traditional stucco in semi cold climates
Last Post 25 May 2010 01:19 AM by Simon_D. 18 Replies.
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RsipgeoUser is Offline
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05 May 2010 08:14 AM
I'm building a couple of houses near the Albany NY area and my client wants a traditional stucco finish. I have heard that traditional stucco is not good in this area and that EIFS is used instead. I have not seen any stucco used in this area. In Philadelphia it is quite common and there are thousands of guys that do it. She really prefers the look of traditional stucco. Can a traditional 3 coat system be used up here? Is there any way to get a drainage plane behind the stucco? One structure will be built with EPS foam SIPs and the other is stickbuilt with closed cell spray foam.
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06 May 2010 08:40 AM
There would be a layer of building felt/tar paper underneath the lath. When people say traditional stucco is not good there, what is the reasoning? I'm assuming it's the cold climate, but I used to live in a cold climate where almost every building you saw had traditional stucco.
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06 May 2010 09:15 AM
Rsipgeo;

I am fro Erie, Pa. originally, a similar climate to Albany and there were plenty of older homes in that area with traditional stucco. 3 coat, paperbacked wire lath & stucco is the best, EFIS is problematic because not to many can properly install
Chris Kavala
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FL. Lic # CBC036455, GA Lic. RLCO000624, LA Lic. # CL33845
Dana1User is Offline
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06 May 2010 10:35 AM
Tread carefully when designing the layup. Read and understand this before proceeding. 

Then short-description:  Stucco is a moisture reservoir. The permeance of each layer in the wall stackup counts.  Newer housewraps and even most felts are too vapor permeable to work in close proximity to the high moisture content and high vapor drives associated with stucco.   A 10mm ventilation/drain gap cures most ills, independent of stucco type or backing type.
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07 May 2010 12:38 PM
This is a very helpful article. What are your thoughts on how this pertains to installation of traditional stucco on ICF forms? Would you still need to use two layers of wrap or an air gap?
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07 May 2010 02:39 PM
Posted By baracudda40 on 07 May 2010 12:38 PM
This is a very helpful article. What are your thoughts on how this pertains to installation of traditional stucco on ICF forms? Would you still need to use two layers of wrap or an air gap?

With ICFs you don't have the mold-on-sheathing issue, but the concrete structural-wall still needs to dry toward the exterior to avoid loading up with ground moisture &  interior air diffusion.  In rainy areas there is some potential for even saturating the outer layer of EPS if you don't some amount of rainscreen gap.  A 3/8" or 10mm rainscreen may be overkill, but some sort of real drain-plane & vent gap counts, even if it's only ~6mm or 1/4".  Without it, wetting events on the exterior keep adding moisture- the high vapor drives of sun on rain or dew-wetted stucco goes through a few inches of EPS fairly readily all spring/summer/fall, but if you put a vapor retarder on the exterior to prevent that, the concrete builds up moisture from the interior all winter.  It has to be allowed to dry in SOME direction.  With a ventilation gap between the stucco & EPS both the stucco & the concrete can dry into the gap, and any condensation forming on the backside of the stucco can escape by running out the bottom.

Semi permeable stucco-like finishes with a permeance between 1-2perms applied directly to EPS can work in a wide variety of conditions but not with the moisture reservoir of tradtional stucco.
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09 May 2010 07:34 AM
Nice article you linked to. It talked about the science AND the solution, which seems simple. What is that particular mesh to give an air gap to the stucco?
Dana1User is Offline
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10 May 2010 10:28 AM
Posted By Rsipgeo on 09 May 2010 07:34 AM
Nice article you linked to. It talked about the science AND the solution, which seems simple. What is that particular mesh to give an air gap to the stucco?
10mm rainscreen products are now becoming quite common in Canada where building codes require them, and in US commercial construction behind stone veneer exteriors, etc.  I'm not sure which version was used in Building Science articles, but if you do an internet search on "10mm rainscreen" you'll find several options, such as this.

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10 May 2010 10:56 AM
This is somewhat off subject, but it would be interesting to see how much of moisture issues could be addressed with active interior pressure management. For example, maintain a slight negative pressure in the heating season, slight positive while cooling or when it is raining.

No wall or ceiling is perfectly sealed and pushing or pulling (from wind, stack effects, exhaust fans, etc) warm moist air in the wrong direction will cause some wet spots.
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10 May 2010 02:41 PM
Posted By jonr on 10 May 2010 10:56 AM
This is somewhat off subject, but it would be interesting to see how much of moisture issues could be addressed with active interior pressure management. For example, maintain a slight negative pressure in the heating season, slight positive while cooling or when it is raining.

No wall or ceiling is perfectly sealed and pushing or pulling (from wind, stack effects, exhaust fans, etc) warm moist air in the wrong direction will cause some wet spots.

Pressurizing or depressurizing the building is just increases the net infiltration.  Most buildings are normally at somewhat negative average pressure due to exfiltration out of ceiling leaks driven by stack effect- it may take significant mechanical depressuring to reverse that flow.  This type of control woudn't change the vapor diffusion issue a bit, but it could conceivably mitigate the worst of the air-transported deposition of moisture problems.  But it would prove to be costly in terms of fuel use. It's probably not cost-effective in the average leaky house- better to spend the money on tightening the place up to save both the house AND on heating/cooling costs.

No wall or ceiling is perfectly air-sealed, but with attention to detail,  in new construction it can come close (even without extensive use of SPF.) Primering seams in sheathing as a gripping surface on which apply tape, foaming & caulking as you go during assembly, pressurizing & sealing the building before insulating or applying the interior wall, etc all pay dividends in longevity & efficiency.
jonrUser is Offline
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10 May 2010 04:39 PM

Pressurizing or depressurizing the building is just increases the net infiltration.  Most buildings are normally at somewhat negative average pressure due to exfiltration out of ceiling leaks driven by stack effect - it may take significant mechanical depressuring to reverse that flow.
Obviously not when we are talking about damaging wall/ceiling in/ex filtration. Pressurizing the interior above the exterior may cause non-damaging (in the summer) *exfiltration* and the stack effect causes exfiltration at the upper areas and infiltration at the lower areas (net flow always averages zero).
This type of control woudn't change the vapor diffusion issue a bit, but it could conceivably mitigate the worst of the air-transported deposition of moisture problems.
And the latter is 100x worse of a problem than the former.
But it would prove to be costly in terms of fuel use.
Pressurization above normal interior levels might very well bring the pressure closer to outdoor pressure, reducing flow through the walls and having little to no effect on the total flow. Can also save energy depending on how you can extract heat from the controlled flow. And in any case, I'll take a little energy usage over mold in the walls.
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15 May 2010 07:58 AM
Interesting. Can an ERV solve most pressurisation issues? SIP walls, Sprayfoamed flat roof and an ERV?
Bruce FreyUser is Offline
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15 May 2010 04:06 PM
My view is that localized wind pressure and negative pressure from exhaust fans and combustion are bigger issues for the typical USA detached single family home than stack effect.  Wind pressure can easily overpower anything you can do with an ERV if you are not tight.

Stack effect is  an important consideration in mid and hi-rise commercial office buildings and we operate outside air fans to try to keep close to neutral pressure at the ground floor in cold climates.  The downside is that you are pushing warm moist air outboard to where it is cold and you risk condensation and ice buildup if you have a leaky (air) building. 

Having a tight building is the best defense in all cases....tall or short... and is the most effective money you will spend..  Ventilation is needed to contol humidity, CO2 and to provide combustion, toilet/kitchen exh make up air. An ERV or HRV should be a part of every well constructed (i.e., tight) house, although stack effect should not be a major issue.

Bruce
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15 May 2010 05:10 PM
I agree, stack effects vary with temp, but say about 5 pascals per floor. Wind can cause 50 pascals (equivalent to a blower door fan) when windy and 10 pascals average. Unbalanced supply/return registers are another easy way to inadvertently pressurize/depressurize a room.

Air quality (including radon, dust, formaldehydes, etc), comfort (drafts), rain infiltration and summer wall moisture are all best with a slight positive pressure (say 1/4 pascal) at all points of the house. But in winter, positive pressure would push moisture into the walls (not so good if there is any wood involved).


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15 May 2010 09:36 PM
Posted By Rsipgeo on 15 May 2010 07:58 AM
Interesting. Can an ERV solve most pressurisation issues? SIP walls, Sprayfoamed flat roof and an ERV?
most ERVs are balanced as to not affect presurization, there are only a few (residential) that can positive pressurize a home.

Chris Kavala
info@southernsips dot com
1-877-321-SIPS
FL. Lic # CBC036455, GA Lic. RLCO000624, LA Lic. # CL33845
RsipgeoUser is Offline
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17 May 2010 09:29 AM
Going back to the rainscreen behind stucco issue - it seems there are several manufacturers but most of it is geared to Canadian code compliance. In short, it is hard to find here in the US. You'd think that in the Stucco failure capital of the NorthEast (Philadelphia) it would be easier to find. Or maybe that's why it's the stucco failure capital.

Assuming the spun plastic mesh between felt is what we are talking about.
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17 May 2010 10:24 AM
Posted By Rsipgeo on 17 May 2010 09:29 AM
Going back to the rainscreen behind stucco issue - it seems there are several manufacturers but most of it is geared to Canadian code compliance. In short, it is hard to find here in the US. You'd think that in the Stucco failure capital of the NorthEast (Philadelphia) it would be easier to find. Or maybe that's why it's the stucco failure capital.

Assuming the spun plastic mesh between felt is what we are talking about.

The spun plastic can be used, but isn't exactly necessary.  "Rainscreen" refers simply to the gap- the mesh is a material that can be used to maintain it.  (The cavity in a masonry cavity walls is also a rainscreen.)  Using spun plastic to establish the well ventilated gap has the benefit of keeping the bugs & other critters from nesting in there, but the simple traditional gaps behind old-school stucco are rainscreens as well. 

More technical definitions specify the pressure boundary of the structure being separated from a ventilated cladding, where the air-barrier & a drain plane is incorporated into the wall inside of the gap, allowing the cladding to to blunt the bulk-water & vapor drives that occur with pressure differentials between interior & exterior.  A pressure-equalized cavity surrounding the building behind the cladding limits the drive into the interior walls of the building proper.  Water from those drives that gets into the rainscreen gap, is managed by the both the ventilation capacity of the gap and the bulk-water rejection of the drain-plane, reducing the amount that gets into the wall due to vapor pressure & wind pressure differences.
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17 May 2010 10:37 AM
Yes, it is important to note that the gap is there to reduce the pressure difference between the two sides of the outer layer. Reduce this pressure (by any means) and you suck far less rain into the layer. Ventilation and drying after it is wet are secondary effects of the gap.


SimonDUser is Offline
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25 May 2010 01:19 AM
In Metal SIP construction the sheet metal skin of the SIP functions as a vapor barrier. All we have to do in almost all climates is apply felt backed metal lath in order to apply stucco. The felt on the back of the lath is only necessary to ensure that the stucco doesn't bond to the skin of the metal SIP. Metal SIPs have a subtle advantage in this respect.
Building Designer PANELfusion, LLC Tampa, FL simon@panelfusion.com "Metal SIP Advocate"
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