We live in an old house. Now, old is relative, but in our part of the country (Maryland suburbs) anything built before the housing explosion in the early 1990s is considered pretty old. Our house was built in 1942 and solidly adheres to the construction standards of that time, which means single-pane windows.
Dual-pane windows were invented in 1935 but weren’t widely used in the United States until the 1950s. Our windows were manufactured by Andersen and they are true divided-light single-pane casement windows. When the house was built, fuel oil was inexpensive, so it was more cost-effective to simply burn more oil than it was to invest in insulation and very costly dual-pane windows – not when you consider that our house has 40 windows incorporating a whopping 136 separate window sashes!!
A single-pane window has an R-value of 0.91. Dual-pane windows usually have R-values starting at 2.0, and super-efficient triple-pane windows can achieve R-values greater than 3. But there’s more to windows than just numbers.
There are three methods of heat transfer: radiation, conduction, and convection. Radiation is the heat you feel from the sun; it warms objects. Conduction is heat that passes through materials; nature will always try to balance the heat on both sides of a barrier.
The last method is most important when considering windows: Convection. When the warm air in a room hits the cold window glass, it loses heat. The now-cold air sinks down, pulling more warm air in behind it. This creates what is called a Convection Current, and it creates drafts that can make a room much less comfortable.
With heating oil hovering at the $4 mark, we started looking for ways to reduce the drafts in our old windows. One of the methods we tried last winter was to cover the window screen inserts with insulating window film; the kind you attach with double-sided tape and shrink to fit with a hair dryer.
These worked, sort of. The main problem was that the air gap they created was nearly 2 inches wide – plenty of space for a convection current to be created in the gap. So we were still losing heat; it was now just a two-step process. They tended to wrinkle, and were prone to tearing. Also, covering the screens meant we couldn’t open the windows for ventilation.
A few months ago Bob started researching other options. He had decided that plexiglass inserts were going to give us the best results, but the issue was finding a supplier that could both cut to very precise tolerances (to the nearest 1/32nd inch), and at a price that wouldn’t require a second mortgage. He finally chose TAP Plastics.
So we started measuring the windows. A regular measuring tape simply wasn’t going to be accurate enough, as we had to be as precise as possible so we could get the tightest fit. Bob found this ruler at the hardware store and it was the perfect tool for measuring inside dimensions.
This is a Lufkin X46 Extending Ruler. It has a brass extension bar that lets you get amazingly precise inside readings. To make sure we got the most accurate measurements, we each took them independently and then compared numbers. If we didn’t get the same numbers, we went back and re-measured. A lot of work, but the end result would be worth it.
We then ordered the plexiglass. For most of the panes, we chose 1/8” thick clear plexi, so there would be less chance of wobbling or warping. For the smaller panes (the ones in the French doors, which we would have to do individually), we chose 3/32” plexi.
Installation
Each piece of plexiglass was protected by two sheets of blue adhesive plastic. To get started, Bob would peel the plastic off one side of the plastic, and then set it in place in the sash, right up against the muntins. This would leave a gap of between 5/8” and ¾” – just right to create a true dead air gap.
While pressing the plexi against the window, Bob started to peel back the front protective plastic. To hold the plexi in place, he used these little things:
They’re called glazier’s points, and they’re meant to hold panes of glass in window sashes. They’re usually covered up by the window putty. In our case, we were going to use them to hold the plexi tightly against the sash. They are just pushed into the wood frame with a flat tool, like a paint scraper. Each sash required about a dozen points.
Once the plexiglass is installed, it is hardly noticeable:
So, did it work?
We’ll let the pictures tell the story.
This is a three-sash window on a north-facing wall. The left sash is the plain single-pane window glass. The center sash has the screen insert with the window film applied to it. The sash on the right has the plexiglass installed in both the lower and upper sashes. It’s a huge difference!
Here’s another example.
This is a French door on the opposite wall of the same room. Only one pane has the plexiglass on it. Can you guess which one?
We now have this plexiglass insert in every single-pane window in the house. The total cost for the plexiglass was about $1800, plus another $40 for many packages of glazier’s points (we’ve managed to clear out the stock in all three area hardware stores). Applying the plexi took about five minutes per sash; it took more time to wash each window thoroughly before the application!
