kneewalls should be sealed as in this southface inst pdf
see page 5.

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

bonus rooms are surrounded on all sides, top and
bottom by extreme attic temps.
as a result these rooms are hard to heat/cool.

first insulate the walls, then use a foil faced foam
sheathing, caulk to top & bottom plates of walls
nail with button cap nails & tape all seams.
make foil/foam sheathing continuous from bottom
plate to ceiling of space below/attic floor. caulk
foam sheathing to make an air tight seal.

this will stop heat/cold gain from attic.
insulation performs @ its rated R-value when
it doesn't have air moving through it, thus the
caulk/tape to seal.

then you can decide how to heat/cool the room.
duct from existing system...window heat pump...
how you going to do that?

best of luck.

Thanks for responding: energy rater la

I see the website recommends sheathing (osb/rigid foam) for my knee wall. I have 9 4x8 pieces of this 1/2 inch--one side aluminum foil). would you be totally against it?

Also, instead of caulk, are you OK with using spray foam around the edges?

Thanks again

I prefer caulk. the foam expands so rapidly that
if often covers rather than seals. not like you can
push it into the gap like caulk, and it shrinks over time.
Caulk is less expensive, and you can get a good long
lasting seal when you buy 35+ year caulk.

sure, use the foam sheathing that you have.
face the foil so that it faces inside the room,
nailed to the face of the stud, with insulation
in stud bay.
don't forget to seal between floors.

post some pics when you've finished. btw...sometimes
I use roofing nails instead of button cap nails.

best of luck

Thanks for responding: energy rater la

I see the website recommends sheathing (osb/rigid foam) for my knee wall. I have 9 4x8 pieces of this 1/2 inch--one side aluminum foil). would you be totally against it?

Also, instead of caulk, are you OK with using spray foam around the edges?

Thanks again

{{!gwi}}

Took me a few weeks but here are some pictures BEFORE

After

Thanks for the advice everyone. Lets see if it was worth the effort and if the wife can keep it clean.

where is the foil?
if you've faced it into the stud bay & have insulation
in the stud bay, you've lost the value of the foil,
as it needs a minimum of 3/4" air space.

nice work though, I see you went with the
foam to seal rather than the caulk.

so the pdf explained the install clearly
for you? as I often share this info, having
feedback helps.

now...how are you heating/cooling the space?

nice clean job!

The foil is facing the heated area, however I was unable to get the max R value due to not providing the 3/4 inch of space. I'm aware...I guess air sealing will have to do for now.

I did use mostly caulk but resorted to foam in areas where the gaps were larger due to diminishing accuracy.....got tired of spending my days off up there.

Previous owner added that area to the central air system although its not very effective.

My wife uses the bonus room as a huge walk in closet...want to make it tolerable for her but not too comfortable where she might want to spend nights there :)

air sealing will make a lot of difference, plus the
added insulation value of the foam board.

I've never seen a dow product with foil. so I was
curious as to where you got it, and your location.
wonder if its something I can special order?
I usually buy R-max polyst. at @ $25 per sheet...
its pricy! (1" is what is stocked, although I read
of 1 1/2 & 2"...it is overkill for my area)

fwiw, facing the foil into the attic space would
have been the way to go...just so you know...
and for anyone else who reads this thread.

so my questions are above & if the pdf
explained well for you the install.
looks like you 'got it'.

I am trying to understand what you have done. Did you apply the rigid insulation on top of the insulation batts?

take a look at the pdf I linked.
this install is on page 5 I believe.

Energy rater:
Purchased @ Home Depot in New Jersey approx $16 a sheet. I only used there because I had a few left from a previous project and did it want to waste....
Pdf was self explanatory.

Mike:
Insulation was hanging off wall in bonus room above garage. Wanted to air seal and increase r value for cheap. And yes rigid over batts.

Looking at the photos the batt insulation was thicker than the 3.5 inch studs. I am going to guess it is an R-19 batt which has a thickness of about 6 inches. Perhaps it is even thicker. When you apply the sheets on top you are compressing the fiberglass which reduces the R value. The sheets you applied have an R value of 3.3. So you may have not increased the total R value, and there is a chance you may have decreased it!

The sealing you added will certainly help. I question how much sealing is needed for a an exterior wall that has no penetrations.

Is the bonus room heated? If not then added insulation is not going to improve the comfort level.

I see a sloped ceiling above. Is that the exterior roof deck? If it is are you going to insulate it?

I totally understand mike, however I'm not investing any more time or money. Going to be optimistic or at the least let my wife think that I made a difference in her bonus room(walk in closet)

What I should have done was nail 2x2 horizontally, then place the foam board onto that as to not disturb the insulation. will fight that battle some other year :)

Room is connected to the central air (Natural Gas). Natural gas prices currently does not justify spending more nights and weekends up there.

Mike the only way to get to the roof deck would be to remove drywall....not going to happen unless my wife gets pregnant with twins :)