Blocked weep screeds

Harry, this is what I understand from your comment thread on the 4/16 meeting notes:

The weep screeds force moisture inside the exterior walls, whether from interior humidity or exterior penetration, to drain out away from the foundation. However, the Sto flexyl coating is being wrapped around the the horizontal joint where the stucco ends, which–since we don’t know how permeable it is–may prevent or slow the dissipation of moisture to a sufficient extent to cause mold or rot in the walls. Additionally, we have at least one layer of impermeable elastomeric paint on non-replaced screeds, over which the Sto is being applied, exacerbating this situation. Vanderslice could not assure you that these coatings have sufficient permeability to allow any trapped moisture to escape in a let me say timely way.
You are suggesting that the Sto applicator NOT wrap the flexyl around the edge of the weep screed, but rather end it ever so slightly above that joint. Indeed, maybe someone should also remove the old elastomeric paint along that line as well.
(I’m thinking “blocked” weep screed isn’t a good term, but “sealed” might be more precise.)

Do I have that right?

About hmarcuse

I've been an environmentalist since my college days in the 1970s (May 1977 Seabrook, New Hampshire occupation) , and am now a professor of German history at the University of California, Santa Barbara.
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3 Responses to Blocked weep screeds

  1. Harry Nelson says:

    Pretty much on target, Harold. Some more specific info is below.

    The weep screeds, when done in a standard way, allow moisture trapped between the standard building paper and the lath/stucco to escape. In normal lath/stucco, the permeance (a measure of ability to allow water vapor to get through) is high, >10 perms. (A perm is a unit of measure of permeance).

    At WCP we have systematically added layers of lower permeance to the outer stucco. These are at least 1 and maybe 2 layers of elastomeric paint, and now in the new construction, Sto Flexyl and Sto Powerwall. My estimate of the permeance of all these added layers is <1 perm.

    Because it is now much harder (by perhaps a factor of 10) for water vapor to escape our walls, compared to the standard stucco design, the weep screed function is more important; it is more important not the seal or block it. Already in my unit at least there has been dampness/mold/or rot near the baseboards in some places, which may be attributable to the reduction in permeance of the elastomeric paint and the blocking/sealing of the weep screeds.

    The qualitative answer that `no water/moisture will enter the walls' ignores the basic design category of these walls: they are called drainage walls because the design is to allow trapped water/moisture a route out of the interior of the wall. Water/moisture has a variety of ways of entering, including leaks in the barrier, which are sure to occur, but also from high humidity areas of the house (like showers), and also up from wet soil under our buildings.

    So my question is simply: can the weep screed be left unblocked and unsealed on our homes?

    If not it is really up to the advocates of non-standard blocking/sealing to make a credible case that the non-standard situation is acceptable. A credible case to me numerical, showing the long term average relative humidity after water entry stays below 70%. But I don't think the cost of such a study should fall on WCP in any way: we have a right to expect standard, time-tested construction techniques, and not improvised short-cuts that saddle us with new future risks.

    If the Sto representative gets Sto engineers to do the numerical studies as part of their sales pitch, that would be great. Qualitative answers aren't much help to me, unless they are accompanied by a full warranty and guarantee for future problems.

  2. Harry Nelson says:

    Here is a standard diagram of how a stucco system works, from the Northwest Wall & Ceiling Bureau’s handbook on Stucco systems. This is one of a variety of handbooks on stucco that describe the way stucco works; this is one of the best diagrams I’ve found:

    At West Campus we have:

    (1) Greatly reduced the water vapor permeance of the stucco, by putting relatively impermeable paint on top of the stucco. My estimate is that the water vapor permeance has been reduced from >10 perms to <1 perm by the 3 or 4 layers (1 or 2 elastomeric + Sto Flexyl + Sto Powerwall).

    (2) Additionally sealed/blocked the weep screed at the bottom. The impact of sealing the screed is enhanced by the water vapor permeance reduction.

    The non-standard situation proposed for WCP could be validated with actual engineering studies, for example using this software:

  3. Harry Nelson says:

    Here is a document on house wraps…

    Click to access K01472.pdf

    Page 2:

    “Building codes require that WRB’s be vapor permeable and establish the minimum allowable level to be around 5 perms. This value is based on traditional building practices and does not necessarily address the requirements of modern energy efficient construction, new cladding systems and materials. DuPont Building Scientists believe that 5 perms are not enough to ensure the kind of consistent performance you need from a WRB. DuPont suggests that a WRB have a moderate to high vapor permeability.

    So why is vapor permeability important? Because wall cavities do get wet, roofs leak, condensation occurs, plumbing leaks, construction materials are installed wet and internal moisture loads can be very high. However it happens, walls get wet and require a way to dry out. When a wall can’t dry out, it becomes vulnerable to moisture-induced damage including mold and rot. DuPont Building Scientists believe in designing and building forgiving wall systems. All WRBs produced by DuPont, are engineered to allow maximum drying while still maintaining the other critical properties”.

    The cumulative vapor permeance at WCP after application of the Sto layers is approximately 1 perm… 5 times lower than recommended in the document above.

    Another interesting document:

    Click to access sma09a.pdf

    Slides 10 (Water Tight Assembly), 23 (Stucco is Vapor Permeable), and 26 (Vapor Permeability Prevents Mold Growth) are particularly appropriate.

    One point: Stucco alone has a vapor permeance of 15-20 perms. Our existing building paper probably has a vapor permeance of 5 perms… modern building papers like Tyvek are as high as 58 perms.

    Keeping the weep screed open is one of the only countermeasures we have to address the huge reduction in vapor permeance we have introduced.

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