Add an Extra Pane to Your Windows

I have an old house, and two of the windows are still single paned. As part of my ongoing energy efficiency improvements, I wanted to reduce the air and heat leakage from them, and reduce condensation on the glass. Since it's not in the budget to replace them this year, I needed a reasonably priced fix.

I decided to add a pane of acrylic on the inside of each window.

Materials:
- Clear acrylic panes (plexiglass, lucite, etc)
- Double sided foam tape (found in the weatherstripping aisle)
- Cove trim (optional) and fasteners
- Caulking
- Dessicant (optional)

Tools:
- Work gloves (acrylic can be very sharp)
- Safety glasses (you like your eyes, right?)
- Measuring tape and pencil/pen
- Exacto knife
- Miter box and saw
- Caulking gun
- Hammer (if nailing on trim)

The first thing to deal with is air leakage. I got out the trusty caulking gun and caulked around the edge of each pane of glass, and filled in any gaps around the window frame. The caulk I used is water based clean up, so a wet finger made sure that I got all the gaps, and a damp rag cleaned everything off the glass. I used white indoor/outdoor 25 year caulk.

Let the caulk cure, per the tube instructions. You want the window and caulk very dry before you install the acrylic.

Measure the inside of your window opening and subtract 1/8". That's the size of your acrylic pane. If you're good at getting perfect cuts, maybe 1/8" is overkill.

Don't remove the plastic/paper protective backing on your acrylic yet. Acrylic is fairly easy to trim to size with a sharp knife, some clamps, and a straight edge. If you decide to use a power saw of some kind, go slow and wear safety glasses. Mark out your cut lines, lay out the straight edge (I like to clamp it in place), and score the acrylic with the knife. The thicker the acrylic, the harder it will be to break. 0.08" thick is pretty easy, 0.22" is a bit of a beast. Make a deeper score on both sides for thicker acrylic.

Using a table or stair edge, snap the acrylic along your score mark. It might help to have someone put their weight on the sheet to keep it in place. The edge of the acrylic may be sharp if you didn't get a clean break. Wear gloves.

I used two types of acrylic on this project. On one window I used 0.08" thick acrylic and on the other I used 0.22" acrylic. The only reason for the difference was what the hardware store had in the dimensions I wanted. For thermal properties, the 0.08" has R-1 and the 0.22" has R-2.5 (approximately). A single pane of glass is about R-1.

I decided to trim the indoor side of the window, to ensure that the acrylic pane didn't fall out or bend. I cut sections of primed cove trim at 45° to fit into the frame.

This is where a miter box becomes your best friend. But, make sure that the angles are right for putting pieces together. If you're cutting 45°, you want to end up with one long side and one short side.
This   \___/   not this /____/

The long side should be the inside dimension of the window frame.

Remove one side of the plastic/paper backing on the acrylic, this will be the side facing the pane of glass.

Optional but recommended: put some sort of desiccant between the old glass and the acrylic. You can buy commercial desiccants, save them from your shoe boxes, or use rice. This will reduce condensation between the panes and moisture damage to the wood. Installing the pane on a really cold day when no one has showered, done laundry, or cooked in the house will also help, since the air will be drier than usual.

To improve the insulation properties, I used short strips of double sided foam tape to hold the new pane on the original window frame (less conduction). Once the acrylic was in place, I caulked around the edge of the pane and the frame, to prevent air movement from the warm, moist interior into the cool air gap between the glass and acrylic.

Peel back the edges of the backing facing the inside of the house, and install your trim.

Caulk around all edges of the trim, to be really sure air isn't getting by. If you're like me and your mitering skills are dubious, this is your chance to disguise your sins.

The clear acrylic is hard to see, and if your trim matches the existing fairly well, it may be that no one actually notices your window improvements. But, you'll have doubled the R-value of the window, and hopefully everyone notices that on the heating bills.

I've had these installed for a month now, and only saw a little condensation on the glass side when we hit -30°C this weekend. Even so, it was was a lot less than last year and a big improvement.

I did these on the inside, but you could easily do this on the outside of your window if you used the appropriate trim and caulking. Or, you could do this with glass.

If you want to remove this later, you could go for peelable caulk.

November 2012 update:
Since I've had these in place for a year, I have some suggestions:
- Use the peelable caulk.
- Remove the acrylic in the summer. There was certainly less condensation on the glass last winter, but there was enough moisture over the course of the year to peel the paint behind the acrylic.
- Use silica gel desiccant. You can find this at outdoor stores (to keep a gun safe dry) or craft stores (to dry flowers).