I'm planning an energy retrofit of my stucco-clad house. The stucco is in only so-so shape, so I think I'm going to take the opportunity to remove it and add a few inches of XPS as a continuous layer of insulation over the poorly-insulated fiberglass-filled 2x4 stud walls. I'd like a new stucco coating over the foam but am worried about the weight. I found this awesome article:
http://www.buildingscience.com/docu...amed-walls

…Which says that with this kind of exterior insulation, the maximum cladding deflection for a heavy three-coat stucco finish should be 1/64" or less to minimize cracking.

But here's my idea: why not use surface-bonding cement instead of a traditional stucco process? It's basically the same thing, and it's already fiber-reinforced to resist cracking. It's also lighter in weight, and you can add liquid latex to it to improve its characteristics. I'm thinking that this covering would basically *be* stucco, but easier to apply and lighter weight. Does this sound sane?

What is between the old stucco and the 2x4's?  Is it plywood, OSB or insulated sheathing?  What shape is the sheathing in?  Fair, good or excellent?  Just curious.

Posted By Alton on 14 Feb 2014 02:54 PM
What is between the old stucco and the 2x4's?  Is it plywood, OSB or insulated sheathing?  What shape is the sheathing in?  Fair, good or excellent?  Just curious.

I'm honestly not sure. I haven't started the project yet; I'm still in the planning stages. The house was built in 1972 and the quality of construction was about average, so I highly doubt there's insulated sheathing there already. It's probably plywood or 1x6s or something. This is in Northern New Mexico so it's a drying climate. I would be very surprised to find much if any water damage. Any of it that's in will just ripped off and replaced with plywood sheeting.

Do the world a favor and use EPS rather than XPS for your insulating sheathing. The modestly higher R/inch of XPS is due to its HFC134a blowing agent, most of which leaks out in the first 40-50 years, at which point it's R value is about the same as EPS of the same density.

HFC134a has a global warming potential of about 1400x CO2. EPS is blown with pentant at about 7x CO2.

From a 25 or 50 year lifecycle point of view it's unlikely the lower energy use of that slightly higher R is ever going to be a net benefit to the planet.

In Europe HFC134a is banned for most uses, and XPS there is blown with CO2, (at about 1x CO2, go figure!? :-) ). But those goods have the same R/inch as EPS of the same density.

The cost per R per square foot of EPS is about 25% less than that of XPS too, so if you're not thickness limited, you can go higher-R on the same budget with EPS for an even bigger net win.

The difference between masonry cladding and surface applied cementicious or textured acrylic finishes on rigid styrene are significant- the latter has no vented gap between the exterior and the foam for penetrating rain to drain & dry into, and some of the acrylic based EIFS systems are much lower permeance than the foam, and are effectively exterior side vapor barriers. If your wall cavities have poly sheeting or foil facers on batts there is some potential for creating moisture traps, so tread cautiously. If there's interrior poly or foil, it needs a back-vented siding approach to have a path toward drying. Even 3" (R12.6) of 1.5lb density EPS has a vapor permeance of about 1 perm, which is sufficient drying capacity, as long as the exterior of the foam has access to exterior air- even a 1/4" space vented both top & bottom is enough.

Assuming you don't have interior side vapor-barrier issues, the biggest risk to the surface applied approach is from bulk-water incursions behind/below the window flashing with very limited drainage or drying capacity. If you take that road use a crinkled-type housewrap between the foam and sheathing which makes it both a drain-plane & capillary break between the exterior channels of the wrap and the susceptible wood. Take pains to lap it properly with the window & door flashings to create a single continuous drain plane layer with no discontinuities, so that any liquid moisture that makes it to the flashing stays on the exterior side of the housewrap.

Thanks for the great reply, Dana1! I hadn't even considered the EPS vs XPS differences and was under the impression that polyiso was the only one that would lose some R-value over time. Thanks for that info! What it sounds like is that my approach is possible but has some details I'll need to pay close attention to.

There is no vapor barrier sheet or foil-faced anything in the existing walls, and the fiberglass batts are all paper-faced. So the wall cavity can breathe interior air. Does your recommendation hold if the EPS sheets are all poly-backed? I assume that would substantially reduce the permeability, no? I'll try to get uncoated EPS sheets.

It sounds like you're recommending that my drain plane be between the wood sheathing and the foam sheets rather than between the foam and the SBC cladding. What if I applied housewrap over the EPS, moving the drain plane closer to the exterior of the wall. Part of this project will be re-doing the windows, so I have no problem installing the new windows flush with the new surface of the thicker wall to keep a continuous drain plane. This way, any water that got through the SBC cladding would hit the housewrap and stop there, and the bright New Mexico sun hitting that thin masonry cladding would heat it up and dry out the water before it could penetrate beyond the foam and touch the wood sheathing. How does that sound?

Posted By ILikeDirt on 14 Feb 2014 05:24 PM
Thanks for the great reply, Dana1! I hadn't even considered the EPS vs XPS differences and was under the impression that polyiso was the only one that would lose some R-value over time. Thanks for that info! What it sounds like is that my approach is possible but has some details I'll need to pay close attention to.

There is no vapor barrier sheet or foil-faced anything in the existing walls, and the fiberglass batts are all paper-faced. So the wall cavity can breathe interior air. Does your recommendation hold if the EPS sheets are all poly-backed? I assume that would substantially reduce the permeability, no? I'll try to get uncoated EPS sheets.

It sounds like you're recommending that my drain plane be between the wood sheathing and the foam sheets rather than between the foam and the SBC cladding. What if I applied housewrap over the EPS, moving the drain plane closer to the exterior of the wall. Part of this project will be re-doing the windows, so I have no problem installing the new windows flush with the new surface of the thicker wall to keep a continuous drain plane. This way, any water that got through the SBC cladding would hit the housewrap and stop there, and the bright New Mexico sun hitting that thin masonry cladding would heat it up and dry out the water before it could penetrate beyond the foam and touch the wood sheathing. How does that sound?

First, the wall doesn't need to "breathe" air, in fact air leaks on the interior side are far more likely to create moisture problems than fix them. Its the ability to dry via vapor diffusion that we're looking for, since that's usually the primary means of drying.  A square inch of air leak on the interior side can move as much moisture into cold sheathing in winter as a whole wall of vapor diffusion through wallboard, but it won't dry out quickly via the same path. (The thermal performance of the wall takes a big hit when it's leaking air too.) Drying of studwall assemblies is almost always primarily by vapor diffusion.

Asphalted paper facers on batts run about 0.5 perms when dry, and are pretty good vapor retarders, but more than 10x as vapor-permeable than 6-mil poly or 2mil foil.  Standard latex paints run 3-5 perms.  When a paper facer gets wet or the air on either side is nearly 100% relative humidity it becomes more vapor open, so something like 5-8 perms, so it's something of a "smart" vapor retarder, that can dry faster when moisture level in the wall cavity is high, and falls off when it's dry.

Most EPS with facers are fairly low permeance, comparable to poly sheeting.  I'm not sure if any of the standard EIFS finishes will bond properly to EPS with poly or vinyl facers, but those facers will significantly impede the ability of the assembly to dry toward the exterior.  Fiber reinforced cemeticious finish and unfaced EPS is probably a better approach than the sand-acrylic gluey stuff.  Most EPS with facers is also low density Type-I stuff, and not appropriate for this type of finish anyway.

Unless you're doing a serious re-mounting and re-flashing of all the windows & doors, the drain plane has to be between the foam and sheathing. If you're moving the windows outward and using the exterior finish itself as the drain plane, it has to be truly perfect forever to work reliably. While there is good, better, and best, I've yet to see "perfect" in anything construction, have you?  The requirement for near-perfection and maintenance of those susceptible places around windows is the real Achilles heel of many EIFS systems.  In less rainy places like NM you can get away with  it a bit better than say, Vancouver B.C. or Seattle, but it's still an issue that requires some attention.

When the sun heats up and vaporizes water inside the unvented finish it finds it's way MORE quickly into the sheathing, not less.  As liquid it moves primarily via capillary draw, and EPS is a pretty good capillary break, but as a gas it moves via vapor diffusion and any micro convection paths it can find.  Housewraps are fairly waterproof to liquid water but highly vapor permeable. Once that moisture gets by the housewrap as water vapor it'll concentrate in the coldest materials inside the wall assembly- most likely the paper facer on the wall board.  Whether it becomes an issue depends on how dry the interior is and how rapidly that water can move through the interior finish paint.

Both XPS and polyiso lose some amount of R with time, but the labeled-R value is something like a 15 or 20 average performances.  The fully depleted R value of 1.5lb polyiso is about R5.8-R6/inch, but it also has a derating curve with temperature to pay attention to in cold/very-cold climates.  The labeled R value is required to be at a mid-foam temp of 75F with a 30F difference across the foam, but while polystrene (EPS & XPS) increase fairly linearly in performance as temperatures fall off, polyiso does only so far, and once it's past the peak-performance knee it's performance falls pretty rapidly with lower foam-temp- it's a very non-linear curve, and the curve varies with how it's manufactured, all pretty arcane stuff, to be sure.

You really know your stuff, wow. I'm a little confused on this vapor diffusion drying stuff. If it's not too much to ask, would you mind explaining this concept in a bit more detail? For example, let's say it's raining and my wall cladding gets wet, and the housewrap is between the sheathing and the foam. What happens?

I wasn't so much thinking of using the SBC cladding itself as the drain plane, but if there's crinkly housewrap or some other ribbed, draining-type moisture barrier immediately underneath it, wouldn't that be the drain plane? I'm sure you're correct that this is a bad idea, but I could use a bit more explanation to help me understand it, if you don't mind.

If, on the other hand, the drain plane is between the wood sheathing and the EPS boards, that means the windows should be on the same plane rather than moved out two or three inches, right?

Posted By ILikeDirt on 14 Feb 2014 10:05 PM
You really know your stuff, wow. I'm a little confused on this vapor diffusion drying stuff. If it's not too much to ask, would you mind explaining this concept in a bit more detail? For example, let's say it's raining and my wall cladding gets wet, and the housewrap is between the sheathing and the foam. What happens?

I wasn't so much thinking of using the SBC cladding itself as the drain plane, but if there's crinkly housewrap or some other ribbed, draining-type moisture barrier immediately underneath it, wouldn't that be the drain plane? I'm sure you're correct that this is a bad idea, but I could use a bit more explanation to help me understand it, if you don't mind.

If, on the other hand, the drain plane is between the wood sheathing and the EPS boards, that means the windows should be on the same plane rather than moved out two or three inches, right?

The rain gets by the siding an hits the foam- if there's a vented rainscreen gap between the foam and siding the liquid water follows gravity downward, and the bits that cling due to surface tension starts drying into the gap (and convecting toward the exterior) immediately. If it's an impermeable EIFS cladding without a vented gap, the moisture gets into the interstitial spaces of the foam, and dries fairly slowly toward the interior.

Yes, the housewrap is part of the drain-plane, but only part- the other part is the  window flashing.  When rain gets by the cladding at the more susceptible window & door locations, that liquid moisture gets directed onto the WRB. When the WRB is between the foam and sheathing, it needs to be a crinkle type, which will allow liquid water to drain downward more readily. The crinkles are akin to a micro-rainscreen gap, but with orders of magnitude less convection (low enough to not impact thermal performance.)

If you're putting the housewrap between the foam & sheathing it's usually easier to flash the windows properly if the glass is roughly co-planar with the sheathing (an "innie" mount) as opposed to co-planer with the siding ("outie" mount). 

Posted By Dana1 on 18 Feb 2014 03:27 PM
 If it's an impermeable EIFS cladding without a vented gap, the moisture gets into the interstitial spaces of the foam, and dries fairly slowly toward the interior.

What happens in an ICF home when the water gets between the EPS/EIFS and concrete? There is no rain gap/drainage plane between the cured concrete and EPS and finished EIFS exterior. Where does the water go?

I assume it gets absorbed into the 6" concrete core, correct?

Surface bonding cement is tougher than stucco because of the fiber, but it is almost impossible to knock the fibers below the surface. That means you end up with a fuzzy wall. I've used the product on icf foundations for years but they are fuzzy.

Posted By Lbear on 18 Feb 2014 10:12 PM

Posted By Dana1 on 18 Feb 2014 03:27 PM
 If it's an impermeable EIFS cladding without a vented gap, the moisture gets into the interstitial spaces of the foam, and dries fairly slowly toward the interior.

What happens in an ICF home when the water gets between the EPS/EIFS and concrete? There is no rain gap/drainage plane between the cured concrete and EPS and finished EIFS exterior. Where does the water go?

I assume it gets absorbed into the 6" concrete core, correct?

It gets into the concrete core which redistributes it via capillary draw up/down and sideways. But if it's chronic & persistent it can also saturate & seep enough moisture through to affect the interior finish wall, which is why it's important to set up your window bucks properly to move the water out, and not seep or drip behind the EPS.

Okay, so I need to keep the water-resistive barrier over the plywood sheathing, because If I add foam insulation, then the plywood will be the first cold surface that any water to penetrate the stucco and get vaporized by the heat of the sunlight will condense on. If I put the WRB over the foam and co-planar with outie windows, then vaporized water will permeate through it and condense on the plywood, which is yucky.

Next question: This strongly implies that I need innie windows so that the windows remain a part of the drainage plane. If I really want outie windows instead, can I add a second WRB over the foam? Any moisture trapped between the two would just be within the foam and would eventually vaporize from heat or drain through the bottom, right?

No wait, that doesn't make sense. With the foam sheathing covering it up, the plywood won't get cold; that's the whole point of adding the foam! The first cold surface is likely to be the outer face of the outer-most foam board, reinforcing the need for a vented rainscreen between that board and the SBC. So if there's anywhere the WRB should go, it's there, especially if it's a type made for stucco that gives you an automatic ventilated gap.

Right?

FYI, this is climate zone 5, but I live at the southern-most edge of a county immediately to the north of a zone 4 county. So it's probably a 4.5. :-p The house does not have air conditioning and and there is no poly air barrier underneath the drywall.