Why This Old House chose concrete block over poured concrete for a new foundation under a jacked-up addition

The old stone foundation sat on clay-loam soil and was too shallow, so the crew hand-dug the trench to 4 feet, below frost depth, and poured a footing 12 inches by 22 inches. The International Residential Code requires footings and foundation walls to extend below the local frost line unless they are protected some other way, and exterior footings to sit at least 12 inches below undisturbed ground.

On that footing the mason lays 10-inch block. Vertical rebar is set into the footing with epoxy and runs up through the cores, horizontal joint reinforcement goes into every other course, and the blocks are laid so their cores line up for grout. Once the wall is up, the outside gets a waterproofing coat, 2 inches of rigid foam and a parge coat.

The video also covers cost: with the addition already on jacks, block comes out cheaper than a poured wall. A cost guide from Metal America Concrete gives general context. It puts block walls 10% to 25% below poured walls in most markets, says block needs no forms and no ready-mix truck schedule, and notes that block remains common for additions and repairs where getting poured concrete in is impractical. The same guide rates poured walls higher for water resistance and for resisting soil pressure.