Tuesday, March 6th, 2007 | Popularity: 4%
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:
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I 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 left under the assumption of “Both/And” rather than “Either/Or.” If you would like to check out the video link or sign up for the weekly video tips and techniques newsletter, click the link above or sign up on the home page http://www.StrawBale.com.
Popularity: 4%
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Andrew Morison is a licensed contractor specializing in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs. You can check these out at http://www.LearnStrawBale.com.
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September 13th, 2007 at 2:18 pm
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, razz@charter.net, Bale Built Energy Designs.
September 13th, 2007 at 2:18 pm
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.
September 13th, 2007 at 2:19 pm
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.
Regards,
Michael Thomas
The IronStraw Group
September 13th, 2007 at 2:19 pm
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.