Yes, I far as I know, you need the walls opened up to add spray foam insulation. The product will form a "bridge" if you tried to spray it in from a access point on a closed wall.

I don't know what area you're in, but in our area, the rule of thumb is $1.00 per sq foot per inch, provided you need a minimum of 250 sq ft 4" thick. That's because it is very labor intensive to set up the equipment and clean the equipment after spraying.

As noted by Danvirsse above.

Kaschka, even if Great Stuff worked in this application (and it really doesn't) you would be spending a few hundred $$ anyways, just to buy enough cans to foam an area that size. You're better off with a foam kit or just biting the bullet and having it done professionally. It hurts the wallet, but you'll be pleasantly surprised at how much it can increase your comfort level in a formerly underinsulated space.

great stuff comes as closed cell in a red can
as open cell in a blue can.

just fyi

best of luck.

best of luck

How droll!

When even the manufacturer says it won't do.

just clarifying what types of great stuff are available
worthy.

seems to me the point has been made that pros
need to do the install.

but
at the very least a determination of access
to kneewall area would be needed.
createing an access, if one isn't there,
and then foam the wall from the attic/kneewall
side.

as surely you know this,with your experience with
foam.

there are more that just foam to seal kneewalls
as evidenced in pdf file from southface.
this is a better way IMO. insulate then
use foam sheathing & caulk etc to air seal wall.

page 5 shows how to seal kneewalls
& other key air sealing area how to's

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

and yes, as always

best of luck.

Hello could you say more about other ways of insulating besides foam . I did have a foam guy come look at it, and give me a quote for foaming the underside of the cathedral ceiling (the slanty part) The knee wall wouldnt be done, but the ceiling inside the closet (on the other side of the knee wall) would.

$500 really isn't too bad for the work hed do, but its a HECKUVA lot of hassle on my part, removing plaster, lathe and grungy old rock wool. My lungs hurt just thinking about it! (Yes, I will wear a respirator.)

What I'm wondering now is 1) is it worth the effort, is it going to make that much of a difference - since the rafter space would only be 4 1/4" (they would fill to the top and I would shave off the overflow). And 2) is foam really the best way to go?

NOTE that there are NO soffit vents and never were - the attic is small and appears to be dry and well ventilated by the 2 gable vents. there is old rock wool in the cathedral ceiling now so the idea then as now was just to plug it up.

Thank you all - your advice / feedback is greatly appreciated!

if you copy & past the link in my post it will show other ways
of sealing & insulating kneewalls.

so you are taking down the ceiling (Lathe/plaster &
rockwool? good to know this information, as it
affects the answers we give you.

what type of foam? open cell , closed cell?
open cell will give you about R-13
closed R-30.
the former wouldn't be advisible, the latter
only in areas where R-30 meets code requirements.

foam is the quickest insulation & air sealing in one
install. that is its plus as opposed to regular insulation
where you have to air seal before insulating.

they would foam not just the ceiling above the room
but on each side of the room. the kneewalls.
look at the pdf.

where are you located?
is this a new to you home?
what is the reason for wanting to foam
insulate? is the room uncomfortable?

best of luck.

Type of foam - closed. The guy said that in our area (wisconsin) 5 inches would be optimal, but 4 would do.

It's an old 107 year old house. I didnt really plan to do this but it was more like - the ceiling plaster was loose and coming down anyway, do we want to insulate while it's open? The prime motivation is ice dams on that side of the house, but also it does get very chilly in that room when northwest winds are blowing.

Plus I just like the idea esthetically of cleaning out the grungy junk out of there and filling it with foam!

This stuff looks kinda interesting but not sure about that air channel in the back - as I say, the soffit vents don't exist so there is no air movement through there.

http://www.reflectixinc.com/basepage.asp?Page=DIY+Cathedral+Ceiling&pageIndex=542

http://www.energyvanguard.com/blog-building-science-HERS-BPI/?Tag=scam

don't waste your time & money on that product.
radiant barriers work by reflecting out radiant heat
in cooling climates. little if any value in cold climates.

best of luck

Oh darn - I suppose that means the cute water heater jacket made out of that stuff won't work either.

Actually - if you could answer just one more question. I understand that it is not insulation and it won't keep cold out, but what I would be using it for is to keep the heat IN and keep it from going out through the roof and melting the snow on the roof which is what causes the ice dams. Wouldn't the heat be reflected back into my house instead of warming the roof? I guess I'm not understanding the difference between radiant heat (sunlight) and other kinds of heat.

Thanks again - was just about to go out and buy that stuff but certainly wont if it doesnt do what I want it to.

that is exactly what it means, this fbf foil bubble foil stuff isn't for water heaters either.

convective heat loss is what you have when
heat is escaping from your house into the attic.

radiant heat is what radiant barriers reflect, ie heat
from the sun.

the third type of heat is conductive. like when you
cook & leave a metal spoon in a pot on a hot stove.
metal is very conductive and the heated metal will
burn your hand, wood on the other hand isn't
a good conductor, which is why you can leave
a wooden spoon in the pot while stove is on.

I'm no cold climate expert by any means. but
air leakage is always the enemy. if you have
holes in the ceiling in a cold climate heat escaping
would contribute to ice dams. recessed lights
oversized cuts around bath & stove vents etc.

putting the insulation in the attic in contact with
the ceiling...after air sealing...is key.
this is why foam sells, it air seals & insulates in
one swoop.

fyi:
http://coolcosmos.ipac.caltech.edu/cosmic_classroom/light_lessons/thermal/transfer.html

best of luck