Terry Ruth wrote:Using similar materials (clay or binder type mainly, and aggregates, brick/COB/mortar) of the same density and plastic index(PI) will be key at the interfaces in this hybrid design so they expand and contract at the same rate or are not too far off each other. High density will be less permeable or "breathable" but will be more structural and visa versa. PI determines expansion/contraction. How breathable and how structural will depend on seismic and wind activity an Engineer can determine structural requirements. If they spec out a high density brick, mesh and bar reinforcements, then other less dense hygroscopic materials like an inner plaster can be used to re-gain breathability. In harsh cold climates, double wykes may be required. There the outer wyke can be a structural rated brick to take loads, the inner can be a brick_COB combo for breathability and anesthetics. The center insulation core/vent cap would serve as a thermal brake an ventilation gap. The core r-value would depend on climate. Roxul makes an rigid mineral board IS that is high perm (30 ish) for applications like this and they are rated to be in contact with soils to also be used as foundation outsulation.
What I do when I'm not sure how designs will perform is build a mock-up in the climate zone, add radiant heat from a heater and/or moisture from a spray bottle and see how fast it dries or if it cracks. I'll even soak it in water or spray it with a hose. Temp and moisture meters always help. A pro lab is always best.
The key to success in this hybrid design will be using similar materials.
Materials of the same density and plastic index (PI) will expand and contract at the same rate. Dissimilar materials expand and contract at differing rates, which causes them to crack, or to separate from each other.
If you use high density materials, they will be less permeable or "breathable" but will be more structural, capable of carrying heavier loads. If you use lower density materials, they will be the reverse: more "breathable", less structural.
Your criteria for choosing how breathable to build your walls versus how structural are seismic and wind activity. An engineer can determine the structural requirements for you.
When you consult the engineer, if they specify a high density brick with reinforcing mesh and bar, then that will be a wall that's very structural and not very breathable. If you want to regain some breathability, then you can use other, less dense hygroscopic materials like an inner plaster.
Masonry is a poor insulator, so in harsh cold climates, you may want to build a double wall system. If you do, the outer wythe can be a structural rated brick to take loads, and the inner wythe can be a combination of unfired bricks and cob, for an example of breathability and aesthetics. The gap between the inner and outer wythes would serve both as a thermal break and a ventilation gap.
The r-value you should try for, that would depend on your climate. Roxul brand makes a rigid mineral board that is high-perm (30ish) for applications like this, and they are rated to be in contact with soils, so they can also be used as foundation outsulation.
What I do when I'm not sure how designs will perform is build a mock-up wall in the climate zone. I add radiant heat from a heater and/or moisture from a spray bottle, and then see how fast it dries or if it cracks. I'll even soak it in water or spray it with a hose.
You can learn good information this way. If you can use temperature and moisture meters while you do it, that's even better. Setting it up and having it tested in a professional lab is always best.
Similar materials or not without it "natural" designs will fail