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?
Moisture
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.
R Value
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.”.
Alternatives
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
Window Quilts
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.
Our Decision
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.
Hardware stores sell a plastic material that can be taped to a window sill to provide an extra layer of clear insulation on a window for winter. This material, when placed well, does not have wrinkles, and is not readily noticable.
This small expense will fit almost any window and will do a great job insulating and preventing any condensation on the window (including when you have the drapes down at night). It may not be glamorous, but it is much more effective and enviromentally friendly than spending thousands on fancy window coverings that create condensation.
The one caveat - this material must be put on when the window is dry. Preferably heat and dry the window with a hair dryer before installation.
Our family has been using this material since I was a child in Northern BC. The plastic was left on all winter, and reused the following winter. For windows that do not open, the plastic was left on all year.
Thanks for that. A couple of comments:
I think that this "plastic wrap for windows" is an excellent product for old leaky windows. For little money, you eliminate air leakage and add R 1 to the window
However, the important expense for this isn't the money, it's the time. To do a whole house would take the better part of a day. The first year, fine. What about the fourth year? I would be concerned about people not getting around to it
Also, I don't think that this really "competes" with shades/blinds. We needed shades for privacy - why not get ones that add insulation (as opposed to, say, venetian blinds)?
Finally, I would dispute the reusability of the plastic wrap that you buy at the hardware store. It gets destroyed when taking it off. Earth's General Store sells some truly reusable stuff, but it distorts the view through the window (still worth it - I've used both products myself).
Conrad
I have the Hunter Douglas Duette triple honeycombs. My infrared thermometer shows the temperature to be 12 degrees warmer than window frame, so it does help retain heat / keep out cold. The triple cells are supposed to have an R-value of 5.69.
I hope you got a good deal on these blinds. A quickie search tells me that 48"x48" Architellas cost $602 in Canada but only $276 in the US. I believe Costco offers 40% off Hunter Douglas products.
If you ever figure out how to automate these Hunter Douglas Powerrise blinds to open with the sun and close at night, let me know. That will help even more with energy savings during the winter.
I agree with the hardware store taped plastic to the window frame,However it is hard to find quqlity now.The one I just put on is really hard to get on as it is cheaper.If anyone knows where I can get some quality seal in Edmonton Alberta I would be much appreciative.
I agree with the first comment, retrofitting the thin plastic films sold in hardware stores is probably the most viable window weather proofing method.
I figured out a new way to fit it a couple of years ago. This works with conventional windows where the space between the window and storm window can be accessed.
Instead of on the inside frame, I put the film between the two window panes. If one gets all wrinkles out the film is completely invisible in this location. It is also protected. For windows that aren't opened in the summer it can stay there for years.
I have wood storm windows, and the tape + film is as easy to mount to the storm window frame as it is to the inside window frame. I mount the film in the fall before installing the storms.
To reduce leakage/drafts I put weather stripping both on the inside double hung windows and on the outside storms.
B.t.w. I have tried and compared most of the other methods mentioned above.
Honeycomb shades: Works, but lack of tight seal to window reduces efficiency. Mine are "Solaris" brand bought at a closeout sale in one of the Edmonton southside strip malls.
Opaque roller shade: Any opaque shade will stop radiant heat flow. This is equivalent to about R1 addition.
Rigid insulation: I cut 3/4" thick polystyrene insulation sheets so they would be a snug fit to the window frame. Unlike comments made above, I got a tight enough fit not to have condensation problems.
Martin
Hello Conrad,
As I think you have mentioned in one of your posts or e-mail exchange on this subject, there are times when living in a passive solar house can be a bit oppressive due to excessive light! We're in our first winter at our superinsulated house, and there are indeed times when we want a window shade to be able to make better use of the south portion of the house on bright afternoons.
I've read your initial comments on the H-D shades and will look into them. I am curious to know whether your views remain the same i.e. that you still consider there is some insulative benefit to the shades despite leaving gaps on cold nights. I assume these are the black-out shades?
As always, thanks very much!
Mississippi John
Mississippi Mills, Ontario
John,
I hope that you're enjoying your house.
Absolutely, there is an insulative benefit to the shades, and I make a note to close them every night (even on the second floor where we don't need them for privacy). I feel tangible proof every morning when I open them and feel a rush of cool air escape from behind them!
Of course, I can't compare the Hunter Douglas ones that I got to other, possibly cheaper shades.
Also, they are not complete blackout shades.
Conrad
Thanks once again for your thoughts - this site is such a valuable resource! House is great and are enjoying one of the nicests winters (ie. lots of snow) that we have had in years.
Happy New Year,
Mississippi John
Mississippi Mills, Ontario
Hi John,
Congratulations on finishing your new home. I've enjoyed following your blog.
About a year ago on a sunny day, I forgot to open the blind in one of the bedrooms with a south-facing window. When I got home around noon I realized my mistake and opened the blind. On a sunny winter day, the inside temperature of my south-facing windows is typically around 30 degrees as measured with an IR thermometer. Immediately after opening this blind, the window was almost 60 degrees. Fortunately, there was no apparent damage to the window. During construction, a contractor had left a down vest against one of my south-facing windows. It sat there for a few cloudy days without problem, but then the sun came out on a cold day and the inner pane cracked due to the stress caused by the trapped heat. Of course, it was the largest window in the house.
I would recommend getting blinds that open at both the top and bottom for at least your south-facing windows and leaving a gap at both top and bottom if you want to close the blinds on sunny days to avoid trapping heat between the blind and the window pane.
B.
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