Exterior Rigid Insulation
Project Location: Western New York between Buffalo and Rochester.
I’ve just framed an addition on to the back of my c.1835 farmhouse. Conventional 2×6 walls at 16″ o.c. with 1/2″ OSB wall sheathing. I have an opportunity to use 3″+/- high density spray foam in the wall cavity, leaving about 2″-2-1/2″ of empty space behind the drywall. The question is, given what’s already in place, how can I best supplement the cavity insulation to increase the total R-value of the wall? I have thought of three scenarios:
1.) Add 1″-2″ rigid Styrofaom panels on the exterior over housewrap. This adds time, materials, and extra details to the trim and siding installation, though. I’m also concerned with sandwiching the OSBwall sheathing between the spray foam and the rigid foam. If It ever got wet for any reason, it wouldn’t be able to dry to the inside or outside. Mold? Rot?
2.) Add 1″-2″ rigid Styrofaom to the interior side of the framing before drywall installation. This seems easier, as I only have to detail windows and door from the inside, a job that has to be done anyway with jamb extensions. We’d just be installing deeper extensions. But, does this assembly also trap moisture between two layers of foam?
3.) Increase the spray foam thickness in the wall cavity to the full 5-1/2″ depth, and install the drywall at the interior as usual, and install siding over the OSB/housewrap conventionally.
If anyone can provide some insight in to cost-effectiveness and workability of any of theses scenarios, or has experience with another way that works, please let me know.
Thanks.
skinnymaplesco
Replies
Naughty naughty, asking a vapor barrier related question without stating where this construction is located!
my suggestion based on not knowing where you live...
Apply 2" if foam over your OSB and tape the seams. The foam is your vapor barrier now. Put a rain screen detail over that to allow water penetrations to drain. If you alreadly placed your windows, redo them to work with the new exterior plane.
On the inside of the cavities, you should blow cellulose in Dense Pack. Any water that does get into your wall can now dry out again to the interior of a house.
This assembly is R30, BTW.
or wet spray cellulose add on top of spray foam inside.
Exterior Rigid Insulation
Hadn't thought of that. Good idea.
Thanks.
check this out:
http://forums.finehomebuilding.com/breaktime/construction-techniques/rigid-foam-under-sheet-rock
A 2004 thread on what you might be thinking.
The beginning is pretty good, but do read further-Mike Smith has a post explaining the Mooney Wall concept. After you get done with at least the first page of posts, feel free to continue.
Further information (and there's a lot of it) on the Mooney Wall concept can be had using the google search box up top of this page. Try Mooney Wall-a plethora of information, questions and answers................
Best of luck.
Exterior Rigid Insulation.
I'm in Western New York between Buffalo and Rochester.
Thanks for the reply.
Applying the 2" of rigid foam on the exterior seems very effective, but is also the most labor-intensive process from the foam through the trim and siding phase. It also seems like the method requiring the highest degree of difficulty on getting a fair amount of details right to avoid moisture issues down the road.
Any experience with applying rigid foam panels on the interior before drywall?
Thanks.
interior foam
I have seen people do this years ago adding insulation to the out side thought it was a bad idea ,
no around here in ottawa ontario canada , people are having issues with this system, rotting walls ,and ants seem to just love the sm for some reason .or do they just love the rotting wood
new constrution as dry as you may think it is ,there are alot of variables to worry about ,I even put saw notchs betwen the studs 2 top 2 bottom to let any moisture out .that is in the out side sheathing .
I have been adding styro to the inside of the walls for years not the outside, I could never understand the logic there
you need 1 vapour barrier here anyway and it is to go on the warm side ,not on both sides .
there is styro made for this purpose and it has strapping @ 16" oc, electrical just goes on the face of the stud over the vapour barier as the does the wireing so no vapour barrier penetrations ,we are trying to keep the heat in in the winter ,and the cool in in the summer ,
now we use polyurethane foam and still use styro on the inside
but i bet many will have a differnt opinion .on styro on the outside
I can send you some details on this system if you would like