What the numbers say
The National Park Service's Preservation Brief 9 cites U-values (heat loss, lower is better) of 0.88 to 0.99 for a single-glazed wood window, dropping to about 0.44 to 0.49 with a storm added. That beats a double-glazed metal window without a thermal break, at about 0.6. Preservation Brief 3 puts it another way: a single-glazed storm only brings the window to about R-2, but that's twice as good as the single pane alone, and studies show a wood window plus storm can approach a double-glazed replacement.
Glass choice matters. The Building America Solution Center says low-e storm windows cost about the same as standard ones but are about 50% more energy efficient than uncoated storms. These are window figures, not a promise about your heating bill, so ask anyone quoting savings what assumptions they used.
Interior or exterior
- Exterior storms protect the original sash from weather and are usually accepted on historic projects, but the outside of the window looks slightly different. Ask for a frame color that matches the trim and a meeting rail that lines up with the sash's. NPS advises against unfinished aluminum.
- Interior storms leave the outside untouched and some can be pulled out in summer. Some friction-fit panels have flexible gasket edges and press into the frame. They need a truly airtight seal, or room air gets behind them and condenses on the cold original glass.
Match the storm's operation to the window: a sliding storm over a sliding sash, not a fixed panel, if you open that window or need it for an emergency exit. Before buying, ask the seller to show you how to open, clean and remove the panel.
The moisture rule installers sometimes get wrong
Whichever layer faces the room has to be the tighter one. With an exterior storm, that means weatherstripping the original sash, and leaving the small weep holes in the storm's bottom rail open. The Solution Center says not to caulk the bottom of an exterior storm, because those openings drain water and let the gap vent slightly to the outside.
If fog shows up between the layers, check that the weep holes are clear and the inner layer is sealed, then look at indoor humidity. Fog sealed inside a double-pane unit is a different problem; see Why there's fog between the panes of a double-pane window.
Fix the window first
From inside, note cracked glass, loose glazing putty, peeling paint, rotted sills and a sash that won't close and lock. The Solution Center says cracked or rotting sills should be replaced before any storm goes on. If the sash is in rough shape, Should drafty original wood windows be repaired or replaced? covers what's worth saving.
In a house built before 1978, assume the old paint may contain lead. EPA's rule says anyone paid to do work that disturbs paint there, including carpenters and window installers, must work for a certified firm. Don't scrape or sand to check the wood yourself.
What to ask for in the quote
- Is the product ENERGY STAR certified for our climate zone, and is the glass low-e?
- What's its air-leakage rating? The Energy Department notes it's reported in cfm per square foot or per foot of edge, and lower is better.
- What sash repair and weatherstripping is included, listed separately from the storm itself?
- Who measures, and who pays if a storm is made to the wrong size?
- Is your firm EPA lead-safe certified, if any painted surface will be disturbed?
For a whole-house order, ask for one window done first so you can judge the look and how easily it opens. Leave second-floor exterior installs to the installer rather than a ladder job.
Philadelphia historic properties
If your house is on the Philadelphia Register of Historic Places, the Historical Commission must review installing, replacing or removing storm windows before you order, though replacing clear glass is exempt. Review is free, and staff approve over 90% of building permit applications without sending them to the full commission. Call (215) 686-7660 or email preservation@phila.gov. How Philadelphia homeowners can check historic designation explains how to look up your property. In Bucks and Montgomery County, ask your township whether your street is in a local historic district.