Another foam sandwich question: retrofit options
First, I am a DIYer. There is no pro involved. That said, I try very hard to educate myself…
I’m in the process of upgrading windows 1 by 1 in my house. When I do this work, I’ve begun to take the approach of stripping all drywall, polyE, and existing (generally poorly installed) fb batts. I then custom cut and foam in polyiso foam board and don’t replace the polyE. However, the house also has 1″ of XPS outsulation with an interior-to-the-XPS 1.5″ air gap and someday, when the stucco gets redone, I’ll probably put more outsulation on. Stucco is directly on the XPS.
Based on some of my readings here, it seems like I’m committing a crime against vapor here. I have darn impermeable polyiso in the stud bays with XPS outside (granted there is the weird air gap behind the XPS). I could go to wet spray cellulose but I’d be going from ~R-33 with poly iso to ~R-19. That hardly makes it worth pulling the bad batts.
I live in a dry climate Zone 5b (Colorado) that generally doesn’t experience wind-driven rain.
I’m inclined to try to get away with this practice, especially since I need to do something about foundation wall insulation in the basement and the idea of using batts or cellulose in that location is very unappealing.
I guess I’m looking for someone to tell me I’m insane or meh, it will probably be fine. I’ve read several of the Q&A questions and a dozen or two of Martin’s articles. Enough to know that this could be a bad idea as I’m not providing a way for the wall to dry but also enough to realize there isn’t one way to do it.
After all, the house has stood for 26 years with XPS on the exterior and polyE on the interior…
One alternative is to do no insulation work until I deal with the stucco, likely years down the road.
Thanks for reading and sharing any opinions or ideas.
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Replies
Keith,
Before anyone can answer your question, we need more information on the air gap between the exterior XPS and the next interior layer. This type of wall assembly is very unusual.
If there is a 1 1/2-inch-deep ventilated air gap between the XPS and your house, then the XPS is useless. You might as well hang the XPS on the clothesline in your backyard with big clothespins.
The only way you might get some benefit from the existing XPS is if the builder of your house created a very airtight sealed cavity between the XPS and the next interior surface. That's highly unusual. Moreover, any air movement through this cavity -- even through relatively small cracks -- would seriously degrade the thermal performance of the XPS.
Yes, my wall system is goofy.
Briefly:
House was built in 1976. Work in 76 seems to be pretty good. Vertical pine siding originally
House was remodeled in 1987. All work from 87 is below my own skill level (advanced DIY) but extent probably means a contractor was involved. Some very bad work. Ex: hidden electrical nuts in walls, penetrated shower membrane, no or bad subfloor/underlayment work, missing air returns, and this weird stucco wall.
The modified the wall in 1987:
1976 Original wall I->E: drywall, polyE, fb batt, soltex/plywood corners, pine siding (have not yet found building paper, tar, felt, WBR)
1987 New exterior shell: from pine siding (still there) out: lath stacks and other misc lumber creating an airgap of ~1.5", 1" XPS, metal lath and 3/8" stucco ("one coat").
On some walls there is a weep screed (not airtight at the bottom), on some walls the bottom of the wall is totally open. On one of those wall, I steel wooled it and can foamed it because the mice were using it as an entry point. That was before I realized that gap could be of value.
With the pine siding extracted in an area of work (under window), I have a 2" gap to either 'shim' out or very conveniently can install poly iso. But in doing so, have made a foam sandwich in this area. It isn't so much about this one 3'x7' area but about how to deal with the rest of the wall.
It is worth mentioning that 90% of the stucco is holding up well but that there are no visible expansion gaps. On the southwest wall, that has resulted in some cracks and bulging. On other orientations, the wall is doing pretty well.
BTW, thanks for the great article about innie/outie windows. I'm dealing with that right now. I find it very surprising there isn't more info on the web about that issue. My 76 windows are all innie due to the stucco upgrade. Windows replaced or moved in 87 are outie.
Yeah, she's a bit of a funky house but she has good bones (2x6, clear spanning trusses, poured concrete foundation, epdm roof).
Keith,
If you are opening the walls from the interior and can gain access to this air space, you need to seal all air leaks in and out of the air space, and/or fill up the airspace with insulation -- ideally, insulation that isn't air-permeable. Because air leakage renders your exterior XPS worthless, the air sealing work is of the highest importance.
The best way to proceed would probably be to expose the air space from the interior and fill it full of spray polyurethane foam.
Hmm, ok. I'll have to think about the spray foam option. Because of my unit of work size, I've avoided spray foam (concern over loss of aged product etc).
Going back to the original question, however, I'd still have a foam sandwich. Or a foam and polyE sandwich.
Air sealing: I had a blower door test done. Despite the 1970s windows and one bad door fenestrations, the house tested 0.25 ach. I'm not sure what to make of that. The energy audit company was not great in other ways so perhaps the blower door test was inaccurate. On the other hand, I've used a thermal gun a friend owns and didn't find anything interesting on a winter night (the usual can light leaks and exhaust fan leaks which I'm slowly dealing with and a couple 80s era skylights that obviously aren't low-E).
It seems like, both thinking about this project and another project I have (basement foundation insulation), that maybe my interior approach is the wrong way to go about it. Perhaps I should be leaving the interior drywall intact in most cases and strip the exterior instead one wall at a time. Leave the interiors fb batts where they are for whatever value they convey (R-19 x corrective factor x loss due to less than perfect install ~R11?). That does leave polyE in the wall throughout the house but as you pointed out in other articles the polyE probably has lots of cuts in it.
New wacky idea, which I understand you probably can't actually endorse.
Cut existing stucco/XPS package off the wall with concrete grinder in sensible pieces. I know from this window work that if I'm careful, I get a combo panel off. (see caveat below) Cuts very easily.
Strip wall of haphazard lathing (airgap) and pine siding (possibly harvest for re-use interior).
Install housewrap on plywood/soltex layer (resist urge to remove soltex, which I really hate)?
Install two layers of 2" poly iso staggered and taped. (~R-26 for the new stuff, interior batts worth ~R-11 = ~R-37...past the point of value in my climate zone).
(Optional?) Install a stucco drain mat product over the 2" of polyiso.
To the extent they survive, screw the well-planned cut panels of stucco and XPS back onto studs through the polyiso with expansion gaps installed in the cuts. Plan B is entirely redoing the stucco.
The catch might be that I may have removed those XPS/stucco chunks in nice pieces previously because I had the wall open and cut/removed a lot of long lathing nails. If every lathing nail is a pull through, I'm guessing that idea is as busted as the stucco. Well, it might be worth an exploratory try as long as I understand that entirely redoing the stucco is Plan B.
That kind of sounds like months of work at the DIYer pace. Ugh.
Remodel work is tough. To bid out stripping the house, adding 4" of polyiso, and replacing the stucco while upgrading the windows would exceed $50k without the window I'm guessing. As a DIYer, the cost is probably closer to a $10k.
Well, thanks for reading and the advice. No easy answer I think?
When trying to fill a 1-1.5" gap you may be better off using non-expanding injection foam (TriPolymer, CoreFill 500, etc) than an expanding polyurethane, which can be done without gutting the wall. The stuff runs about R5/inch, and is fairly water-vapor permeable, but air-tight. If there's nothing in there but a few random 2x lumber blocks as spacers it would add an average R7-R8 to the stackup.
While expanding polyurethane pours might get you there too, the risk of cracking the stucco is high, and a full cavity fill isn't as guaranteed. The maximum pressure with non expanding injection foams just isn't all that high, and is controlled/driven by injection system during installation.
Since you have a ~1-perm vapor retarder (the XPS) and the cavity, the all the drying of the stucco is to the exterior. As long as the pine siding/sheating is dry when you install it you should be just fine, but, since it would otherwise need to dry to the cavity, it will only dry very slowly through injection foam. Inspecting and correcting the window flashing details could be an important preliminary. (With high windows and 2' roof overhangs it's less critical than with minimal overhang.)
Dana,
Zero overhang. Walls have metal flashing caps: naturally with screws through the top, since they did nothing 100% correct during the 87 remodel,