· by Michael Blair, Nashville's Windows & Doors
A lot of houses in East Nashville, Sylvan Park, Belmont and Edgefield still have their original wood double-hungs. The glass is wavy, the sash cords may be broken, and the room is drafty in winter, so the first quote is usually for replacement. Often the better answer is to repair the sash and add a storm window, which stops most of the air and keeps the original window doing what it has done for a century. The real decision is whether that storm goes on the inside or the outside, and it is worth settling before the heating season starts.
Why a storm window does the work
An old single-glazed sash loses heat in two ways: through the glass, and around the sash where it rattles in its track. The air leakage is usually the bigger of the two. A well-fitted storm window closes both, adding a still air space in front of the glass and a second seal around the whole opening, and it does it without removing the frame, the trim or the plaster around them.
The sash itself stays repairable. Glazing putty can be renewed, sash cords rehung and weatherstripping added to the meeting rail. None of that is possible with a vinyl insert that replaced the sash, and the old wood, milled from slow-grown timber, tends to outlast the replacement if it is painted and kept dry.
Exterior storms
The familiar version is an aluminum frame with two or three tracks, mounted to the outside of the window casing. Its great strength is protection: it keeps rain and sun off the sash and the sill, which are the parts that rot. Modern units are slimmer than the old mill-finish ones, come in colors that disappear against painted trim, and can be ordered with low-E glass.
The drawback is how they look and how they are cleaned. They change the face of the house, and the outside of the prime window can only be reached by removing panels. If the house is inside one of Metro’s historic or conservation zoning overlays, exterior changes to windows are reviewed by the Metro Historic Zoning Commission, so check with them before ordering anything that mounts to the facade.
Interior storms, and the condensation rule
Interior storms are a glass or acrylic panel in a slim frame that presses into the inside of the window opening, usually held by a compression gasket. They are invisible from the street, they seal very tightly against the jamb, and they come out in a few seconds for spring cleaning or for opening the window on a mild day. For historic facades they are often the path of least resistance.
Whichever side it goes on, one rule decides whether the glass fogs: the tighter layer belongs on the warm side. Warm indoor air carries moisture, and it will condense on whatever cold glass it reaches. With an interior storm that seals well, room air never gets to the cold outer sash. With an exterior storm, the prime window should be the tighter of the two, and the storm needs its weep holes open at the bottom so moisture can escape outward.
- Interior storm: seal it tight, and the old sash stays dry behind it
- Exterior storm: tighten the prime sash, keep the storm’s weep holes clear
- Fog between the layers in winter means warm air is leaking outward
- Moisture on the room side of an interior storm points to high indoor humidity
Getting it done before the cold
Late September is the sensible time to measure. Storm windows are made to order, and old openings are rarely square, so every window is measured on its own in several places. Ordering now usually means installing before the first hard freeze, which in Middle Tennessee tends to arrive in late October or November.
Houses built before 1978 are likely to have lead paint on the sash and trim, and any work that sands, scrapes or cuts it falls under the EPA’s lead-safe renovation rule for contractors. Mounting an interior storm usually disturbs very little. Repairing the sash first disturbs a lot, and that part should be done with lead-safe practices and cleanup, not just a dust mask.