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Community and Q&A

Acrylic stucco and wall system

Peter Schneider | Posted in Energy Efficiency and Durability on

I live in Canada (zone 7a) and am in the process of renovating a 1960 home down to the studs. We have decided to put 3″ of EPS on the outside of the original sheathing with another layer of sheathing on the outside of the foam. With the 2×4 walls I have been thinking about spray foam (closed cell originally but am now considering open cell based on reading the Musings of an Energy Nerd book concern around closed cell) insulation. For the exterior I am thinking about using acrylic stucco which is water resistant/proof.

I have a couple of questions as a result:

1. What underlayment is recommended when using acrylic?

2. Is closed cell actually viable between the studs or would the vapour potential within the walls not breathing to the inside or outside be a problem?

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  1. GBA Editor
    Martin Holladay | | #1

    First, can you tell us your name?

    I'm not sure what you mean by "underlayment." On the exterior side of the new rigid foam, you're planning to install either plywood or OSB sheathing, I assume, and that's good. On the exterior side of this new sheathing, codes require the installation of a water-resistive barrier (generally, housewrap or asphalt felt, although other WRBs are possible).

    For more information on WRBs, see All About Water-Resistive Barriers.

    If you are installing an artificial stucco product (acrylic), you obviously need to follow the instructions provided by the stucco manufacturer. Most stucco application instructions require the use of at least two WRB layers.

    Beyond the minimum requirements of stucco product manufacturers, it's worth paying attention to the stucco crisis (wall rot problem) facing the insurance industry. In light of stucco failures, GBA recommends that all new stucco installations include an air gap between the stucco and the WRB or wall sheathing. For more information on GBA recommendations, see To Install Stucco Right, Include an Air Gap.

    Finally, you might want to abandon your plan to install rigid foam and a new layer of sheathing, and instead call up a certified EIFS installer. EIFS is a combination of rigid foam and synthetic stucco. EIFS can only be installed by certified installers -- it's not a do-it-yourself approach. The EIFS installer will take legal responsibility for any future water-entry problems related to the EIFS installation.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Is closed cell actually viable between the studs or would the vapour potential within the walls not breathing to the inside or outside be a problem?"

    A. If you are planning to install exterior rigid foam, I advise the use of a vapor-permeable type of insulation between the studs. Options include cellulose, mineral wool, fiberglass, or open-cell spray foam. I don't recommend the use of closed-cell spray foam if exterior rigid foam is used. For more information, see How to Design a Wall.

  3. Peter Schneider | | #3

    Thanks Martin for both responses. My name is Peter Schneider. I am trying to find out how to setup preferences (name and notifications) but am not currently a paid member which may be the issue.

    I have been reading yours and other materials on the wall systems and the issues with classic stucco. It has been a source of many learnings and one of the reasons that I have been looking at the synthetic stucco option. The problem is that other than EIFS there is not a lot of discussion around synthetic stucco's use.

    The synthetic stucco properties and what I have read about traditional wall systems (use of Poly on the warm side of the wall) all lead me to concern around application of these properties (water resistance being one). The greatest concern is that of having the equivalent of to vapour barriers (one on the inside and another on the outside) that trap moisture between the two of them. The next concern is around the air gap requirements and if the same are required for a synthetic stucco as for a classic stucco (noting that your articles, including To Install Stucco Right, Include an Air Gap, are all directed to classic stucco scenarios).

  4. GBA Editor
    Martin Holladay | | #4

    The fact that synthetic stuccos have a low vapor permeance is, indeed, sometimes problematic. I would certainly advise that any synthetic stucco application include the same type of air gap that GBA recommends for traditional cementitious stucco.

  5. Expert Member


    Synthetic stucco, a "face sealed" cladding system, has been cited in the official report and countless lawsuits as one of the major causes of BC's Condo Crisis - the widespread failure of building envelopes that occurred here several decades ago. I don't know if modifying the substrate by adding a rain-screen gap alleviates the risk, but before I would consider it in this climate, I'd make sure it wasn't being installed the way it has traditionally been. That way that caused all sorts of problems.

  6. SwitchgrassFarmer | | #6

    Synthetic stucco information for systems other than EIFS is quite sparse, I feel your pain.

    On the ICF foundation of my home we used the cement board synthetic stucco method, but with a woven rainscreen material behind it:

    The big "Kleenex-Xerox" like name in synthetic stucco is Dryvit. Out of curiosity I looked to see whether Dryvit had anything new in this area. Dryvit says: "Outsulation systems can be installed in either “barrier,” “moisture drainage” or “pressure equalized” configurations, and these systems are engineered to perform in all climates and on all types of structures."

    A little Googling shows that Dryvit's "RESIDENTIAL MD SYSTEM, An Exterior Wall Insulation and Finish System with Drainage Mat" may be close to what you are after: (Note that is a UK link.)

    Dryvit also has a cement board system with drainage spacers:

    BTW, we found it difficult to find a synthetic stucco contractor who had done an installation on anything other than EIFS. Through just plain dumb luck, one day a light commercial EIFS contractor happened to show up at my farm as a customer, and it turned out he had that type install experience!

  7. Peter Schneider | | #7

    Thanks everyone for your help and quick response. It appears that the drainage spacing is a must regardless of the type of siding one uses. We will be integrating it regardless of which way we go.

    Given the information from Andrew I have more reading to do.

    Thanks Malcolm for the note on Vancouver. I remember those failures and was not aware that they were synthetic related. Vancouver is a unique climate in addition to all else. In Calgary we do not get the same sort of humidity as Vancouver and the cold here posses different problems.

  8. Peter Schneider | | #8

    On another front Martin, I managed to find where to change my Profile so my name should show going forward. Also found out why I was not getting notifications (Mac Mail spam filter).

  9. Expert Member

    Peter,You're right - Calgary gets away with things we never would here. Carefully detailed you should be fine.

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