28 March 2011

Why Infill Stawbale?

Strawbale walls can be constructed in two basic ways: loadbearing or infill.  Loadbearing strawbale, as its name implies, involves constructing strawbale walls in such way that they support all roof and any second floor structural loads directly on the bales themselves.  The first strawbale structures in 19th Century Nebraska were built this way because wood was a scarce commodity.  Infill strawbale involves building some independent structural system - stud walls, post-and-beam, etc. - to support all structural loads, and then strawbales are infilled between or around that structural system.

So the question is: if you can use the strawbales as your structure, why would you build any other way?  It turns out there are two main benefits to building infill strawbale structures instead of loadbearing structures.



EASE OF PERMIT APPROVAL
Infill strawbale meets the current building codes throughout the U.S.  That's right, infill strawbale meets the current codes!  See my blog post on this topic.  Loadbearing strawbale walls, on the other hand, are taken on a case-by-case basis by your permit officials (unless you live in a region that has passed a separate loadbearing strawbale code).  Because strawbale as a loadbearing material is not in current U.S. building codes, you need to demonstrate precisely how the structural capacity of the strawbales will support the building.  This usually means you will need the assistance of a structural engineer to demonstrate to the permit office that the strawbales will indeed support the structural loads of your building.  This may mean you incur an additional cost (for the engineer) and your permit may require additional time for review.


THE WEATHER!
If you live in a climate where it rains during the construction season, then infill strawbale may make your life less stressful when you build.  When using infill strawbale construction, the roof is completely on and leak proof before you begin installing your strawbales.  This means that all strawbale installation is protected from rain during construction.  The concern with water infiltration is only on the top of the bales, not the sides.  So as long as you install entire sections of wall from floor to ceiling, your bales should stay dry during construction until you are ready to plaster.  (NOTE: the side of the wall that faces out is not a concern unless you lay your bales on edge; if you lay your bales flat, with the straw ends poking inside-to-outside, then any rain will only penetrate and inch or two into the bales and then come right back out, leaving your bales nice and dry inside where it matters.)

I am not advocating one or the other technique for building with strawbales.  Both methods work nicely and both result in beautiful, well-insulating walls.  However, I find it helpful to take all variables into consideration when deciding which construction technique is right for you.



RESOURCES
I think these two books are the best resources to learn about how to build with strawbales in wet climates.  All of the authors live in snowy, wet, humid climates.

         
click the book covers above for more info or to purchase

2 comments:

  1. Hi, We are planning on building 16 Straw Bale houses so found your blog short but sweet, quick question though.. With infill do you need the hazel pins to stabilise the walls? we estimate we would need around 7000 4ft straight sticks but that is based on the load bearing method commonly used here in the UK. We live in a wet part and are group-building so in-fill make it easier with british "summers"being what they are. Thoughts???

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    1. I do still pin the strawbales to each other when I do infill. It is not really necessary structurally if your framing has some type of lateral bracing. However, it really helps with safety during construction, and the pins lock in the strawbales to each other so it saves you time adjusting the bales later. I use bamboo pins, as that is a readily available invasive material available here in the Eastern US. But the hazel pins are fine as well. Technically you can skip the pinning for infill strawbale, but then a) don't sit on the walls as you build, b) be sure your top row goes in super tight, and c) be sure that your structural system has lateral bracing built into it. Hope this helps.

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