Jen’s Boot Camp and Allerton Abbey Experience

When we were stuffing cob into the little gaps between logs during last years ATC, there was a spot or two where you could tell there was an air gap behind the logs. My guess is that there still is a little bit of air right behind the logs in the space where the logs curve in to touch the log above and below and that air insulates the mass against the charging. The wood itself being more insulative relative to the soil mass also limits heat transfer through the thickest part of the logs which are most likely touching the soil mass. John Hait had the benefit of using the concrete shell against the mass so no insulation value there, and the smooth surface allowed soil to settle against the wall and limit air pockets.

So it'll be a learning point to remember for the next wofati build, as soil is added against the outer logs to form the berm, to be sure it gets tapped in snugly against the back of the logs to fill in any air gaps. Maybe splitting the logs in half lengthwise to reduce the insulation between soil and inner living space would help, but at a cost of wall strength. Having posts every 4 feet instead of 8-10 feet would allow using milled boards, which would certainly make it it easier to get that thermal contact, but does involve more posts and milling versus more exterior logs.

Mark, you are completely correct. That is definitely one of the things everyone wishes had been done differently for the Allerton Abbey build. Definitely something we want to work on for the next wofati!

I wonder too if it might have been good to put a thermal break between the ground and the dry soil mass.  A membrane with a horizontal wall of logs and then another membrane before laying down the dry soil layer might be an option for that.  Another thing to consider might be no vertical horizontal logs, but rather the use of horizontal fibers/branches to hold the thermal mass from subsiding and then cob to finish the wall so that there is no thermal break between the mass and the living space.  Don't know....just thoughts.

Here are the ATI graphs after our return from our trip—you can see the cluster of days when we artificially heated the Abbey, a second, lower cluster when we were living and cooking in the Abbey but not using the stove for heating, followed by a steep drop while we were gone, and ultimately a plateau around 40F when the temperature stabilized and actually began to come up slightly on its own before our return (although the sharper upward trend at the end is due to our beginning to live and cook in the Abbey again).

(BRK Day 67)