I'm not sure where to pot this, but I'll start here. We are adding and addition to our 100 year old home. It will be a 400 sqf basement (16'x25') with one finished floor above it. It will connect to one wall of the existing house. The basement will be about 3' exposed on the West end and 5' exposed on the East end. The long wall (25') will be Southern exposure. My question is; what would be the technology to build the basement with? My designer likes the Weaver pre-cast walls. I would like make it as energy efficient as possible without going to some super expensive solution. Any suggestions? Thanks, Ron C

Insulating Concrete Forms (ICF).

How do you cover the exposed exterior?

What is your location/zip code (for climate & weather, which can affect recommendations)?

There are various finishes that can be applied to the exposed EPS of ICFs (a whole topic of discussion on the ICF forum on this site), including wood clapboards, if that's the look you're going for.

In some instances it can make sense to do the whole thing (including the floor above the basement) as ICF, and with a long southern-exposed wall the thermal mass of a concrete wall can affect the net heating & cooling efficiency by a measurable amount.

What is the first-floor construction, including siding and interior finish?

21771, Mount Airy MD

The inside of the basement will conditioned living space as well as the upper floor. The exterior will be covered with Hardy board.

Maryland is US climate zone 4, and the long-term economics of an R22-25 whole-wall R value (all thermal bridging of structural elements) would make some sense.  R22 is about the minimum-R for most ICF vendors, and the thermal mass would give it a slight performance edge over a low-mass R22, but not necessarily an R25 low-mass solution. If doing basement-only as ICF you could cut back to R16 (there are a few vendors that will do 2" EPS, and add an inch of rigid EPS or XPS to the above-grade portion.

To hit the~R25 range for a stick-built upper wall would mean 2x6 with blown/sprayed cellulose (or high-density fiberglass) and 1.5-2" of exterior rigid foil-faced iso sheathing (seams taped, edges sealed with spray foam) outside the structural sheathing.  You can use 1x4 furring on the outside of the foam, through-screwed 24" o.c. to the studs to support the fiber-cement siding using minimal length fasteners for the siding to avoid fully penetrating/thermallly short-circuiting  the foam.  With 1.5" foil-faced iso the whole-wall R will run nominally ~ R24, but the foil facer delivers another ~R1-1.5 or so on average performance as long as it has 3/4" of air-gap between it and the siding due to the radiant-barrier effect.  With 2" iso you'd be at ~ R27+ rb effect.  Any more than that becomes not cost-effective in the longer term, but at ~ R25-ish with this (relatively) low cost method there's an economic rationale in US climate zone 4.

If you go ICF basement, foam-clad stick built on the upper floor, place the foundation sill such that sheathing foam either aligns perfectly with foam of the foundation (or slightly outside the foundation foam), to ensure the cavity between the foam & siding always drains properaly. Either way you'll need to use Z-flashing  if you're installing the housewrap between the foam and structural sheathing above rather than let exterior moisture to drain to the concrete.

On any exposed section of ICF you can Tapcon the 1x furring  to the foundation 24" o.c. for hanging the siding, much as with the stick-built. Allow both top and bottom venting of the cavity between the foam & siding for maximum drying- when the sun hits dew or rain-wetted Hardie the moisture drives are pretty intense for awhile, but allowing the siding to dry in both directions keeps both the siding and the structural walls drier and happier.

In a conditioned basement you'd also want at least R7.5 under the slab for both heat loss and summertime mold under finish-floor considerations in that location.  If you're going all out and building in a radiant floor, make that R10 minimum, R12 not-insane.  The radiant-floor guys like to use XPS under slabs because you can then staple the tubing to the foam and it'll hold, but EPS is usually cheaper per unit-R, and has sufficient structural loading for any residential application under a code-minimum slab thickness.   With an ICF foundation, butt the slab-foam directly up against the wall-foam to eliminate thermal bridging to the footing &  subsoil, essentially floating the slab.

If it makes architectural sense, pitching the roof along that southern section so that the slope faces the sun, and even wiring the pitch for future photovoltaic power may make long term sense.  PV prices have been crashing lately, and if not today, at some point in the next decade or so the economic of PV may become compelling. (With local & federal subsidies that include selling the production-credits it's fairly compelling right now.)

If you're going with a vented roof design, setting it up so that you can put at least 18" of cellulose (~R60) at full depth right out over the top plate of the studwall or ICF has an economic rationale, as well as an enhanced summertime comfort aspect. Code-min is R38, but R38 in fibeglass really doesn't cut it for comfort- that ceiling will get pretty warm in summer. But higher-density fiber + higher R helps quite a bit. (Low density fiberglass blowing wools are pretty crummy on comfort even at R50+, and dense-packing or high-density batts get pricey compared to blown cellulose.)  With any cellulose product insist on "borate only" fire retardents, to avoid any corrosiveness issues with sulfate-based fire retardents should it ever get wet.  Borates reduce the ant/termite/wasp risk too, since it's toxic to the gut-flora of wood-boring insects that allows them to digest the wood.