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>From <br><br>Moisture Properties of Plaster and Stucco (render) for<br>Strawbale Buildings<br>John Straube<br><br>download here:<br>http://members.westnet.com.au/ejt/pdf/Straube_Moisture_Tests.pdf<br><br>"6 Conclusions<br>Based on the test data and literature review, several conclusions can be drawn:<br>1. A 450 mm (18”) thick strawbale should have a vapor permeance of<br>approximately 110 to 220 ng/ Pa•s•m2 (2 to 4 US perms).<br>2. Cement:sand stuccos are relatively vapour impermeable. In fact a 38 mm<br>(1.5”) thick cement : sand stucco may act as a vapor barrier (i.e., have a<br>permeance of less than 1 US Perm).<br>3. The addition of lime to a cement stucco mix increases permeance. As the<br>proportion of lime is increased, the permeance increases. Pure lime:sand<br>stuccos are very vapor permeable. The permeance of a 38 mm (1.5”) thick<br>cement : sand stucco can be increased to 5 or 10 US Perms by replacing half<br>the cement with lime and to 15 to 30 US Perms by using a pure lime : sand<br>stucco. The addition of even a small amount of lime (0.2 parts) may<br>increase the permeance of cement stucco dramatically (e.g.,from under 1 to<br>3 to 6 US Perms).<br>4. Earth plasters are generally more permeable than even lime plasters. The<br>addition of straw increases the permeability further. A 38 mm (1.5”) thick<br>earth plaster can have a permeance of over 1200 metric perms (over 20 US<br>Perms), in the same order as building papers and housewraps.<br>5. Applying an oil paint to a moderately permeable 1:1:6 stucco will provide a<br>permeance of less than 60 metric perms (1 US perms) and thus meet the<br>code requirements of a vapour barrier.<br>6. Earth plasters were not found to have significantly different water<br>absorption than cement and lime stuccos. The earth plasters, regardless of<br>density and straw content, resisted 24 hour of constant wetting easily,<br>although the topmost 1/8” of surface became quite “muddy”. In a real<br>rainstorm this behavior may cause erosion.<br>7. Lime washes appear to be somewhat useful for reducing water absorption<br>while not reducing vapor permeance. The lime wash over earth plaster did<br>not dramatically lower water absorption but will increase the mechanical<br>strength of the plaster after wetting, i.e., they will increase the resistance to<br>rain erosion.<br>8. Based on Minke’s and Straube’s earlier tests, siloxane appears to have little<br>or no effect on the vapor permeance of cement, cement:lime, lime, and<br>Moisture Properties of Plaster and Stucco for Strawbale Buildings EBNet<br>BalancedSolutions.com 34<br>earth plasters while almost eliminating water absorption. The use of<br>siloxane can be recommended based on these earlier tests.<br>9. Sodium silicate did not seem to have much impact on water uptake or vapor<br>permeance. This additive may hold earth plaster together, or increase its<br>erosion resistance, but as tested it had no noticeable impact on moisture<br>properties.<br>10. Linseed oil at 2% in an earth plaster mix is not a very effective water<br>repellent and does act to restrict vapor permeance somewhat. It may add<br>some strength to an earth plaster in the wet state. Heavy applications of<br>linseed oil to the surface of finished earth plaster will, based on Minke’s<br>tests, reduce the water absorption to almost zero, but will markedly decrease<br>vapor permeance.<br>11. The test methods described here appear to provide repeatable results, and in<br>general compare well to previous tests on different samples by both the<br>same (Straube) and different researchers (Minke)."<br><br>> From: dave@howorth....uk<br>> To: strawbale@amper.ped.muni.cz<br>> Date: Tue, 12 Apr 2011 22:36:15 +0100<br>> Subject: Re: [Strawbale] moisture<br>> <br>> On Tue, 2011-04-12 at 09:17 +0300, jonas kacerauskas wrote:<br>> > Hello again dear friends,<br>> > I want to specify one important moment for all of us: all the time<br>> > from different sources i was achieved information that lime plaster is<br>> > more "breathable" than clay plaster, whats why i was tought to put<br>> > clay plaster on interior and lime on exterior..the last time i saw<br>> > this in UK in the library built by Amazonails...so i am very suprised<br>> > about Andrew information...<br>> > best regards:)<br>> <br>> My understanding is that the vapour permeability of clay is somewhat<br>> greater than that of lime. So I'm surprised that you found the contrary<br>> in Amazonails' library. Do you remember the reference?<br>> <br>> There's a table in this document<br>> <http://www.greensteps.co.uk/tmp/assets/1163178050906.pdf><br>> <br>> One thing to be careful of is that the permeability for liquid water is<br>> different to the permeability for water vapour. For liquid water, lime<br>> is sometimes added to clay to increase permeability (as a field<br>> dressing, for example). But the discussion here is about vapour<br>> permeability.<br>> <br>> Cheers, Dave<br>> <br>> <br>> ____________________________________________________<br>> European strawbale building discussion list<br>> <br>> Send all messages to:<br>> Strawbale@amper....muni.cz<br>> <br>> Archives, subscription options, etc:<br>> http://amper.ped.muni.cz/mailman/listinfo/strawbale<br>> ____________________________________________________<br>> <br>> <br> </body>
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