Slip straw building

Hello all!  I am in the process of purchasing 80 acres in Northern California. There is an incomplete stick built house/structure on the property already.  I’m inheriting someone else’s plan for a house so until I have the ability and time to build the  wofati-esq home of my dreams I am planning on finishing the 2x6 I insulated building.  I will have tons of time, less monies.  I was looking at slip straw as an insulating fill in between the studs.  My question for the forums was as follows (now that the back story is there).  I have acres and acres of invasive Himalayan blackberries.  I need to clear many of them away.  Has anyone ever used the chopped dried canes as a building medium ? I was thinking of using this plant that I have too much of in conjugation with some smaller particles sawdust and wood chip to make a mix for the slip straw fill. If this is a terrible idea please let me know why?

I've never used dried canes in slip straw before. Slip straw works as an insulator because the hollow stalks of straw create lots of little air pockets. If the dried canes are hollow and durable enough not to be completely crushed when packed into the wall they should be fine. I would avoid adding sawdust and wood chips to your slip straw. Wood has a really poor R-value and it looks like your stud bay is only six inches deep so you want your slip straw to be as insulative as possible.

That will be a lovely house when finished.
Hemp crete has slightly different properties and if your blackberry stuff is similar, then it may work.
How do you plant to process the plants you have already ?
Do you have any clay around?
What is the climate like in your region?

John C Daley wrote:That will be a lovely house when finished.
Hemp crete has slightly different properties and if your blackberry stuff is similar, then it may work.
How do you plant to process the plants you have already ?
Do you have any clay around?
What is the climate like in your region?

blackberry is similar to the hemp as far as the canes go.  I was planning on cutting them into 3-5 inch chunks and mixing with slip in a cement mixer.  There should be clay around I saw a few patches when I was up there a week or so ago .  Is northern humbolt redwood country.

Hi Tatton,

I would not pack slip-straw (light-straw-clay) into a cavity that has a very low vapor permeable barrier on one side--that's asking for the straw to begin decomposing because the mix can't dry out quickly enough.  

A 12" wide slip-straw wall is pretty much the maximum LSC wall depth for this reason, and the walls are left to dry from both sides until ready for plastering, usually at least three months of warm, dry summer conditions, and more often eight-to-ten months later in areas with damp winters and less than super dry summers.  

Theoretically, a 6" wall will dry to one side only (the interior) since the plywood (presumably 1/2"? CDX ) has a very low vapor permeance--it'll allow water vapor to escape, but probably not fast enough to prevent moist, warm conditions from decomposing the straw.  You could drill a bunch of holes in the plywood to make it more vapor permeable, but there's a limit to that as the plywood is probably the material that supplies shear (out-of-plane force resistance) for the structure.  Too many holes and it won't hold up as expected.

Unless someone on this forum has done what you propose in a climate similar to yours and has checked to see that in fact the slip-straw survived without signs of decomposition...I'd look for a different insulation here.  Unless you want to take a chance and possibly have a very costly do-over on your hands.

Jim Reiland
Many Hands Builders