psheraton at hotmail...
Tue Apr 12 23:51:14 CEST 2011
Moisture Properties of Plaster and Stucco (render) for
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
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
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
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
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 at howorth....uk
> To: strawbale at amper....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
> 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
> Cheers, Dave
> European strawbale building discussion list
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