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Hey Sarah, Code definition 100.1 for conditioned floor area (CFA) is the floor area (in square feet) of the enclosed conditioned space on all floors of a building, as measured at the floor level of the exterior surfaces of exterior walls enclosing the conditioned space. Hope this helps!

Thanks Kaitlin. Thats what I thought, but was hoping they measured inside the walls. Seems like it doenst make sense on thick walled buildings....say a straw bale house. its not space that needs to be conditioned.

Oren Leiman , nice to see you here! I agree that this situation requires careful thought and assessment before moving forward. These walls have been sitting there for 90+ years, with heat from the interior freely moving through open wall cavities and drying out the backside of a potentially leaky stucco assembly. Blowing insulation into these walls will reduce heat transfer (and air movement behind the stucco assembly) and could cause moisture issues. But it also could be fine... can you tell us more about the situation: 1) what's the integrity of the stucco? does it have a coating? 2) any evidence of interior moisture damage, especially at windows? 3) have you done any exploratory testing to see what things look like from the inside?

Thanks Katy!
No obvious leaks from old stucco. Major renovation begins in fall.

In New England, USA, i put vapor barrier behind sheetrock during ac remodel. …, 30 years ago

Some of them are not HVI rated but still provide the benefits of a balanced ventilation system with heat recovery. There are efforts underway to modify code to simplify their code compatibility. For example, some are rated to European standards and it may be as simple as recognizing those. A work around is to model them as balanced without heat recovery. These require fan power inputs and those ratings from manufacturer spec sheets are acceptable if they are not available in the HVI database. Although, there's an archaic ASHRAE rule requiring a minimum amount of duct work on exhaust fans if not properly rated for sound—really have to get lost in the weeds to find that. To avoid this, install both the ductless systems and a bath fan with continuous operation capability for compliance. There will likely be a penalty losing w/o heat recovery, but at the ADU scale it's not as significant. Almost certainly not an issue when using a HPWH. Hopefully they simplify ductless ventilation requirements in the next code cycle.

thanks Nate and good idea

I added the requirements last time around to my original post. He said the windows also had to be higher standard, but I think the numbers were easy to meet.

It is currently a gas on demand...so perhaps that is a ding.

Thanks, Dan McDunn  McDunn Construction , this is great. So after installing a few times, is this your new go-to slab edge method?

I like this method as it gives the same basic feel as an exposed slab edge, and it eliminates any weird connections of material or corners. The first one we did we had the foam proud of the siding and had weird returns into door jams and so forth. This detail eliminates all of that clunkiness. This just puts the face of foam in the same plane as the concrete would be in a typical house without slab edge insulation. Copper is sweet but expensive, so if you are going to paint it use bonderized sheet metal for better paint adherence.

The termite shield could still go in behind the sheet metal. Although, the foam is not a food source, and the mud sill in PT, and the adhesives used for the foam and the copper, and the sill sealer below the PT, I believe combine to pretty well eliminate termite risk. Can't hurt though.

We have done what Katy suggests on 2 recent ADUs. On A third ADU we just kept the existing gas water heater and connected it to the ADU. All three have a demand operated recirculation pump. It never made any sense to put a heat pump hot water heater in an ADU...biggest ADU I've done is about 500 SF. I think I read that electric on demand are now allowed in small buildings. Katy...did I hear that from you or your guide? I forget how small the building has to be to allow that, but it might make environmental sense in many cases.

Sarah, in theory, electric tankless WH are allowed for homes ≤500 sq. ft. but as explained in the e-guide (see pg 104) this may not fly from the standpoint of the entire home meeting Title 24 energy code, since all of the compliance now needs to come from the envelope but even with great windows, slab insulation and other above-code specs, you may exceed both the heating and cooling budget and ultimately not be Title 24 compliant... (mainly since ADUs have more window area than allowed prescriptively due to how the code works.)