Tired of your single-pane and leaky windows, but can’t afford to replace them? Here’s an easy way to dramatically improve the performance of your windows and reduce damaging condensation without spending much money: Make removable interior storm windows using wood casing, window film, and foam tape.
First, acquire some wood-trim molding. Contractors often throw out scraps of this material. All trim should be finished before cutting.
Determine whether you want the interior storm window to fit inside the existing window casing or on top of it. In general, it’s easier to friction-fit the interior storm window inside the existing window frame. To do this, measure the inside of the window frame horizontally and vertically and subtract a quarter inch from each dimension.
Cut the wood to length with 45-degree angles at each end, with the thick edge of the trim to the outside. If using square stock, you can use a rabbit joint (also called a lap joint) at the ends. Lay the pieces out and make sure the frame is square and the proper size. I like to use some nails tapped into a workbench at the outside edge of the frame to hold it in place and keep it square.
Cut a slot in the pointed end of the trim, deep enough to make the spline work, but not so deep you cut through the finished face of the inside corner of the frame. It’s important that this spline cut be made in the same relative location on all of the trim pieces so each corner pair matches. Cut the spline slots the width of your spline material (thin plywood that is the thickness of your saw blade works well for this).
Put the frame pieces between the nails again and glue the splines in place. Trim the spline and clean up the glue. Lightly sand all the rough edges of the trim, then “dry fit” the frame into the window opening. Next, put double-stick tape on the outside edges of the trim. Install the film as recommended by the manufacturer and shrink the film. (You can find interior storm window film at home improvement stores.)
Last, determine the best thickness of foam tape to use. You want it snug, but not so snug that it rolls the tape off when you install the storm window into the window frame.