Make an Old House More Energy Efficient With Inset Storm Windows

We live in a 1950's house with original single-pane windows. It's not really energy-efficient and I had been looking into options for replacing the windows for a while, but I wasn't looking forward to the price or the giant pain of getting on a ladder, prying out the old windows, and putting in the new ones. Plus, none of the sizes available at the home stores fit the window openings exactly, which would mean that I would either have to fill in the gaps or special-order the right size windows. Then during a recent cold snap, it got so cold outside (7 degrees at night) that we actually had frost on the INSIDE of some of the windows- the windows were so cold that the humidity in the house condensed onto the windows and then froze.

With frost inside my house (and the dogs parked in front of the space heaters), I wanted something that could be effective quickly. I had previously built some plastic covers for a few of our windows based on this: http://www.hammerzone.com/archives/energy/conservation/basics_1/window_cover.htm. They're pretty good for keeping down drafts but hard to remove and store without breaking so I was hoping for a more permanent solution.

The local hardware store had storm windows in exactly the right size for our existing windows- 36x39 and 36x63. They're made by Larson and meant to be installed on the outside, on top of the existing windows, but looking at them I could tell they were designed to be installed on vertical windows, whereas installing them horizontally would mean some seams would be vulnerable to water. Figuring I might be able to caulk the seams, I bought one of each.

It was dark and cold when I got home, and I didn't feel like getting up on a ladder outside to see if the windows would align correctly. I brought them inside and put them up against the inside of the windows. Not only did they clearly fit well, but they almost completely filled the opening and covered the windows. Instantaneously, the whole room felt warmer as the new glass blocked the chill from the old windows. I figured the effect might wear off after the storm windows sat in the opening all night, but the next morning the boys' room still felt noticeably warmer than we're used to. I pulled out my iphone infrared thermometer and the surface of the storm window glass was 62 degrees- a full 10 degrees warmer than the single-pane window in the next room over. I slid open the storm window and checked the temperature on the old window behind it, which registered 46 degrees. So the little air gap in between the two windows resulted in a 16 degree difference. With two windows and 24 total square feet of glass in the room, we were looking at a major improvement.

So what next- just leave the storm windows in the opening until a good chance to get on the ladder? Heck no- I wanted to find a way to keep a good thing going.

So- how to install the windows on the inside? They weren't designed for that, and couldn't be placed directly against the old windows because of the way the locks and sashes were designed. I played around with some scrap wood and found that I could install a ring of 1x2s all around the opening to press the storm windows against. I put the 1x2s a little more than 2.5 inches in from the wall so that I could install 1x3s on the outside to build up the window trim. I used wood 1x2s on the bottom in case of any condensation or leaks, and I used MDF for the top and sides. I intentionally left about a 1/8 gap at the end of each piece because I was installing on a dry day in January and don't want them to swell up on a humid day in July.

There was many years of grime built up on the inside of the old windows. I found that microwaved vinegar was amazing at cleaning it up.

After I dry-fit everything, I primed and painted all of the trim and put it all back in. I nail-gunned the 1x2s into place and put a thick layer of white window caulk all around the ring. The storm window inserted on top of the caulk and it was clearly a complete seal- no draft or chill whatsoever. I double-checked that both the storm window and the old window could be opened and closed, then ran another bead of caulk around the edge of the storm window and then installed the new 1x6 window sill on the bottom and the 1x3 trim on the top and sides.

To give the new windows a Craftsman look, I added 1x4 trim to the bottom
and sides and a 1x6 cap. (The paint is Glidden Marshmallow White if you're wondering). Pre-primed MDF for everything except the window sill.

Everything looks great, and you can't even tell there's two layers of window unless you look from an oblique angle. Honestly, even when I do see the gap it doesn't bother me, because that 2-inch cushion of air makes the whole upstairs feel so much nicer. Once I had the design down, I finished 4 windows in 2 bedrooms in a single weekend. All without ever carrying a big window up a ladder in the cold and trying to install it. And the price was right- the 36x39 storm windows were only $45, and the 36x63 ones were $59. Compare that to over $100 each for new windows, which couldn't be installed until warmer weather and would probably require scaffolding to get them in place. All of the trim was about $15 per window.

Phase 2 began the next weekend with the basement windows. Theoretically I could have stood on the ground and installed 2 of these in the manufacturer's recommended way, but it would have looked weird without the same thing above them, and I still wasn't sure about the seams of the metal with it installed horizontally. The basement windows already had nice wooden trim on the insides, which meant that I couldn't just slide the storm windows in. I was worried I would have to rip out the existing trim and redo everything to make it fit, but I tested out my jigsaw on a corner of the aluminum trim of the storm windows with a specifically-for-metal blade and it cut right through. I had to cut off about 1/2 inch from one side and 1/4 inch from the bottom, but once that was done they fit just right. If you try this, you can't just lay down the windows and jigsaw through the metal- it'll bounce all over the place. I was finally able to clamp it in between a 1x6 and a piece of plywood and get a nice straight cut.

Once the storm windows were trimmed to fit the openings, installation zipped right along. I painted the 1x2 ring of trim and installed it a bit inside the existing windows and test-fit everything (in a couple of spots I used 3/8" thick trim instead of the 1x2s because of the thickness after the jigsawing). I put in a thick layer of caulk (I ended up using about 1/2 tube of caulk per window) and slid the windows in, then screwed them in. Use a wet washcloth instead of your finger to smooth the caulk around the cut metal.

Once I got going, I was able to finish 6 more windows the second weekend. Total cost per window is about $60-80, depending on size.  The whole house feels nicer, and the heat isn't coming on as often. I'll update in the summer on how it does in the heat.