Building Science Notes

A brick rowhouse or brownstone can stand for 130 years and then be damaged by a well-meaning renovation. The owners insulate the walls from the inside, close everything up, and enjoy a warmer house. Years later the face of the brick starts to flake off, or a floor begins to sag where a beam end has rotted inside the wall. By then the cause is hidden behind finished plaster.

It doesn't have to go that way. You can insulate a historic building safely and keep its character, if the work is designed for how old walls behave.

What goes wrong

Most of Brooklyn's rowhouses have solid brick walls. In winter, heat leaking out through that brick keeps it warm and helps it dry after rain. But insulating the inside without thinking it through means the brick gets colder and stays wetter for longer. That can lead to:

• Freeze-thaw damage. Water soaked into cold brick freezes, expands, and breaks off the face of the brick (spalling). Mortar crumbles.
• Rotting beam ends. Floor joists sit in pockets in the brick wall. A colder, wetter wall can rot them where they can't be seen.
• Mold. Warm, moist indoor air that gets behind the insulation can condense on the cold brick and grow mold behind the finishes.

None of this shows up right away. It can take years.

Why this matters now

For a long time, New York's energy code simply left many historic buildings out. That was partly to protect them: the wrong insulation can cause exactly the damage described above, and careless upgrades can strip away the details that make a building worth keeping.

That has changed. Since 2026, a renovation of a historic building in New York City has to meet the energy code for the work being done, like any other building. Exceptions are still possible, but only feature by feature, and only where meeting the code would harm the building's historic character. Each one has to be justified in a report.

And now NYC landmarks and buildings in the city's historic districts, which have always been subject to energy code requirements, are able to take advantage of this exemption in order to preserve character-defining features.

So for more old buildings than before, the question is no longer whether to insulate during a renovation. It is how to do it without causing harm.

The right way

• Fix the water infiltration first. Repairing gutters, leaders, copings and flashing, and repointing with a mortar that suits the old brick, are necessary first steps to keep bulk water out of the building. After we fix this, we can determine if interior insulation is acceptable.
• Test before deciding. Check the brick and model how the wall will handle moisture over many years (a hygrothermal analysis, using software such as WUFI), rather than guessing.
• Use insulation that lets the wall dry. We use only vapor-open insulation on the inside of solid brick, so moisture that gets into the wall can dry back out toward the room. The right material and thickness depend on the brick, the exposure, and the climate. More insulation isn't always better.
• Make the inside airtight. Keeping warm, moist indoor air out of walls is as important as the insulation itself. Selecting the right air barrier materials is critical.
• Ventilate. A tight house needs a steady supply of filtered fresh air, and exhaust from kitchens and bathrooms. Mechanical ventilation also keeps indoor humidity in check, which protects the walls.
• Protect the beam ends. Detail the joist pockets so the wood stays dry.
• Preserve the building's facade. Insulating from the inside leaves the facade untouched. We usually replace the windows with Passive House quality units that match the look of the originals: triple glazed, airtight, and much quieter.

The takeaway

Comfort, durability, and preservation don't have to compete. When the walls, windows, ventilation, and water management are designed together, the house is warmer, quieter, and healthier to live in, and the old brick lasts longer. Each part does more than one job.

This is how we approach Passive House retrofits of old buildings. If you are planning work on one, get in touch before the walls are opened.