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