Dual Pane Interior Mylar Storm Windows

The pictures below show a couple methods to make dual pane interior storm windows using Mylar film.

The advantages of the two pane Mylar film method are:

The dual panes add about R2 to the windows current R value, instead of the R1 that a single pane storm provides. This cuts the loss from a double glazed window about in half.
The Mylar film is quite clear and does not distort the view
These storms are very light weight and easy to install and remove -- or they can just be left in place.
These storms can be made for odd shaped windows.

I used 2 mil Mylar purchased from www.MirrorSheeting.com at a cost of about 20 cents per square foot per pane.

The windows are made by making an edge frame from 3/4 inch strips of Medium Density Overlay plywood. The Mylar film is then placed on a flat surface, and tensioned with tape on the edges to remove any wrinkles. A bead of silicone caulk is applied around the top surface of the frame, and it is placed on the Mylar film and weighted. After the silicone has cured, the process is repeated for the 2nd layer of film. You end up with two layers of Mylar film separated by the 3/4 inch edge frame. The frame can then be installed with dabs of silicone, or Velcro tabs, or small screws. The pictures below show the full process.

An alternative to the Mylar film might be polycarbonate sheet. My understanding is that rolls of thin (0.01 inch?) polycarbonate sheet can be purchased for about $1 per square foot. My understanding is that the material is available with a UV resistant coating to extend the life. The material is said to be optically very clear, and would probably be easier to form a wrinkle free surface in than the thin Mylar film. If you have any experience with this material, or know of a good source for it, please let me know - Gary.

An alternate frame that utilizes a hardware store kit for making window insect screens is also shown.

I consider this whole thing still in the experimental area, so if you have any thoughts or ideas that might improve the design, please let me know -- Gary.

How-To

Make an edge frame.

Measure the inside of the existing window frame. Deduct 1/4 to 1/2 inch from these dimensions to allow clearance to install the new storm.

I used some scraps of Medium Density Overlay plywood for the edge frames -- this material works well because it is stable and warp resistant. But, other materials may do just as well.

The edge frame -- this is for a window with a peak on the top.

Assemble the frame on a flat surface.

Detail of top joint.

The corner joints are made with simple butt joints that are glued and nailed.

Be sure to check that the frame is square, and adjust if necessary.

The waxed paper is to prevent the frame from sticking to the surface it is resting on.

After the glue cures, paint the edge frame to match the window frame color

Lay Out the Mylar Film and Stretch It Flat

Cut a piece of the Mylar film that is a couple inches larger than the edge frame all the way around.

Lay the film down on a flat surface.

Stretch the film out flat by pulling the corners out with tape. For smaller windows, one piece of tape on each corner is enough -- for larger windows use a piece on each corner plus a piece in the middle of each edge.

Put enough tension on the tape to pull any wrinkles out. You may have to go around and adjust the tape tension a couple times.

The picture below shows the Mylar pulled out flat with the edge frame already positioned over it.

Apply Silicone To Edge Frame

Apply a bead of clear silicone caulk to the outer surface of the edge frame. Then, carefully position the edge frame over the stretched Mylar film, and lower it onto the film. Press down the edge frame all the way around. Then weight the edge frame down with any handy weights until the silicone cures.

The storm window edge frame placed on top the Mylar film. The paint cans are just to weight down the edge frame while the silicone caulk is setting. Note the blue tape that is used to stretch the Mylar flat.

After the silicone has cured, remove the weights and trim off the excess Mylar with a razor blade knife.

2 nd Pane

Do the 2nd pane in the same way you did the first pane.

Be sure that the inside surfaces of the Mylar film are clean and dust free before you silicone the 2nd pane in place.

Install the Storm Window

Install the storm window in the existing window frame.

I used two small screws that just barely penetrate the Vinyl frame of the existing window. Very little is needed to hold the storm windows in place.

The final product is good. The Mylar is quite clear and non-distorting. The only deviation from "perfect" on mine is that there are some very minor wrinkles that are visible in certain lighting -- for a $30 fuel saving per year, I can live with that :)

The installed storm window.

Alternate Using Window Screen Frames

Here are a couple pictures of an alternative that uses a window screen frame kit for the edge frame. These kits are meant for replacing window insect screens. These kits are available in most hardware stores, and come in a couple different sizes. They include the aluminum edge pieces and plastic corner pieces. This makes a pretty easy way to make a storm window frame. The kit I bought at Home Depot was about $13.

Picture shows the aluminum pieces that make the edge of the frame as well as the plastic corner pieces that slide into the aluminum edge pieces to make the frame.

You will need to cut the aluminum edge pieces to the proper length to fit your window, and then assemble the frames using the plastic corner pieces.

Once the edge frame is assembled, build the storm window in the same way that is described above for the wood edge frames.

The aluminum edge frame being siliconed down to a Mylar sheet

Performance

The 25 sqft Mylar window pictured above cost me about $10 for the Mylar ($0.2/sf * 2 panes * 25sf) plus about $5 for the edge frame lumber and silicone, for a total of about $15.

For this window applied over my current double pane windows, and for my climate:

Cost of the window about $15.

For my climate and fuel cost (8000 deg-days, $2 per gallon Propane, over dual pane windows)

Saving per year = $33

Ten year saving = $520 (assuming 10% fuel price increase per year)

Greenhouse gas = 210 lbs per year reduction

So, the simple payback is about one half heating season in my situation. You can use the Insulation Upgrade Calculator to estimate your saving.

Note that the payback on single glazed windows in my climate would be around one month of the heating season!

Here is the Insulation Upgrade Calculator results for my case:

Any ideas for improvements on this?? Gary.

Life

Life of Mylar inside storm windows ...

Gary 2/28/07

Updated later 2/28/07 to correct cost of Mylar (thanks to Randy for catching this!)

Updated February 6, 2009 to add note on Life.