Don't replace those old windows before you try window inserts

bay window
CC BY 2.0 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, a new double glazed window 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; double glazed units will not, as they lose their seal and the argon leaks out, as the vinyl 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 preservation and conservation, where replacement windows ruin heritage houses, at great cost to owners and for very little long term gain.

But double hung windows are very hard to seal, 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.

The window insert to the rescue

One solution that has been around for a while is the window insert, an acrylic window that fits inside your existing windows, 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.

window section Lloyd Alter/CC BY 2.0

Indow 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 that 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 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.

bay windowGreening Homes/CC BY 2.0

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

Drarfts© Indow

However the Indow insert 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.

measuring 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. Voila: a trapezoid is on the screen.

Installing indowLloyd Alter/CC BY 2.0

A few days ago he returned with the inserts. They are delivered with a protective coating on the acrylic which is peeled off; then the window is pushed into place.

window-sillLloyd 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).

jasper is comfortableLloyd 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.

Get more information at Indow.

Tags: Energy Efficiency | Preservation


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