I recently did a remodeling project that involved replacing a three-season sunroom with fully conditioned living space. Since the budget was tight, we planned to keep original sunroom’s footprint and build on the existing uninsulated brick pony wall. Unfortunately, when we dug down to confirm the footing depth, we found that it was barely 24 inches below grade. The inspector gave us two choices: We could tear out the existing foundation and wall and pour new 36-inch-deep footings, or underpin the existing foundation to the required depth.

It was evident that the existing foundation didn’t need much additional protection from frost—it had already lasted three decades without heaving or cracking—so I researched a more cost-effective solution, which called for installing a continuous layer of rigid polystyrene insulation against the foundation wall, extending to the base of the footing.

I emailed the building inspector copies of the documentation I’d pulled together, including tables from the IRC. The tables confirmed that when the exterior is insulated with a layer of 1-inch polystyrene, the footing depth of a heated building can be as little as 12 inches below grade in our area of Southeastern Pennsylvania—with no requirement for extending the insulation horizontally. The inspector agreed to the plan, and the construction went off without a hitch.

A version of this article originally appeared in our sister publication, The Journal of Light Construction.

Learn more about markets featured in this article: Phoenix, AZ.