[Strawbale] window construction

[Jeff Ruppert]

Rob,

I must respectfully disagree on a few things.  My perspective is a 
little different.
>
>
> The means to avoid creating that thermal bridge would be the same as with  
> any other wall system and that is, as you imply, by including a thermal  
> break. The most obvious first step towards that end is simply to not  
> utilise a solid lumber, full wall-width buck and insulate.
>   
I am looking at a typical house plan on my desk right now that is about 
1800 sf in floor area.  It includes 28 buck posts (doors and windows).  
Each buck represents 0.33 sf (1/2" wide plywood * 8' tall) of wall area 
when standing inside the building looking at the walls (the plywood on a 
wide post is the only continuous part through the wall, and can re 
reduced in area with a little thought), for a total of 9.3 sf of exposed 
buck area as "thermal bridge."  There is approximately 180 linear feet 
of exterior bale walls, which represent approximately 1440 sf of wall 
system.  The percentage of buck "thermal bridge" area to the overall 
wall system is a little more than 0.6%.  In reality it is a bit larger 
than this due to windows (which have a total R-value of maybe 3).  The 
ORNL thermal test gave us a wall system R-value of 1.45/in for a bale 
wall.  According to a table I have of R-values of different materials, 
plywood has an R-value of 0.63.  Even though this is less than the bale 
wall, it is still significant across an 18" wall, for a total R-value of 
11.  Of course if you are using this system there will be a box-beam, 
which also has plywood.  This area would add approximately 15 sf of 
"bridge" area.  The total may be around 1% of the wall system area at R-11.

I do not believe that a thermal subtraction of less than 1% of the 
overall super-insulated wall system will make enough difference when 
comparing losses that occur when doors open and close, or at windows.  
The order of magnitude is way different, and IMHO, the effect is not 
worth writing off a worthy bale-wall framing system. 

Now, this effect can be reduced, if you are still bothered by it, by 
decreasing the depth of your posts 1" on the interior and stuffing the 
vertical void with straw - simple!

> ...ill-conceived idea from the  
> get-go and would best be avoided. 
I disagree with this blanket statement.  I believe the idea has merit 
and is best used on smaller windows.


> On the negative side, IMO , firstly it demonstrates a disrespect for the  
> materials and the labour that went into making the cut-out section,  
> treating both as "commodities" and disposable at that.
>   
I think using 4x4 posts and single beams, and notching bales around 
them, is hugely disrespectful of a bale wall system.  The stacking of 
bales takes weeks instead of days, and you end up with a mountain of 
loose straw (fire hazard) at the end of each day for weeks.  The total 
notching of bales for a narrow post system is not being quantified, IMO, 
in terms of labor costs. 

For smaller windows, cutting the bales out is no big deal.  The trick is 
to not plaster the window area to begin with so there is no cutting of 
plaster or mesh.  The cut opening will be larger than the window unit to 
allow for a buck, mesh, flashing and plaster.  The edges of your second 
coat of plaster can be held back at any distance from your cut opening.


> Second, it makes for a situation which is extremely difficult to flash and  
> seal in an effective fashion, which ultimately leads to  
> performance-related problems and failures later on for the homeowner and  
> that, is disrespectful of the homeowner.
>   
Windows can be flashed using this method.  You just need to think about 
it for a minute.  Without going into great detail, it just does not have 
to be like this.  We have done this with much success.

> Third, using an abrasive blade to cut through oftentimes  
> metal-mesh-re-inforced Portland cement plaster creates an incredible  
> amount of gritty, permeates-everything, destructive (ie to motor bearings)  
> and harmful-to-respiratory-systems dust... and creates sparks that have  
> the potential to ignite any loose straw, or worse, drive hot molten metal  
> into the straw where it would go unnoticed and could cause smouldering  
> straw for a long time after.
>   

Not necessary!
> Fourth, pounding on a newly-plastered wall to knock out the cut-out  
> section is an excellent way to introduce cracks into otherwise good  
> plaster.
>   
Not necessary!

Dave, great questions!  To answer your questions about cutting the 
windows out, no, the bales do not fall out into the opening, and no, you 
do not have to follow bale edges.  You do want to do it after the second 
coat.  In the past we have made scrap-wood templates for the windows and 
placed them on the walls so we know where the edges of our plaster will 
end up.  Then you can use a chainsaw, or hay knife, whatever for 
cutting.  You can pre-mesh the corners of the openings if you are 
worried about cracking. 

I wish to not let fear about this method be spread around without some 
qualification.  I think it has its place and should not be totally 
dismissed.  We have done a bunch of windows of all sizes this way and 
have learned much from the experience.  If anyone else has done this 
themselves many times, speak up.  Rob, how many windows have you cut out 
yourself?

Jeff