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| Strawbale Archive for June 2002 |
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| 241 messages, last added Tue Nov 26 17:43:05 2002 |
[Date Index][Thread Index]
Re: SB: Fly ash
Can you give us the title of that stucco book?
-Guy-
----- Original Message -----
From: "John Kingsley" <johnk@magma.ca>
To: "John Salmen" <terrain@shaw.ca>
Cc: "Strawbale List" <strawbale@crest.org>
Sent: Wednesday, June 26, 2002 4:23 PM
Subject: Re: SB: Fly ash
> Hi John,
>
> Regarding map cracking, from a stucco handbook I have, it says:
>
> Map cracking usually caused by:
> a) inadequate sand in the mix,
> b) inappropriate gradation of the sand,
> c) excess water in the mix,
> d) inappropriate formula,
> e) lack of lath, or
> f) failure to gap plywood or OSB board.
>
> Note: The stucco handbook is geared towards applying stucco
> to stick houses, not SB houses.
>
> My understanding is that the most common reasons for map
> cracking is a) and c).
>
> I've never heard that too much lime can cause map cracking.
> Although d) above maybe implies the cement-lime ratio may
> be important. In another spot in the handbook, it says:
> The higher the portland cement, the greater the likelihood of
> plastic shrinkage cracks.
> This is the opposite of what you say below. Who knows?
>
> I'm no plaster expert. Just relaying what I have read in books.
>
> On my 6:1:1 wall I had very little map cracking. Although I did
> add some poly fiber to the mix, perhaps this helped with the
> cracking.
>
> Regarding vapour permeability, I wanted my exterior wall
> to be as vapour permeable as possible. That's why I went
> with the 1:1 portland:lime ratio.
>
> My thinking is that no wall is perfect - every one will have
> cracks of some sort. And the water will eventually get in all
> walls, no matter how it is built. So build the wall as vapour
> permeable as possible on the exterior so that the water vapour
> can exit (when it heats up by the sun). Note: this is from the
> perspective of a someone who lives near Ottawa
> (not sure about other climates).
>
> cheers,
> John
>
> John Salmen wrote:
> >
> > I'm curious to hear more about this as my natural inclination is in the
case
> > of cement renders to eliminate the lime altogether and that is how I
have
> > worked with it. Traditionally a 'good quality' 3 coat stucco did not use
> > lime at all or a very small percentage (10-15% of cement max) to improve
> > workability. You use a cement stucco if you want the benefits of a
cement
> > stucco which are stiffness and impermeability. You don't want to detract
> > from these properties as you compromise the inherent properties of the
basic
> > material and the wall. Too much lime in a cement mix increases
permeability
> > but also can promote map cracking (which is also a water entry source)
so
> > you end up with a potentially overly porous wall.
> >
> > Now the properties of a lime render are different; flexibility and
> > permeability. Is less prone to cracking in stress and can be
self-healing of
> > cracks - a whole different material with different benefits.
> >
> > I'm definitely open to other opinions and information on this.
> >
> > John
>
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