Straw Bale Wall Breathability: Everything You Need to Know

Written by Andrew Morrison

girl blowing bubblesI recently posted a new “straw bale minute” video clip in which I argue that bale walls don’t breathe the way they have long been described as breathing. There is a divided crowd on this one. My understanding of how bale walls work is that the plaster is too thick to allow moisture to move through it. That does not mean that moisture laden air does not still find its way in and out of the walls.

Numerous outlet boxes and other penetrations supply ample space for such sir infiltration to take place. The idea of pushing moisture laden air through the plaster seems, to me, to be a false thought per recent studies and a PhD thesis I have read. Still, others disagree. For example, check out a response I got to my video clip:

“Maybe bales up in your neck of the woods don’t breathe after plastering, but they sure do breathe down here in the Southwest! I can’t tell you the number of times I’ve seen ample evidence of that. Just one eg: a gray water system that went through a straw bale wall broke and released probably 100 gal into the straw. I ran a sleeve around the 2 inch pipe, fixing the leak—and then just waited.

Within 2 months, and with no other assistance than being exposed to sun and wind, all traces of the moisture was gone. I had occasion to open up the wall from the inside a bit later, and was able to confirm that the moisture had transpired THROUGH the cement stucco. By the way, I specialize in straw construction here in New Mexico, have built about 50 houses and over 300 privacy walls.”

Clearly, the jury is still out on this one. I would like to hear more about how the author of this comment could know that the moisture moved THROUGH the plaster as he indicates. If he is correct, I would love to investigate this further and see how the two findings might be able to exist together. I like to live my life under the assumption of “Both/And” rather than “Either/Or.”

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11 Responses

  1. Hey Andrew, I’ve started to watch your site and hope to visit you in Jacksonville soon. I agree whole hearteldly with the strawbale and stucco system breathing avenue. Its what makes building with strawbales one of the huge bounuses. Have you ever been in a styra-foam seald building. With in minuets you can tell that it’s air tight. Its an argument I’ve had with many foam builders. They may claim better R-value for the building but with out a positive pressure ventalation system operating or windows open you may as well sufficate yourself. I have been studying on strawbale construction for quite some time now one of the first books I read that addressed studies on the breathablility of the strawbale with stucco was “The Straw Bale House” written by Steen,Steen and Bainbridge, back in the 90’s. You probably know them. Look forward to meeting you some day, [email protected], Bale Built Energy Designs.

  2. I have received a number of emails about the breathability of bales. Clearly there is disagreement about how the system works. I will continue to monitor the discussion on this and other lists and post new information as it shows up. Thanks for all the input.

  3. Recent data exists about breathability of plasters, especially the work done at EBNet. Supported primarily by a grant from the California Department of Food & Agriculture, EBNet has completed an extensive series of tests and research on the material properties of straw bale structures.

    Among their conclusions regarding plaster and stucco for strawbale buildings are:

    1. Cement:sand stuccos are relatively vapour impermeable. In fact a 38 mm(1.5”) thick cement : sand stucco may act as a vapor barrier (i.e., have a permeance of less than 1 US Perm).

    So in one respect Andrew you are correct, moisture would not easily move through cement stuccos; however:

    2. The addition of lime to a cement stucco mix increases permeance. The addition of even a small amount of lime (0.2 parts) may increase the permeance of cement stucco dramatically.

    In which case, the opposite occurs, moisture would pass through a stucco.

    and additionally:

    3. …siloxane appears to have little or no effect on the vapor permeance of cement, cement:lime, lime, and earth plasters while almost eliminating water absorption.

    So one solution to achieve the breathable walls that are desireable for strawbale is to use lime in the mixes and at the same time use a siloxane sealer to prevent exterior moisture penetration.

    It should also be noted there is a difference between a vapor barrier and a moisture barrier.

    Michael Thomas
    The IronStraw Group

  4. Thanks Michael. There is definitely some communication happening these days with the value of cement based stuccos and how they compare with lime based plasters. For me, there are a lot of factors involved and vapor is only one. Ease of installation, quality of installation, quality of material, and need for breathability all play roles. I will continue to watch and take part in the conversation. Thanks again for your input.

  5. I would like to put cement board on the inside and outside of the strawbale house to then finish with stone on the inside and out vs. no stucco. Is this possible with a strawbale out or would there be a problem with breatability? Thanks.

  6. Hi Dinah. I would avoid the board entirely and plaster the bales with a scratch coat. Use the scratch coat as the base for your back buttered stone work and attach the stones directly to the scratch coat/bales. Use lime so that the joints will still allow for breathing of the walls.

  7. Hi Andrew,
    My husband and I are in the design/planning stage, we have plans drawn and are working out the kinks and contacting engineers while we wait for our septic approval (it is a complicated design and a stubborn health dep. so we are waiting for this before moving forward)
    More on topic, we have been researching what plaster/stucco to use, and have gotten a quote from TransMineral (wonderful people) but have been looking into Stuc-O-Flex, and they say their product has a breahtability of 13 perms. Here are their stats:

    Stuc-O-Flex Elastomeric Acrylic Finish provides a protective weathering membrane in a pre-colored, extremely durable, fade and mildew resistant coating. The industry’s highest levels of 100% Acrylic Polymer ensures un-matched resistance to surface cracking unlike standard finishes. An unlimited selection of colors (over 10,000) in four distinct aggregate choices create a wide variety of texture opportunities from the heaviest southwest stucco to the more contemporary sand finish.

    Maximum Crack Coverage & Bridging Abilities
    Breathable (WVT) = 13 Perms Average
    Calcium – Marble Aggregates (Prevents Rust)
    No Silica Sand
    Coating Integrity – Exceeds 30 Year Exposure
    Comprehensive ASTM Third Party Testing
    America’s First Elastomeric Acrylic Finish
    500 Million Feet in Service World Wide
    Equally Effective Regardless of Climate
    (-70°F to 180°F)
    Class A Fire Rated
    New Construction, Retrofit or Repair

    My question is, does a 13 perm rating give the breathability desired in a stucco on straw bales?? Any input is appreciated!! Also very interested in hosting a workshop and/or purchasing some level of your consulting services…Talk with you soon!

  8. Hi Katie. Good luck with your permitting process. Sounds like quite an effort will be required. I would not recommend using the Stuc-O-Flex as it will not provide enough breathability for the bales. The elastomeric does a great job of sealing the walls to the elements, but it also traps potential moisture in the walls. Keep in mind that no plastic or vapor barriers other than the plaster itself are used on the wall interior.

  9. I am recoating a strawbale 3 foot wall which encircles my yard. I have had the wall for 18 years and it consistently cracks and requires patching or new mesh and painting.

    I just had the mesh and stucco recovered and am considering using an elastomeric based coating on the top of the wall only. I would paint the sides suing regular outdoor paint. The elastomeric would only cover about 15% of the wall’s surface.

    Would the wall be able to breathe properly when topcoated with elastomeric paint?

    I live in Tucson, AZ.

  10. Hi Rosa. Because you are not using the wall as a insulation aspect (i.e. it is a garden wall) it won’t really matter if it breathes. The bales can rot inside the wall (not saying to aim for that, but it could happen) and the structure would not be affected assuming there are adequate structural elements in play. In other words if the mesh and foundation and plaster are in tact. That said, I don’t think there will be rot in the wall as a result of the elastomeric in a climate like yours.

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