[GSBN] High Performance Strawbale

[Chris Magwood]

Hi Alfred,

The creation of bale buildings that address infiltration has been my 
main focus for the past few years of design/build, and we've got it down 
pretty well now. Our last blower door test had 1.2 ACH/50, and 
afterwards we realized that the plumbing stack was open! So the walls 
and wall junctions were really tight. The house we did for Habitat for 
Humanity last year 
http://endeavourcentre.com/our-portfolio/habitat-for-humanity-house/ had 
some issues around the straw/clay walls we did, but the bale walls were 
tight and the house ended up at 2.1 ACH/50.

In particular, I've been focusing on how to achieve this level of air 
tightness without lots of yucky "product." For the bale walls, here's 
what we do:

- a strip of air barrier runs along the top edge of the wall, running 
along the bales, under a strip of mesh, so that the barrier is covered 
by the plaster and ties into the barrier in the ceiling. This is the 
biggest leaky place in most bale buildings.
- depending on the floor/foundation type, we'll use a similar strategy 
at the bottom of the wall for wooden floor systems or good sill 
gasketing and a caulk line on the interior and exterior edges for slabs.
- at windows, we don't use spray foam but a light mix of hempcrete to 
seal around the windows. Barrier strips are used here too under the 
plaster seam.
- electrical outlets get site-made barrier "hats" and we caulk around 
the wire once it's been put through the barrier. We've also used the 
plastic "R-2000" electrical boxes but we don't trust the neoprene seal 
for the wire and caulk these too.

It's really pretty darn easy to get a bale wall air tight. The beauty of 
the continuous plaster is that there aren't that many seams, and the 
ones that do exist are easy to predict and do something about! It's a 
much less difficult and troublesome process than making a frame house 
air tight.

As a side note, the prefab panels we've been making have these barrier 
"tails" already embedded and ready for joining to other building elements.

The insulation value question is about to come and haunt us in Ontario 
too. In January, the codes here raise the required R-values and I'm sure 
we'll be facing some questions from plans reviewers who use the lowest 
possible figures that are out there. I'm hoping that we can set up some 
full-scale testing here to get good figures on a full wall panel, and of 
course we'll share any results we get.

Chris "Starving for Air" Magwood

On 11-10-03 11:49 AM, Alfred von Bachmayr wrote:
>
> Greetings all,
>
> Thank you for including me in this elite group of building scholars.
>
> I wanted to get feedback on some performance aspects of strawbale 
> building that I have been confronted with by energy modeling and 
> regulatory entities that question its thermal performance.  This 
> resistance appeared when I contracted a Hers rater along with 
> mechanical engineers who, in generating thermal models of a strawbale 
> building I had designed, consistently gave it low ratings for R-value 
> and infiltration.  In researching the R-value they had given to the 
> walls, I found that from the resources they had found, the value was 
> somewhere between R-23 and R-27.  I got a copy of the newly proposed 
> International Green Conservation Code and found they rated bales laid 
> flat at R-1.3 per inch and on edge R-2.  That equates to R-23.4 for 
> flat bales (2 string) and R-28 for bales (2-string) on edge.  I don't 
> think this gives and accurate performance prediction of  the 
> building.  In addition they considered the buildings to have high 
> infiltration losses because of their opinion that they cannot be 
> tightly sealed.  Their models indicated that the building could not be 
> considered high performance due to the use of the straw thereby making 
> my job of getting the building to the required Hers rating value much 
> more difficult.  I am thinking I am not alone in this problem.
>
> This made me realize that we need to generate more convincing 
> documentation as to the performance and design detailing of straw 
> buildings.  There appears to be little resources out there that 
> engineering professionals access that address the infiltration and 
> insulation values of straw.  This is important because without a 
> sufficient HERS rating and a positive blower door test (now being 
> required in many jurisdictions) a permit and/or a CO will not be issued.
>
> What I would like to hear from you all is the location of resources 
> documenting tight houses out of straw (like the passive houses) and 
> specific techniques you have used to tightly seal up straw buildings. 
>  Specifically what techniques have you used to seal around the windows 
> and doors, at the bottom and top of the walls and at penetrations of 
> the interior finish skins for electrical boxes.  Are there buildings 
> out there that have gotten their HHI ratings down below 2 BTU/SF/DD?  
> If so what techniques did they use and what insulation and 
> infiltration rates were used?
>
> Thank you for your help.
>
> Saludos,
>
> Alfred
>
> VON BACHMAYR ARCHITECTS LLC
>
> 1406 Bishops Lodge Rd.
>
> Santa Fe, NM 87506
>
> 505-989-7000
>
> 505-984-1479     fax
>
> 505-470-6141     mobile
>
> vbarch at comcast.net
>
>
>
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-- 
www.chrismagwood.ca
www.endeavourcentre.org

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