Accidental deep energy retrofit
Hi,
I’m doing a gut renovation of a stucco-sided house in the high desert. We changed most of the window sizes to improve the passive solar performance of the house, which means a lot of stucco repairs. Now my contractor is telling me we should remove all the stucco and start from scratch, rather than trying to tie new tar paper to old tar paper at the studs. It’ll cost a little more, but we’ll get a better result.
So now I’m thinking we have an opportunity to add outsulation. What I’m visualizing is: Install plywood sheathing around all the framing for sheer (there’s just wire and paper right now, which means the stucco is proving sheer?), put two layers of tar paper over that, furr the window openings out with 2x2s, install 1.5″ polyiso boards, install and flash the new windows and then do regular cement stucco on top of that.
Have I got that right? Is there a better configuration? I can’t find any information about polyiso under stucco that isn’t EIFS. Obviously all this is going to cost more than anticipated, so we are trying to control costs, but I want to get it right.
Thanks for your help!
Reanna
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Replies
I recently had stucco guys apply cement stucco right over new EPS foam insulation on my New Mexico house. The WRB was under the foam, so they lathed right over the foam and applied the stucco. In order to avoid having to fur out the window boxes (I also installed new windows), I had them bullnose the stucco so that there is a nice rounded edge leading deeper into the wall to the windows. If you want to install your windows on the same plane as the new stucco rather than inset a bit, you should really put your WRB layer over the foam, not under it--otherwise you'll have to extend it around the furred-out window boxes which will be a nightmare. If you use polyiso as your foam, I have a further recommendation: use foil-faced polyiso and then tape the surfaces with high-quality foil tape, and use that as your WRB. Then, build a furring-strip rainscreen assembly as if you were going to install siding, and have the stucco guys fasten their paper and lath to the furring strips. This will allow the radiant barrier on the face of the polyiso to work for you, and it will keep moisture in the stucco (when it rains during the monsoon season) in the stucco, rather than trying to push its way through the WRB and into the foam.
You can get away with using the foil surface of the polyiso as your WRB because of the secondary WRB provided by the paper-backed lath that your stucco guys will have to use. Between the two of those and the high desert climate, water won't make it into the foam unless there are flashing errors around your windows or the wall-roof intersection, especially if you have a flat roof.
I like the idea of bull-nosing the stucco around the windows. Sounds like an aesthetic improvement. So you flashed the windows over the WRB and not over the foam?
What did you use for your WRB?
Another question: do we know how foam holds up to baking under the stucco on 110 degree days, for many years? Do some foams do better than others?
Reanna,
In your area -- if your climate is very dry -- it's possible that wood-framed houses finished with stucco rarely have water-entry problems. In other parts of the country, lots of stuccoed wood-framed houses have rot.
In a dry climate, you might be able to skip the rainscreen gap between the stucco and the foam. However, including a rainscreen gap is always the best way to go. For more information on traditional stucco installed over a rainscreen, see To Install Stucco Right, Include an Air Gap.
Martin,
We get an average of 4" of rain a year, usually in just a handful of heavy rains, which means the stucco is usually able to dry out quickly. We have the house completely open and haven't found any indication of water damage on the outside of the studs, and the original part of the house is 60 years old.
We also have low-slope roofs with fiberglass insulation and no ventilation. The wood there also looks fine. In some ways our climate is too forgiving -- local builders don't seem to take detailing very seriously. Even air-sealing and insulation details that would improve energy efficiency.
Reanna
Martin,
Here in the high desert, just about every wood-framed house is finished with stucco, including those with flat roofs. For the most part, they are fine due to the extremely low annual precipitation. 7" per year where I live.
Reanna,
Double-layer tar paper WRB for me. I flashed the windows to the WRB with good window flashing tape. Then the foam went up over that, followed by lath and stucco, with no ventilation gap. If I had to do it all over again, I would have used polyiso instead of EPS and gone with the radiant barrier and vented gap. Not because of any water retention or entry problems, but because the radiant barrier would help a ton in the summer, especially with the harsh western sun when the stucco gets to be like 130 degrees. I do worry a bit about the unprotected EPS under there. Polyiso is definitely more resistant to high temperatures than any flavor of polystyrene is. Originally I asked for mineral wool, to reduce expansion and contraction and alleviate temperature worries, but nobody had ever heard of it, and the polyiso bid was a good deal more, so I acquiesced and went with EPS. It's a shame that we don't have available the kinds of rigid mineral insulation boards that are common in Europe.
I agree that the bullnosing is a nice aesthetic effect. I've attached some photos of my windows.
Thanks for the photos, Nate!
I'm confused about radiant barriers. I've read in different places that they're amazing, or that they're not worth the money. Are people talking about different applications, or is it controversial?
It is controversial because they are overrated and not worth any additional money spent on them in a heating climate/season and this site tends to have a northeastern USA cold climate tilt. In an extremely sunny and dry cooling climate/season like we have, radiant barriers outboard of the insulation are great and can make a big difference in felt comfort and reduce cooling bills, especially if you don't have a superinsulated building envelope. And if the radiant barrier is basically free because it's already there, laminated to the surface of the foam, there is no reason not to use it.
Finally, see if you can get them to put some fly ash and ground granulated blast furnace slag in their stucco mix. These additives will increase the strength of the stucco and reduce its porosity, making it more durable, which is especially important if it's a thin layer and it's being installed over an airspace rather than solid backing. It also reduces efflorescence, which can be important given the alkaline aggregates available in the southwest. Finally, these additives are usually cheaper than cement, so the stucco company can actually save money by replacing cement with them. Fly ash and GGBFS can replace cement on a 1:1 basis; up to about 30% with fly ash and up to 85% with GGBFS. You still need some cement of course. Talk to the GC or the owner of the stucco company and point them to these resources:
http://slagcement.org
http://www.everything-about-concrete.com/fly-ash-concrete.html
Could be a good competitive edge or cost-cutting move for them. And of course, replacing cement with recycled waste materials is good for the environment too.
Hi Nate,
What did you use to attach the lathe to the framing through the insulation?
The stucco contractor came out this morning and told me 2 things: bull nosing will cost about $50 per window, because he'll have to buy metal for it. And that he'll need to hand-nail through the insulation, because his staple gun can't hold long enough staples.
Reanna
4" cap-headed nails were used to fasten the insulation to the framing. Your stucco contractor should be putting extra metal lath around the windows regardless of whether or not you inset them and bullnose the corners. Otherwise the stucco will be more likely to crack.
Hi again!
Contractor is telling me the only way he can do stucco over foam is to use western one coat (and possibly that he can only use 1" of foam, instead of 1.5"). I have not heard good things about this product (even from him!). I wish I could find a wall section drawing to send him! Any ideas?
Reanna,
You can always switch to EIFS if you can't find a stucco contractor willing to do what you want.
My article on the topic (and the comments below the article) discuss these details: To Install Stucco Right, Include an Air Gap.
Attached below is a detail from GBA's detail library. Although this detail doesn't show a rainscreen gap between the metal lath and the building paper, I think that such a gap is an essential component of a durable stucco installation.
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It is a completely simple process. After the foam is up, attach lathe with 2-4" nails (depending on the foam thickness) pounded into the framing, using offset spacers to keep the metal from touching the surface of the foam. Apply combined scratch and brown coat. Wait 3 weeks. Apply color coat. It's no different from applying stucco over anything else. I disagree with Martin about the necessity of an air gap in your climate. In general it is absolutely a good thing, but installed over foam and with only four inches of rain a year, the wood in your walls will be just fine.
Martin - are there drawings showing the bottom of the wall? I think that's where the contractor is tripping up.
Let me also say that I think that if you are using an air gap directly behind the cladding with no other support, stucco is a bad choice. The thickest stucco is three quarters of an inch thick and the thinner one-coat applications can be thinner than a half inch thick. Stucco is basically a thin, weak, non-structural form of concrete, and concrete is weak in tension. Tension is when you push on something that's unsupported. Stucco installed over an air gap will have no solid support except at the furring strips. With no solid backing behind such a very thin layer of stucco, it will be very easy to simply push on it and crack it. If using a rainscreen cavity for the wall assembly, I think stucco is a bad choice and I would recommend a cladding with far more tensile strength or flexibility, such as wood clapboard, vinyl siding, brick veneer, or stone veneer. Perhaps you could build a non-structural veneer wall out of thin adobes or slump blocks, which would visually fit in well in most parts of the southwest, and be far stronger than stucco too.
Reanna,
See below. More detail drawings are here:
http://www.wallsystemdesign.org/content/exterior-system-details-stucco
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Nate,
It sounds like you've changed your thinking since your earlier post, where you said you would like to have left a radiant barrier gap?
I was not planning to do the gap, just stucco directly over foam.
Another option we discussed was stained exterior ply (smooth T1-11) with battens, so I guess that's the fall-back, but aesthetically I prefer stucco at this point, because we are using reclaimed redwood and corrugated metal on other parts of the house, and the three materials make for a nice contrast.
Yes, I thought about it some more and realized that you'd take a huge durability hit by installing stucco without a solid backing. Stucco right over foam with no air gap will be just fine in your neck of the woods.
It's funny, the stucco we are removing is just backed by paper, no sheathing behind it. It's a pain when you want to replace a window or whatever, but not at all fragile in the way you're talking about.
What my contractor and the stucco rep he talked to are concerned about is the weight of the stucco hanging on the staples an inch and a half out from the framing. They don't think it can be done.
I'm talking to the contractor about using nails and spacers instead of staples, and it seems like he's open to it. I don't want to do anything stupid, but it seems like it *has* been done, at your place and presumably elsewhere. It's just not very common, so everyone's being conservative.
Nate, I agree with your comments about stucco taking a hit in strength without solid backing. Just to clarify though - tension is the force a material experiences when it is pulled apart, not pushed. Pushing an unsupported stucco wall would induce compression in the surface being pushed, and tension in the material on the opposite side.