The temptation has always been there for eco-house builders. It’s those damn windows; they are just so useless once the sun goes down. There must be some way to insulate them once they no longer need to be seen through, right?
The answer is yes, but not cheaply. The biggest problem is moisture. If you insulate a window from the inside without a perfect air seal between the heated space and the cavity between the window covering and the window, moisture-laden air will flow into said cavity. When that happens, the moisture will condense on the window. Take it from green building pioneer Rob Dumont:
Back in the 70s I had a small house with lots of south windows. I experimented with interior rigid insulation on the windows. Some observations:
You need a very tight air and vapour barrier on the warm side of the insulation. I did not have either, and condensation would readily form on the window. I used a 2 inch thick piece of beadboard as the insulation. The windows were double glazed sealed units with an R value of about R2. Even more condensation would form on the window when the insulation was removed, as the warm, moist air could then, unimpeded, hit the window. The condensation would run onto the sill. I actually got quite sick from the mould that grew on the lower corners of the windows. Never again.
We felt compelled to buy window coverings as soon as we moved into the Mill Creek NetZero Home. Being totally exposed at night through our gargantuan south windows, even in our bedroom, was all the encouragement that we needed. The product that we zeroed in on was Hunter Douglas’ Duette Architella shades. Hunter Douglas claims that “these shades have a state-of-the-art, patented, honeycomb-within-a-honeycomb design offering the industry’s highest level of energy efficiency”. They claim an R-value of 7.73 for their most insulative shades (source).
That sounds pretty impressive, right? And in fact it sounds legitimate on some levels – the three airspaces created by the honeycomb design should indeed stop heat transfer to a non-trivial degree. The elephants in the room that the company ignores are moisture build up and air movement.
As Rob Dumont puts it: “I very strongly doubt that an interior curtain can have [a high] R value. Convective air leakage around the perimeter of the curtain will readily move air past the sides, top and bottom of the curtain and dramatically lower the R value.”.
Again, from Rob Dumont:
I do think that exterior insulation is a possibility. However, Harold Orr put it in perspective for me. An exterior door mounted on the outside of the window has a materials cost of about $200 and probably about $50 worth of labour to install and weatherproof. At $250 the cost is about $14 per square foot, and that would not include the cost of any actuator to allow the exterior insulation to be controlled from indoors. We did have a house in Saskatoon that used a sliding shutter on the south side with barn door hardware. It worked all right, but the sliding shutter used a lot of valuable real estate on the south wall.
Well-fitted exterior shutters circumvent the moisture problem, but on a house like ours, with huge south windows on the second floor, I can’t see a way to control them or a place to put them during the day.
mcnzh - there is no room for exterior shutter on this south wall
It seems that the only product that tackles all of the issues is made by a little company from Vermont. They produce the Window Quilt Insulated Shade:
Window Quilt Shades
Window Quilts are tightly sealed on all four sides when they are drawn down, thereby stopping convective air flow and eliminating excessive moisture build-up. They claim a legitimate value of R-5, which is an excellent improvement for a window.
We seriously considered buying them for our house, but a couple of things stopped us. First of all, they look like diapers. I’m sorry, but at some point aesthetics do play a role. Secondly, at around $10,000 to cover every window in our house, and considering the marginal gains that we were looking at getting due to the high R values of our windows, it didn’t seem like a good expenditure. Window Quilts seem like an excellent product for a retrofit though.
In the end we chose the Hunter Douglas product. They are good-looking and functional, and they seem durable.
I do think that they contribute somewhat to reducing heat loss. On nights that are warmer than –5 degrees Celsius, we completely close them, and moisture build-up isn’t a problem. Furthermore, we can feel the cold air spill out when we open them in the morning, so they must be providing some benefit (the same is true for our cheap Ikea drapes in the bedroom, by the way).
For nights colder than –5, we leave a space at the bottom for air to circulate in and out of . If we forget to do that, this is the result:
on a very cold night, ice has formed behind the insulating shades
Many window coverings can provide a modest benefit to reducing heat loss. Window Quilts can provide a significant benefit, doubling the R value of many windows when they are closed. Since we needed them anyway, we decided to buy the Hunter Douglas Duette Architella product, because it’s probable that they provide a slightly bigger benefit to some other products.
For people who want an more in-depth and technical discussion, see this email discussion between me, Peter Amerongen, Bob Heath and Rob Dumont.