Don't Replace Those Old Windows Before You Try Window Inserts

They do a remarkable job at improving indoor comfort at far less cost.

Three paned window looking out at a neighborhood street

Treehugger / Lloyd Alter

As a committed Treehugger, one of the most important goals of my recent house renovation was to reduce energy consumption per person. The first thing many people do in a renovation is replace all the windows, even though study after study have shown that it has just about the worst bang for the buck of anything you can do. It also doesn't even make that much difference; a single-paned window has an R value of perhaps 1 and a new double-glazed window is between 2 and 4 unless you go very, very expensive.

Then there is the issue of character and appearance. My 100-year-old house has beautiful 100-year-old windows, with divided lights at the top that give the house its charm. They will also last as long as the house, whereas double-glazed units will not. They lose their seal and the argon gas (added between the two panes to minimize heat transfer) leaks out as the vinyl frames or finger-jointed wood deteriorates.

The salesmen keep pitching the energy savings of replacement windows. It's a huge problem for those of us concerned about architectural preservation and conservation, where replacement windows ruin heritage houses, and at great cost to owners for very little long-term gain.

But double-hung windows are very hard to seal and the spaces where the counterweights go are big empty wind tunnels. Air leakage becomes a far bigger problem with them than heat loss through the glass.

Window Inserts Fit Inside Existing Windows

One solution that has been around for a while is the window insert, an acrylic window that fits inside your existing windows and is often held in place with snap fittings or magnetic strips. I have been considering them for years, but worried about the fit (thanks to settlement over the years, all these windows are parallelograms, not rectangles) and the look of the strips that held the windows in.

Section of window pane and frame sitting on a table

Treehugger / Lloyd Alter

Indow Window Inserts

Then there is the Indow window. It has a compression tube around the edge that holds it in place so that nothing needs to be fastened to the window frame. This also seals it really tightly to the frame so that there is no air leakage around it. But what I found most impressive in discussions with Indow was their measuring system, where they promised that they could deal with parallelogram or trapezoid windows.

In their literature, Indow promises an almost doubling of the R value of my single-glazed windows, from R-1 to R-1.87. That's not a lot, but not much worse than replacement windows that cost a whole lot more. But as I mentioned earlier, that is only one of a number of factors that affect comfort, which is really a misunderstood concept. Engineer Robert Bean explains on the blog Healthy Heating that your body absorbs or radiates heat from the surrounding surfaces:

The less efficient a building, the greater the temperature difference between your skin and the temperatures of the walls, windows, doors, floors and ceilings. It is the temperature differences between you and the building which causes discomfort.
Heat vision view of a three-paned bay window
Greening Homes / CC BY 2.0

These walls and windows are COLD, as seen in this thermal image taken during the month of January, pre-construction. It was always uncomfortable in this bay window. Even the piano was suffering.

Bar chart showing effectiveness of reducing draftiness

The Indow insert, however, promised to significantly reduce draftiness to well below even what new windows would deliver. And although noise from the street is not a problem where we live, even the standard inserts offer significant noise reduction. They don't feel cold, because acrylic is not as good a conductor as glass.

Man measuring a bay window
Lloyd Alter / CC BY 2.0

Michael Ruehle, an Indow authorized dealer with GREENheart Buildings Inc., arrived with a little laser measuring device and a netbook running an online program, and measured the length and width of the windows. Then he used the clever little device to measure the diagonals, and entered all the data into the program. Voilà, a trapezoid is on the screen.

A few days later he returned with the inserts. They were delivered with a protective coating on the acrylic which was peeled off, then the window was pushed into place.

Close up image of windowsill with panel
Lloyd Alter / CC BY 2.0

You can see here how seriously distorted the window frame is, compared to the window itself. Yet each of these three inserts fit perfectly, sealing tight to the frame. The difference in comfort was immediate and palpable, and within half an hour the temperature on the floor rose 3 degrees. (The hot water rads have a lot of thermal inertia, so it takes a while for thermostat to cope).

Fluffy dog sitting on a couch in front of three-paned window
Lloyd Alter / CC BY 2.0

As for comfort, the best judge of the subject in our house also noticed the change.

I must disclose that Indow provided these window inserts for my review, but I am seriously impressed at how accurately they fit and how much of a difference they made. I have ordered them for the other existing old windows on the front of the house, and expect that they will make a huge difference in the apartment upstairs—saving energy, increasing comfort, and preserving the historic character of my home.