The "old" part of my house (built in the 50's) has 1/2" foil-faced polyiso on the outside that a previous owner has added at some point as exterior sheathing directly to the studs. There is no plywood or OSB. Cavities are filled with cellulose, sheets of polyiso were not taped at the seams. Here's a picture ( might have to click the link if I can't get the picture to show). http://i3.photobucket.com/albums/y80/strawmyers/House/IMG_0997.jpg I have 50 4x8' sheets of 2" of Type IX EPS that I had purchased for an underslab project that I won't actually be using, now. I intend to now put this on the outside with the seams taped. Do I need to pull off the foil-faced polyiso first and replace with OSB from a vapor permeance standpoint; or can I just apply over the polyiso? If it was 2" of Type II I'd feel fine; but I know the Type IX has a lower permeability. Thanks!

You should be ok.
Tape the seams of the iso- I would use Zip tape, but there are other alternatives. Add the eps on the outside if there is no sign of moisture problems already In the walls. The foil faced iso is a vapor barrier and an air barrier if the seams are sealed. Adding 2” of type 4 Eps on the outside should make the assembly less likely to have problems in the future, and get you good assembly in almost all climate regions. If you are in a rainy region use a rain screen detail outboard of the eps, if not, use it anyway. You look like you have good overhangs so it should be fine from bulk water intrusion, but a rain screen detail is a nice assurance
Where are you located?
Cheers, Eric

Sorry, I should know better. I'm in central Indiana. I already wrapped the existing polyiso in Tyvec: the above-posted picture was taken during that project. I didn't tape the seams at that time because I was afraid it would cause more harm than good since the foil facer is a vapor barrier. I have every intention of taping the EPS with some Siga tape once it is installed. I'll leave the house wrap in its current location because the windows/doors are already detailed to it and I will not be moving them out when adding the foam and furring strips. Thanks for the piece of mind!

Not taping the seams of the iso doesn't appreciably improve the drying capacity of the assembly- it only allows air to leak more easily through the wall at the studbays nearest the seams. Air movement through the wall goes both directions, but is as likely to move moist air to a cold spot where it will condense or adsorb and cause a problem as it is to remove moisture from an already problematic area.

Taping the seams with the appropriate FSK tape (aluminum duct tape) would have been the right thing to do.

The 2" of Type IX EPS on the exterior of the iso is fine, and would be fin even if it's vapor permeance was zero (instead of the ~1 perm that it actually is), since you're not trapping anything that can be damaged by moisture between the foil facer and the EPS.

Better yet, the additional ~R8.5 of R-value means that the inner facer of the iso will now stay above the dew point of your conditioned-space air, with less wintertime moisture accumulation in the stud bay, and the polyiso will now perform near it's rated R even at the cold-weather extremes.

So is it worth the (huge PITA) process of pulling the house wrap and taping the polyiso, then reinstalling the house wrap and re-detailing the penetrations before installing the EPS?  My assumption is that, once the EPS is installed and taped, that will retard any airflow anyway.  I have foil tape leftover from redoing the duct work; so I'd rather take the time to do it right if it's going to make a big difference.  And for the record, I do intend to stagger the seams of the EPS with the seams of the existing polyiso when installing.

How about the upper and lower borders of the EPS: worth running a bead of "foam board adhesive" along the top and bottom edge before installing to "seal" those areas?

My 2" thick sheets of 25psi Type IX borate-treated EPS say R-10... you're not telling me mfg might actually fudge the numbers a bit, are you?

taping the old layer and caulking the new layer as you propose would be a good idea. Do it now while you have the chance.

Just broke my heart, Bob  Okay, guess I'll undo all of the housewrap work I did 2 years ago, tape the iso, and redo the housewrap again.  As you said, now's the time to do it.  Thanks as always to everyone for the input!

Posted By strawmyers on 25 Mar 2014 10:17 PM
So is it worth the (huge PITA) process of pulling the house wrap and taping the polyiso, then reinstalling the house wrap and re-detailing the penetrations before installing the EPS?  My assumption is that, once the EPS is installed and taped, that will retard any airflow anyway.  I have foil tape leftover from redoing the duct work; so I'd rather take the time to do it right if it's going to make a big difference.  And for the record, I do intend to stagger the seams of the EPS with the seams of the existing polyiso when installing.

How about the upper and lower borders of the EPS: worth running a bead of "foam board adhesive" along the top and bottom edge before installing to "seal" those areas?

My 2" thick sheets of 25psi Type IX borate-treated EPS say R-10... you're not telling me mfg might actually fudge the numbers a bit, are you?

If it's generically labeled R10 they need third party ASTM C518 test data to stay on the right side of the law, so it could be that it's somewhat denser than most Type-IX EPS or it's slightly thicker than 2". 

Type-IX EPS typically 2lbs per cubic foot density and comes in at about R4.5/inch in ASTM C518 testing, and would hit R10 at about 2.25" are you sure that isn't 2-1/4" stock? (Measure it!) If labeled for sheathing applications using ASTM C578, it hits R10 at about 2.125", but it should explicitly spell out the mid-foam temp as 40F for that R-value (eg: http://www.foam-control.com/downloads/brochure/Foam-Control-EPS-R-value.pdf )

I'll measure it... but it was supposed to be 2"; and for the embarrassingly large chunk of money I had to give up to special order and ship the stuff, they better have gotten it right!

Measured 10 randomly-selected sheets; and they all came in between 1 15/16 and 2".

BTW, is there a "prefered" brand of foam board adhesive for sealing the upper and lower margains, as indicated above?  I had intended to use the Siga Wigluv tape for taping EPS seams based on recommendations I've seen in other posts.

I've used a number of foam-board construction adhesives, and they seem pretty similar. Loctite PL300 (available through the big box store chains) is probably as good as any.

So, if I'm removing the existing Tyvec anyway to tape the polyiso, is it worth it to replace it with back-draining house wrap before putting the EPS on top? Intuitively, I would think the air space behind the wrap would render the EPS worthless as an insulator; but I recall another thread where you (Dana) had said that is not the case.

Any drain wrap needs to go on the outside of the foam for that reason. I use drain wrap if I'm using horizontal strapping for wood shingles, but it is unnecessary for vertical strapping.

so from the outside:
cedar shingles
horizontal strapping
drain wrap
EPS
polyiso
sheathing
wall studs