Hello everyone,
I’m building a 16 x 20 addition on an existing timber frame in climate zone 6 (central Maine). As part of the process, I would like to experiment with some traditional building techniques such as an exterior lime-based plaster and interior clay-based plasters. The building will be conditioned and occupied.

I would like to use a dustcrete infill around conventional framing. Dustcrete is an analog of hempcrete but uses softwood sawdust (in my case) as the filler and a lime-based binder. Like hempcrete, this material is layed up wet and when dried is plastered on both interior and exterior surfaces. The structure will have large overhangs to protect the plastered walls. I have poured my footer but have not yet poured the foundation walls because wall thickness is in part depending upon my decision on this topic. 

Dustcrete, like hempcrete is often described/marketed as a monolithic wall with decent temperature swing modulations. However, on a per-inch basis the R-value is low compared to more mainstream materials (around 1.5 per inch depending upon sawdust type, packing density, etc.). I’m considering building walls that have dustcrete on both the exterior and the interior, with a layer of either rockwool or wood fiber insulation in the center to raise the R-value.

My primary concern with this system would be the length of drying time for the dustcrete and the amount of moisture that the insulation would take on before being able to dry completely. On each side of the insulation I would leave the wooden laths used for forming and keying in the dustcrete, which would potentially leave a small air channel. I could leave a wider airspace between the layers if that would be beneficial.

Of course, the other consideration is foundation wall thickness. If I go the monolithic route (no additional insulation beyond dustcrete) I’ll likely pour a 12″ thick wall. I could go with a thinner wall (and much less concrete) with the “sandwich” wall.

1. Has anyone had any experience with this type of wall assembly and/or what are your thoughts about the insulation being able to withstand the water content during the drying period? I would prefer to use wood fiber but would opt for mineral wool if the initial wetness is too risky. I’m also going to reach out to Timber HP to get their take on using their product in this fashion.

2. Despite the overall lower r-value of the wall without the added insulation, do you think overall wall performance would be better in the monolithic scenario? In other words, is it worth the trouble to add this insulation? 

Thanks for your help,
Eric