[GSBN] Looking for experiences with AAC and Rastra

Thu Dec 23 17:49:35 UTC 2010

There is no problem with EPS below grade. Widely used and is code approved in Canada. Mostly good marketing by Dow leads people to choose XPS. 
Use draining backfill or drainage dimple sheet and the ICF rarely sees any water anyway. 
Some ICF guys recommend peel and sticks as waterproofing: I don't and believe a drainage gap membrane is higher performance as well as cheaper and more reliable. 

John Straube
519 741 7920
Sent via BlackBerry

-----Original Message-----
From: Derek Roff <derek at unm.edu>
Sender: GSBN-bounces at greenbuilder.com
Date: Thu, 23 Dec 2010 10:29:27 
To: (private, with public archives) Global Straw Building Network<GSBN at greenbuilder.com>
Reply-To: "(private, with public archives) Global Straw Building Network" <GSBN at greenbuilder.com>
Subject: Re: [GSBN] Looking for experiences with AAC and Rastra

What about moisture issues with Rastra, and other insulated concrete 
form (ICF) systems that use expanded polystyrene?  ICFs are recommended 
by many people for below-grade work (pun intended).  Yet expanded 
polystyrene can take on a significant amount of water.  I'm imagining 
that this could compromise the foam, through cracking and spalling, as 
well as reducing the insulation values.

Is this a problem, or have the manufacturers found a way around it?

Derelict

--On Thursday, December 23, 2010 9:01 AM -0800 John Swearingen 
<jswearingen at skillful-means.com> wrote:

I worked as a volunteer on a project with Rastra, a temple with high 
walls.  As Derek says, it's difficult to love--the dust is awful and 
the blocks are, well, grungy blocks.   This was probably one of the few 
applications where the massive concrete gridwork inside the forms made 
any sense, but the mass is all inside the insulation, and so doesn't do 
that much good otherwise.  

It's amazing how Rastra persists as a popular choice--I think it's 
because at first look it seems like Legos.  I've had to deliver my rap 
on it several times to clients who were initially enamored with it.  I 
wouldn't build with it.

John "RastraRap" Swearingen

On Wed, Dec 22, 2010 at 9:09 AM, Derek Roff <derek at unm.edu> wrote:

I've worked with Rastra a few times.  I don't like it.  I don't believe 
their insulation figures, even after they have scaled them back. 
Working with Rastra gives me a powerful reminder of some negative 
aspects of concrete and styrene, the component parts.  Cutting and 
shaping is possible, but it produces a lot of dust.  The dust is awful 
to be around.  I don't know if it is fair to say that the material is 
fragile, but it certainly surprised us multiple times with sudden 
breaks.  Working with ten-foot long, 150 pound blocks carries a 
significant injury risk to a small, amateur crew.

I've worked with two different kinds of Autoclaved Aerated Concrete 
(AAC) blocks.  They are more pleasant to work with than Rastra, for me. 
The ones that I used were very consistent in size and shape, with crisp 
edges and corners, which could be chipped in handling.  A few blocks 
cracked when dropped, but they seemed fairly strong.  I have concerns 
about the embodied energy, the need for two different kinds of massive 
kilns to make them, and their insulation claims.  It seems like every 
company talks about the amazing properties of their 2 pounds per cubic 
foot blocks, but the ones they actually sell have a higher density, 
which they often don't list.  The ones that I have used are still 
pretty light, but very different from what the industry keeps promising.

AAC blocks have a very rectilinear, industrial feel, which is the 
antithesis of the organic houses that I like.  But is seems reasonable 
to me to consider this option for foundation/stemwalls.

Derelict

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu

--On Wednesday, December 22, 2010 10:41 AM -0500 Chris Magwood 
<chris at chrismagwood.ca> wrote:

Hey all!

I'm wondering if anyone out there has much or any experience working
with Autoclaved Aerated Concrete (AAC) blocks and/or Rastra blocks
for foundations?

Neither product is readily available here in Canada, but I'm
preparing some info on insulated foundation options and would like to
include something about these products if I can get some info. I've
obviously web-researched both, but I'm looking for actual
experience... did they meet your cost expectations, were there
peculiarities about the install process, how would you rate them
environmentally, etc?

Feel free to respond to me off-list, as I know this isn't exactly SB
stuff. However, I'm hoping that among the vast experience included on
this list would be some hands-on stories about these materials.

Happy holidays!

Chris

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John Swearingen
Skillful Means Design & Construction
2550 9th Street   Suite 209A
Berkeley, CA   94710
510.849.1800 phone
510.849.1900 fax

Web Site:  http://www.skillful-means.com
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