Replies

1. 
   Aedi | Apr 24, 2019 04:16pm | #1

   You are correct in worrying about your builder's proposal, their assembly is extremely ill advised and will not work. The <1" of spray foam is basically useless from an insulating perspective, but is an air and vapor barrier. Since it is so thin, it will get cold enough for condensation to form on its interior surface. This is bad in the best of cases, but since he uses a layer of interior polyethylene, that water will not be able to dry to the interior and will be trapped in the assembly. It would not take long for mold and rot to follow. Also what's with the double layer of building wrap?

   Your proposed assembly will work, though there might be a better solution.

   In general, I am not a big fan of flash and batt. While the spray foam is better performing than batt insulation, the thermal bridging of the studs severely undermines that. Best case, using 2" of closed cell spray foam adds about R4, on a whole assembly level. Since you are using Zip-R, you don't really need the air sealing benefits of foam either.

   As you correctly note, R9 exterior foam is not sufficient to prevent excessive condensation, so the layer of closed-cell spray foam is necessary. However, Zip-R is also available in R12 panels, so you might be want to look into that. Alternately, you could use a couple layers of regular foam or mineral wool boards on the exterior in place of Zip-R.

   Edit: Modified this post after re-reading OP and realizing he had already noted concerns.

2. GBA Editor
   Martin Holladay | Apr 25, 2019 06:58am | #2

   Tech,
   Your assembly is better than the assembly suggested by your builder, but your planned WRB (the Zip R) might not meet code. Most building codes require two layers of building paper or asphalt felt under stucco. (Check with your local building official on this point.) Building paper is cheap, so you might want to install two layers of building paper on the exterior side of the Zip R -- after taping the Zip R seams, of course.

1. 
   Aedi | Apr 25, 2019 10:31am | #3

   I didn't realize that two layers of building paper was a common requirement in codes. I don't know much about building codes in Minneapolis, but I took the time to find the relevant section in the 2018 IRC (2510.6):

   Water resistive barriers shall be installed as required over wood based sheathing and shall include a water resistive vapor permeable barrier with a performance at least equal to two layers water-resistive barrier complying with ASTME2556, type I. The individual layers shall be installed independently such that each layer provides a separate continuous pane and any flashing intended to drain to the water resistive barrier is directed between layers.

   Exceptions:

   1.Where the water-resistive barrier that is applied over wood-based sheathing has a water resistance equal to or greater than that of a water-resistive barrier complying with ASTM E2556, Type II and is separated from the stucco by an intervening, substantially nonwater-absorbing layer or drainage space.

   2.Where the water-resistive barrier is applied over wood-based sheathing in Climate Zone 1A, 2A or 3A, a ventilated air space shall be provided between the stucco and water-resistive barrier.

   I believe our friend's proposed assembly falls under the first exception, and makes good sense from a building science perspective.

   I just had to refresh myself on this, so for everyone who is curious: The purpose of the double layer of building paper is to create a drainage gap between the stucco and the sheathing. It ensures that if the stucco remains bonded to the first layer of building paper as it dries, it pulls away from the second, keeping the sheathing from being in direct contact with a reservoir cladding. It is a clever, cheap way to keep an assembly safe, but when there is an air gap (edit: between the stucco and WRB) already in the assembly it is not necessary.

   Building science corp, as always, has some good resources on the topic:
   https://www.buildingscience.com/documents/reports/rr-0105-brick-stucco-housewrap-and-building-paper/view
   https://www.buildingscience.com/documents/insights/bsi-029-stucco-woes-the-perfect-storm
   https://www.buildingscience.com/documents/building-science-insights/bsi-102-coming-stucco-pocalypse

3. GBA Editor
   Martin Holladay | Apr 25, 2019 10:44am | #4

   Tech,
   More info here: "To Install Stucco Right, Include an Air Gap."

4. Expert Member
   Dana Dorsett | Apr 25, 2019 11:19am | #5

   There is no mention of the stud depth, and it matters from a wintertime dew point control perspective.

   >"I have a double sill plate and (3) large sliding doors and lots of windows in this home, so I would like to get some exterior insulation to help with the thermal bridging."

   Thermal bridging? The thermal bridging of the framing is "in the noise", probably smaller than the rounding errors in the thermal performance of three large glass sliders and lots of windows. Spending the ZIP-R and closed cell foam money on higher performance glass is the better investment, even in a 2x6/R19 type wall.

   Take the time to model this using BeOpt or some other performance tool to get a handle on what upgrades make a meaningful difference, and which don't:

   https://beopt.nrel.gov/home

5. 
   Peter Yost | Apr 27, 2019 06:19am | #6

   In the builder-proposed assembly it is true that the interior surface of the 1-inch foam flash will be the first condensing surface if the rest of the cavity is air-permeable insulation BUT if the 6 ml polyethylene is the interior vapor retarder (and there is an interior air barrier--either the PE or air tight drywall), the PE as a solid Class I vapor retarder will not pass enough moisture for significant condensation on the first condensing surface.

   Yes, the PE will also not accommodate much if any interior drying of the assembly, but for many years, Canadian builders have been detailing interior PE as both a vapor retarder and air barrier with success. The builder assembly would not be my first choice, and I would bump the 1-inch flash to a 2-inch one, but I bet it would work so long as there is that interior air barrier.

   Peter