Do•It•Yourself Interior Storm Windows

Do you feel cold standing next to your windows? With easy to make interior storm windows, you can feel warmer while saving money on your fuel bills. These storms will increase the R-value of your windows by around 2.3, however they will reduce the solar heat gain by about 15%. For most windows in Maine, this is a good trade-off. Not only do this save energy and therefore money, they also increase comfort and reduce outside noise.

Directions

Measure: The storm windows should be put as close as possible to the glass of your windows (½ " is best). However it should be placed such that it contacts the frame all the way around, if necessary it can be placed on the window trim, and held on with clips. Having determined where you will put the storm measure carefully the width and height of each window. Measure in at least two places, and average the readings, then subtract the thickness of your weatherstrip ½" (which will compress the 2 thicknesses by half giving a nice seal). The short side will need to be shorter by the width of two pieces to make the butt joint.

Frame: 1" x 2"s can be made by ripping down a 1" x 4". The wood does not need to be high quality, #3 pine works fine. But pre-primed is easy for frames which will be painted (or white). And clear pine looks great unpainted. The corner is made with a butt joint held with two 3" deck screws in pre-drilled holes at each corner. The long sides should be ½" shorter than the window opening (for the width of a two compressed weather strips). The short sides should be shorter than the window opening by ½ and two widths of the frame wood (for the butt joint). Screw from the long side into the short side (as shown). Sand the frame smooth and knock the edges off slightly. Now is the time to paint, or finish, if desired. Also be sure to label which window the frame goes into, on the top outside surface.

Films: This works best if done in a clean environment. Put the double sided tape around the edge of the frame (on the face). Long edges first, and no overlap. Leave the paper on for now. Cut the film to cover the frame plus a little for slop. The film is folded, if it is hard to open, put a piece of tape of each face of a corner, and pull apart. Starting at one short end, remove the tape, and carefully stretch the film and place it over the exposed tape. Press it down with the back of your fingernail. Next, do the other short side, pulling just enough to get the wrinkles out. Proceed with the two long edges.

Once it is firmly in place, trim the excess film at the outer edge. Then shrink the film using a hair dryer (a heat gun can be used if you are very careful. Stay 6" to 8" away from the film until you see how it goes. Slower is better than melting a hole in the film. It should end up drum tight with no wrinkles. Looking at it obliquely to the light will show any remaining wrinkles. Starting in each corner seems to minimize any wrinkles. Repeat with the other face (or install film on both sides, and then shrink). Try to make sure that the inside faces of the film are clean, as once assembled, the inside can't be cleaned.

Tape: The 2" tape is used to cover the film edge and the outer edge of the wood frame. Be sure to leave a bit to wrap around the corner. One piece for each side (4 pieces all together) works best for me. Use a short piece of tape to create one or two tabs to make it possible to remove the storm. Start on the back side, about 10" up from the bottom, wrap the tape outwards across the face and then the edge. Then extend out away from the window about 1.5" and fold back on itself. Make sure that the edges of the tape are lined up; make a crease at the fold. Finally, continue onto the front face.

Weatherstrip: The weatherstrip (½" wide, ½" thick) is placed on the outer edge of the frame. For each piece, start at the outer edge of the previous piece, ensuring an nice square corner. If the window is to be installed against the window trim, it should be placed on the back face, of course.

Use: The storms should be swapped out with your bug screens when you are starting up your heating system. (Removing the screens will save you about 1% to 3% of your fuel bill by itself). Store them for the summer where you store your screens. Be sure to lock the windows before installing the storm; windows do not seal properly unless they are locked. Arrangements should be made to ensure easy removal in any window which is an emergency egress.

Savings: Given the prices above (which are for small quantities) a typical 3 by 4 foot window would cost, $9.59 and probably require less than an hour to build. The savings depend on what kind of windows you currently have and what kind of fuel you are using. For single pane windows, the savings would $202 in electricity, $135 in oil, or $69 in wood. For double pane (soft coat) lo-e windows, the savings would be $68 in electricity, $45 in oil, or $23 in wood. All of which would give a payback at less than one year (at today's prices, imagine prices in ten years). Theses savings are tax free, repeat every year, and increase along with fuel prices. Triple pane, or south facing hard coat lo-e double pane windows are the only windows that might not benefit from this approach.

The film is susceptible to tearing if it is poked with anything sharp, but with proper care there is no reason these can't last at least ten years. Over the past 11 years, since these made their debut at 2008's expo, over 25 thousand of them have been made by Mainers and for Mainers, and around the world. I and, the Midcoast Green Collaborative are proud to have been instrumental in helping Mainers through these past winters.

Variations:

Over the years, many changes have been made to my original design, by Guy Marsden, Charlie Wing, Habitat for Humanity, and others. Here are my opinions of the differences.

• Mounting Position:
  1. Next to sash: This is the best location, within a ½" of the glass, which provides the optimum insulation value.
  2. Within the Jam: If obstructions prevent having the storm next to the sash, it can be placed anywhere within the window jams. Be sure to get it parallel with the window.
  3. Against the trim: If necessary, the window can be made larger to overlap the window trim. Note: For many windows this will mean being outside the vertical and top trim, but still inside the window sill at the bottom. The weatherstripping will need to be put where the storm meets the trim (i.e. on the back face). Some method of affixing the storms to the windows will be required as well; be sure to compress the weatherstripping by ½ of its thickness.
• Weatherstrip:
  1. Polyethylene Foam: This was a change from Guy Marsden, and seems to be the best option for general use. ½" x ½" is recommended. This product allows a generous amount of slop in window frames while maintaining a tight seal. The only issue is that it is not easily available at the hardware store. You can order it in a special kit just for this application on-line at Foamtapes.net (Contains: 500 feet of weatherstrip and 1000 feet double sided foam tape) or a la carte. Or contact me for exact quantities.
  2. Polyester Foam: This foam is slightly more robust and more ultra-violet resistant, it is however about twice the price. Only recommended where those characteristics are important (such as for skylights).
  3. EPDM (rubber): D-shaped weatherstripping is very robust, but is only available in 3/8" thick, and does not have as much leeway. This leaves very little room for error and makes inserting a removing windows a pain. Only recommended for permanent installations, and possibly where water or physical abuse are a problem.
  4. V-strip: Thus vinyl weatherstrip does not have much leeway in making a seal, often requiring supplemental means of keeping the windows in place. It also doesn't make an air tight seal. Not recommended.
  5. Rubber Foam: This weatherstripping is closed cell, providing a more air tight fit, but has little compressibility, and adhesive is not strong enough to stay on when inserted or removed. Not recommended.
• Corner Joints:
  1. Lap Joint: This is probably the best joint, glue makes it strong, it looks nice, and ensures a right angle. Sadly, it requires special tools, and takes a lot of time and effort. Only recommended for fine woodworking versions.
  2. Butt Joint: This is the simplest joint. Long screws (I use two per corner) are used to make the connection, without glue. This joint is plenty strong, quick, and easy to make with minimal tools. Care should be taken to ensure the corner are right angles. This is my recommended method.
  3. Miter Joint: Some people think this joint looks better than a butt joint, but it is hard to get a nice right angle, and the wood has a tendency to split when two screws are used. If you like the look, you might try it.
  4. Miter Joint with Corner Brace: As above, but requiring an additional piece and additional work. I don't feel that the brace adds any required support to the joint, as the windows are kept in shape mostly by the shrunk film. Additionally, the brace impinges on the glass area of the window, reducing solar heat gain, and view. Not recommended.
• Affixing the Film:
  1. Clear Double Sided Tape: This is the stuff that comes in the kits at the hardware store. It requires a clean surface and some pressure (rubbing) to ensure good adhesion. Large rolls are available. Some people have trouble with this tape, but I find it the best compromise, as it allows face mounting, and is invisible on cross-braces. I don't use the hardware store stuff, I buy large rolls from Uline, which is much better stuff.
  2. Foam Double Sided Tape: This is stickier than the clear, but it has an appreciable thickness, if used on the edge of the frame (which is the only way to have it not show on non-white frames) it increases the window dimensions by about 1/8"; plan accordingly.
  3. Window Screen Spline: A groove is made in the face of the frame, and the film is held in place with a spline used for window screens. This is tricky to do, and requires specialized tools. A race between students doing this method, and tape methods seem to indicate that it is about twice as slow. It requires miter joints, and the wood is not hermetically sealed, possibly leading to moisture issues. Not recommended for amateurs.
• Location of Double Sided Tape: The tape can be placed at the outer edge of the face of the frame, or on the edge of the frame itself. I think the former is easier, others claim the opposite. I don't think there is much of functional difference, so do whatever you find easier. Note: if you are using opaque foam tape, edge mounting will hide that.
• Wood Type: Almost any wood is viable for making these windows, from #3 (knotty) pine, up to select hardwoods, and including finger jointed trim wood. The only issue would be warped or twisted wood, or knots so big that they compromise the structure, other than that it is a personal choice.
• Wood Finish: The wood is sealed within the film and the tape, and therefore requires no finishing. However, in the interest of aesthetics, if you wish to finish your wood, such as matching the paint of your trim, or staining raw wood, this can be easily done. Make sure that finish is completely dry and cured before assembling the storm.