Storm Windows and Interior Inserts: Insulating Original Windows

Most of the way. A double-pane replacement window insulates because two layers of glass trap a sealed space of still air or argon, and a Low-E coating reflects room heat back inside. A storm window over an original single-pane sash does the first part: two layers of glass with a pocket of air between. If the storm glass carries a low-E coating, it does the second part too.

What a storm cannot do is seal the pocket the way a factory glass unit does. The space between the storm and the sash is vented slightly, by design, so that moisture can escape. It is also deeper than a factory unit, so a little convection happens inside it. The result is a window that insulates well, but not quite as well as a modern unit with the same glass, and one that still depends on the original sash closing reasonably tight.

The other thing a storm does is protect the old sash. Paint lasts longer, putty stays put, and the wood stays dry. On a house where the original windows are part of the look, that is a large part of the value.

A modern exterior storm is an aluminum frame that screws to the exterior casing or blind stop, holding one or two glass panels and a screen in tracks. A two-track storm has a fixed upper glass and a lower panel that slides up, with a screen behind it. A triple-track lets you raise or lower the glass and the screen independently for ventilation. The frame is caulked to the casing on the top and sides and left open at the bottom, with small weep holes so any water or condensation drains out.

That bottom vent is important and often gets caulked shut by a well-meaning homeowner. Once the pocket is sealed at the bottom, moisture that leaks past the old sash in warm weather has nowhere to go, and you get fog and eventually rot on the sill. Leave the weeps open.

Aluminum storm frames are thin and do bridge some cold, so the frame itself will frost lightly on the worst nights. It does not matter much because the storm frame is outside the wall; what matters is that the glass panel covers the whole old sash and the frame seals tight to the casing on three sides.

Low-E is a thin metallic coating that lets visible light and sun warmth through but reflects long-wave heat, the kind radiating off your walls and furniture, back into the room. On a replacement window it sits inside the sealed glass unit. On a storm window it is a hard, durable version of the coating on the glass panel itself, facing the pocket.

The EPA estimates that ENERGY STAR certified low-E storm windows installed over single-pane clear glass save about 10 percent on annual heating and cooling on a national average, and that swapping clear-glass storms for low-E ones saves more on top of that. The Department of Energy's Building America program has field-tested low-E storms and found similar results, with the largest gains in cold climates.

The ENERGY STAR storm window specification has separate criteria for the Northern zone. Certified storms for the North use a high-gain low-E: a coating that reflects room heat but does not block the sun, so winter solar gain through south and west windows still helps heat the house. That is the right choice for Minnesota. A low-gain, tinted or reflective film sold for hot climates is the wrong one here.

Clear-glass storms are still worth having over single-pane glass. But if you are buying new storms, the low-E upgrade is the difference between a storm that keeps out wind and a storm that also keeps in heat.

An interior insert is a panel of clear acrylic edged with a soft silicone tube. It is custom-cut to the inside of the window opening, usually to the face of the jamb between the stops, and pressed into place. The tube compresses against the jamb on all four sides and holds the panel by friction. No screws, no tracks, no changes to the original window. Pull it out in spring, store it, press it back in October.

Acrylic is used because it is light, clear and insulates a bit better than glass of the same thickness. The compression tube is the important part. Old openings are out of square, and a rigid frame would leave gaps; the tube fills them. Made-to-measure inserts are laser-measured to each opening for this reason, and the good ones seal tightly enough that the draft at the sash disappears.

Because the insert is inside, it is invisible from the street, which is why they are common on houses in historic districts and on windows with exterior details that a storm frame would cover. They also work on windows that have no place to mount an exterior storm, like inswing casements and some steel-frame windows.

The limitation is that the insert covers the whole window. You cannot open the sash with the insert in place, so a bedroom egress window needs a plan for getting the insert out fast, and any window you ventilate in winter is a poor candidate.

Both add the same thing, a second glazing layer with still air behind it. They differ in where they live and what they let you do.

A storm or insert is the right move when the original window is worth keeping: old-growth wood sashes that close, true divided lights, a frame that is solid and square, and a house where the windows are part of what makes it what it is. It is also the right move when the budget covers twenty storms but not twenty replacement windows, because a low-E storm over a sound single-pane sash gets you much of the comfort at a fraction of the disruption.

It is the wrong move when the problem is the frame, not the glass. A storm does not fix a rotted sill, a jamb that has pulled away from the wall, a sash that no longer meets the stop, or a frame so out of square the sash locks will not engage. Putting a storm over that traps a leaking window behind a second layer, and the moisture that gets in has a harder time getting out.

It is also the wrong move if you want the window to disappear as a maintenance item. A storm and an original sash are two things to paint, two things to re-putty, two things to adjust. A quality replacement is one, and its glass unit, frame and weatherstripping come as a tested package with a whole-window U-factor on the label; for the Northern zone ENERGY STAR sets that at 0.22 or lower. If the original windows are ordinary post-war sashes with no character, replacement is usually the better long-term answer.

• Storm or insert: sound sash and frame, character worth keeping, historic exterior, budget for glass but not for full replacement, noise from a busy street.
• Replacement: rot at the sill or lower jamb, racked frame, sash that will not close, single-pane aluminum sliders, windows you are done maintaining.
• Either: fogged double-pane glass from the 1980s and 1990s in a sound frame. A storm helps, but the failed unit stays failed; compare against an insert replacement.

Adding a storm or insert over a neglected window wastes the money. The old sash is still the primary seal; the storm is the backup. Go through the original window first.

Check the glazing putty on the outside of each pane and re-putty any that has cracked or fallen out. Check the interior sash stops and parting beads; if the sash rattles in the frame, add spring-bronze or V-seal weatherstripping in the jamb so the sash rides tight. Make sure the sash lock pulls the meeting rails together. Replace broken sash cords so the upper sash closes all the way. Then paint or prime any bare wood.

Look hard at the sill and the bottom of the jambs and the interior stool. Probe with an awl. Soft wood there means water has been getting in, and a storm will not stop that; the sill or stool needs to be repaired or replaced first, and the source found. Once the window is tight and dry, a storm or insert over it does what it is supposed to do: a still layer of air over a sound window.

JMB Builders is a licensed Minnesota residential building contractor (BC809005) based in Zimmerman, working across the Twin Cities metro. We install storm windows and storm doors you have bought, repair the sills and stools under old windows, and replace windows when that is the honest answer. Send photos of the window inside and out and we will tell you which. Call 763-204-2099 or send photos through the site.

• Storm door installation, labor only, on a storm door you bought — jmbbuildersmn.com/services/doors/storm-door-installation-labor-only
• Rotted interior window sill? Stool replacement and what the rot means — jmbbuildersmn.com/services/window-trim/interior-window-sill-rot-replacement
• Fall weatherproofing checklist: weatherstripping, storms, sweeps and sealing before snow — jmbbuildersmn.com/services/insulation/fall-weatherproofing-package
• Insert and full-frame replacement windows, measured and ordered by us — jmbbuildersmn.com/services/full-service
• Send three photos per window, written quote within one business day.