In the Northeast, we have the benefit and burden of a large stock of uninsulated masonry buildings of various typologies and conditions. Leaving these buildings as they are is untenable with the global effort to reduce carbon emissions, and will not address climate shifts, the fabric of community, or the health of occupants. We will explore insulating from the interior, exterior, or both. In all cases the approach and design must be informed by retrofit feasibility, durability and toxicity of materials, installation cost, embodied carbon, emissions, labor capabilities, and overall envelope performance including freeze/thaw damage.