# lath and flashing



## ICE (Feb 4, 2018)

_R703.7.3 Water-resistive barriers. Water-resistive barriers shall be installed as required in Section R703.2 and, where applied over wood-based sheathing, shall include a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper. The individual layers shall be installed independently such that each layer provides a separate continuous plane and any flashing (installed in accordance with Section R703.4) intended to drain to the water-resistive barrier *is directed between the layers*. 


Exception: Where the water-resistive barrier that is applied over wood-based sheathing has a water resistance equal to or greater than that of 60-minute Grade D paper and is separated from the stucco by an intervening, substantially nonwater-absorbing layer or designed drainage space. 
_
Directed between the layers seems odd_.  _What am I missing here?


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## conarb (Feb 4, 2018)

u





ICE said:


> Directed between the layers seems odd_.  _What am I missing here?



That's news to me, we always put both layers under or over the flashing.  Is that something new?  I don't recall ever seeing it.


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## cda (Feb 4, 2018)

Check page 11 under stucco


Not sure if it helps


http://www.dupont.com/content/dam/a...ets/K16279-Residential-OpenStudWRB_102811.pdf


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## Francis Vineyard (Feb 5, 2018)

http://dli.state.mn.us/CCLD/rm/PDF/1309_IRC-109.pdf


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## tmurray (Feb 6, 2018)

Only if there is a drainage space behind the stucco. 

Correct me if I'm wrong, but isn't stucco essentially adhered to the building paper?

This would mean that water would not be able to get out of this assembly if the stucco is less permeable than the building paper and sheathing. If you're using modern stucco, this is more than likely the case. This type of wall system is what cause Vancouver's leaky condo crisis in the 1990s.


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## conarb (Feb 6, 2018)

tmurray said:


> Only if there is a drainage space behind the stucco.
> 
> Correct me if I'm wrong, but isn't stucco essentially adhered to the building paper?
> 
> This would mean that water would not be able to get out of this assembly if the stucco is less permeable than the building paper and sheathing. If you're using modern stucco, this is more than likely the case. This type of wall system is what cause Vancouver's leaky condo crisis in the 1990s.



T Murray:

The theory for two layer Grade D paper is that the wet stucco "crinkles" the top layer creating a drainage plane between the layers, so the water drains down between the layers without going through the bottom layer into the building.  When I started in the trades there was no two-layer paper, we just used 15# asphalt impregnated felt, the water that saturated the paper was able to dry because of a breathable wall system, water leaked in and dried out.  Then codes added insulation in the walls  trapping water and holding it in, then codes added plywood sheathing sealing the walls up so they couldn't dry out and all Hell broke out.  I flew up to Vancouver in the late 90s to see it, your first fix was to drill 2" diameter holes, top and bottom, in the plywood sheathing, then because of energy efficiency you went to rain screens to allow walls to be sealed up but allow them to drain.  By this rational Tiger's _R703.7.3 _is crazy, like many of our codes, you should thank God every day you get out of bed that you don't have to enforce our crazy codes.  Tiger, take that our of the Tiger code.


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## my250r11 (Feb 6, 2018)

From my 2009 Code check book


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## tmurray (Feb 6, 2018)

conarb said:


> T Murray:
> 
> The theory for two layer Grade D paper is that the wet stucco "crinkles" the top layer creating a drainage plane between the layers, so the water drains down between the layers without going through the bottom layer into the building.  When I started in the trades there was no two-layer paper, we just used 15# asphalt impregnated felt, the water that saturated the paper was able to dry because of a breathable wall system, water leaked in and dried out.  Then codes added insulation in the walls  trapping water and holding it in, then codes added plywood sheathing sealing the walls up so they couldn't dry out and all Hell broke out.  I flew up to Vancouver in the late 90s to see it, your first fix was to drill 2" diameter holes, top and bottom, in the plywood sheathing, then because of energy efficiency you went to rain screens to allow walls to be sealed up but allow them to drain.  By this rational Tiger's _R703.7.3 _is crazy, like many of our codes, you should thank God every day you get out of bed that you don't have to enforce our crazy codes.  Tiger, take that our of the Tiger code.



Yes, the way stucco was done before was that the building envelope layers of the got more permeable as you moved from the inside out. The sheathing was less permeable (plywood) than the paper, which was less permeable than the stucco. Even using the board sheathing was not a big deal, because even though it was super permeable, everything else was really permeable too. But they've changed all the permeabilities. now it actually decreases on the way out. Your sheathing (OSB) is more permeable than the paper, which is more permeable than the newer stuccos. Still nothing happens until we insulate these walls. The decrease in the heat flow is the big factor. But this doesn't mean we shouldn't insulate our walls, it just means we shouldn't do stupid things with low permeance building materials.


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## conarb (Feb 6, 2018)

Back when we used board sheathing even good contractors bought the cheapest, green so it will shrink allowing ¼" gaps between the boards, and full of knots that are loose and falling out so we got plenty of air circulation, then we went to line wire with no sheathing for maximum air circulation, I still use line wire and have my engineers design lateral loads in other ways, mainly steel frames since whatever we do we get steel moment frames now, here is an example of a house with line wire, other ways I allow air to circulate is to get the engineer to allow sheathing on the interior, and I never want insulation touching either sheathing or paper is get permission from my field inspector to hold the insulation 2" away, as well; as getting permission not to seal the electrical and plumbing holes to allow air in:  










Note the mold growing on new 1-1/8" plywood after a rain, stupid California made them take the formaldehyde out of plywood and now mold grows on it if it gets wet, my air quality consultant tells me that they are getting mold readings from OSB that has never gotten wet, just exposed to the moisture in the air.   I had my hardwood floor guy come in with his huge floor sander and take it off before we waterproofed the deck, they can't do that with OSB, it all has to be replaced since the spores are down inside the particles.  BTW T Murray, those windows are German triple pane fabricated in Vancouver, so I sent about $300,000 American dollars to the Canadian economy, flashings are doubled Vycor, 12" first layer, 9" second layer.


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## mark handler (Feb 7, 2018)

tmurray said:


> Only if there is a drainage space behind the stucco.
> 
> Correct me if I'm wrong, but isn't stucco essentially adhered to the building paper?
> 
> This would mean that water would not be able to get out of this assembly if the stucco is less permeable than the building paper and sheathing. If you're using modern stucco, this is more than likely the case. This type of wall system is what cause Vancouver's leaky condo crisis in the 1990s.


​
The stucco should not adhere to the paper. It is meshed into the lath, and the lath is attached to the structure. the top layer of paper should restrain it, while it dries.
The main purpose of building paper is to keep water from contacting the substrate and structural support members—very commonly sheathing like plywood or oriented strand board (OSB), wood or metal studs—so that these materials stay dry.
Metal can rust and wood can rot.
Also, wood is prone to expand and contract with changes in moisture, so it’s essential to keep sheathing dry to provide the plaster with a sound substrate. Minimizing the changes in moisture minimizes the stresses that might be placed on plaster from behind. In addition to structural considerations, excess moisture within a wall creates a potential for mold or mildew inside buildings.
Building paper prevents moisture-related problems in stucco walls.
Several industry documents, such as PCA’s Portland Cement Plaster/Stucco Manual, EB049, ACI’s Guide to Portland Cement-Based Plaster, and building codes across the country, recommend two layers of paper. During construction, paper can be damaged. Two layers of paper provide greater assurance that water won’t get to the sheathing or support members. Paper should be lapped like siding, meaning that upper layers are placed over lower layers. This facilitates drainage toward the outside. Where the edges of paper-backed lath meet, connections should be lath-to-lath and paper-to-paper.
Building paper should comply with the current requirements of UU-B-790a, Federal Specifications for Building Paper, Vegetable Fiber (Kraft, Waterproofed, Water Repellent, and Fire Resistant). This specification differentiates weather resistive Kraft papers by types, grades, and styles. Grade D is a water-vapor permeable paper. Grade D paper with a water resistance of 60 minutes (or more) works well for stucco applications, and is often preferred to Grade D paper having the minimum 10-minute resistance required by UU-B-790a.
Some specifiers are turning to house wraps for stucco underlayment. While these materials may be more rugged than paper—and therefore less prone to damage during installation—a single layer is still not adequate according to many industry professionals. At best, a hybrid system, with the house wrap closest to the sheathing and covered with the paper, seems to be an acceptable alternative.


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## tmurray (Feb 7, 2018)

mark handler said:


> The stucco does not adhere to the paper. It is in the lath, the lath is attached to the structure. The main purpose of building paper is to keep water from contacting the substrate and structural support members—very commonly sheathing like plywood or oriented strand board (OSB), wood or metal studs—so that these materials stay dry. Metal can rust and wood can rot. Also, wood is prone to expand and contract with changes in moisture, so it’s essential to keep sheathing dry to provide the plaster with a sound substrate. Minimizing the changes in moisture minimizes the stresses that might be placed on plaster from behind. In addition to structural considerations, excess moisture within a wall creates a potential for mold or mildew inside buildings.Building paper prevents moisture-related problems in stucco walls. Several industry documents, such as PCA’s Portland Cement Plaster/Stucco Manual, EB049, ACI’s Guide to Portland Cement-Based Plaster, and building codes across the country, recommend two layers of paper. During construction, paper can be damaged. Two layers of paper provide greater assurance that water won’t get to the sheathing or support members. Paper should be lapped like siding, meaning that upper layers are placed over lower layers. This facilitates drainage toward the outside. Where the edges of paper-backed lath meet, connections should be lath-to-lath and paper-to-paper.Building paper should comply with the current requirements of UU-B-790a, Federal Specifications for Building Paper, Vegetable Fiber (Kraft, Waterproofed, Water Repellent, and Fire Resistant). This specification differentiates weather resistive Kraft papers by types, grades, and styles. Grade D is a water-vapor permeable paper. Grade D paper with a water resistance of 60 minutes (or more) works well for stucco applications, and is often preferred to Grade D paper having the minimum 10-minute resistance required by UU-B-790a.Some specifiers are turning to house wraps for stucco underlayment. While these materials may be more rugged than paper—and therefore less prone to damage during installation—a single layer is still not adequate according to many industry professionals. At best, a hybrid system, with the house wrap closest to the sheathing and covered with the paper, seems to be an acceptable alternative.



Mark, Have you looked at this? https://buildingscience.com/documents/building-science-insights/bsi-102-coming-stucco-pocalypse

Also, careful Conarb, you and Dr. Lstiburek made some common points...


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## mark handler (Feb 7, 2018)

The theory (and actions) of bonding exterior plaster to the paper is a major part of the problem.
Exterior plaster should be "bonded" to the lath. Not to the paper.
Exterior plaster is water and air permeable, it is not the waterproofing.
A major error is to paint the
Exterior plaster with elastomeric paint which traps the water in the plaster with nowhere to go.

I also do not recommend exterior plaster in extremely wet climates. It has no opportunity to dry out.


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## tmurray (Feb 7, 2018)

exterior plaster was permeable, but modern ones are not. See http://nparc.nrc-cnrc.gc.ca/eng/view/accepted/?id=fd418a70-1cdb-48e1-b424-22201117f552

The vapour permanence needs to increase by 13,000 times before it is no longer considered a vapour barrier. The wet cup transmission is twice as good as dry cup, but still far below what would be necessary for a "safe" design by a margin of thousands.

I'm also struggling with the concept that the plaster is not bonding the the building paper. Isn't it installed touching the paper? It's portland cement. Isn't that like saying that the concrete in the slab should only bond to the welded wire mesh, not the vapour barrier it's sitting on?


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