Houses built a hundred and fifty years ago were built out of brick or stone, then had an air space, lath and plaster, often made with horse hair. It had a lousy U-value, or thermal transmittance, the amount of heat that goes through the wall.

Old houses (typically built before the mid-20th century) were designed with a "breathable" philosophy rather than an "airtight" one. Before modern central heating, homes relied on fireplaces and natural ventilation; walls were built thick using solid brick or stone without a "cavity" to stop moisture. These solid walls are naturally porous, allowing water vapor to pass through. If you add modern closed-cell insulation to these structures, you trap moisture inside, which causes dampness, mold, and rot in the original lime mortar or timber frames. Furthermore, building materials like wood and early stone were chosen for structural integrity rather than thermal resistance (R−value). In 2026, the focus for insulating these homes is on "breathable" natural fibers like wood fiber or lime render, which improve warmth while still allowing the house to "exhale" moisture as it was originally intended to do.