed. haven't worked up the bid
for that one yet, may just charge straight fee per hour.

can you post some pics of your areas to be sealed?

Would the foam board be on the warm side of the studs and covered with drywall. For that price you might as well use spray foam to air seal the wall.

no the foam board would be to the attic side. insulation in stud bays & sheet rock to the inside of the room. as shown in link pics.

spray foam would do the same thing, if the installer takes the
time to move insulation at attic floor & seal between floors.

homeowner has to weigh the cost of foam & how fast it is
vs the cheaper material & high labor costs.

every house is different, in some homes it is next to impossible
to run the hoses for the spray foam.
some sf companies won't take smaller jobs, although in the
slower times of year may.


hope this clarifies

Sorry for not responding sooner, I don't get to check this forum that often. I'll take some pics either today or tomorrow, judging from what you are saying the price seems fair.

Not sure why gardenweb is rotating the photo, but this is the type of insulation that will be covered