Energy Conservation: An Interior Window Cover Reduces Drafts, Saves Money

By Bruce W. Maki, Editor

The idea for this window cover occurred to me in 1980 or 1981. When I was a college student, I saw a display in a shopping mall for an interior "storm window" product that was customized for the size of each window. They wanted over $100 for a medium-sized window cover made of Lexan (a thick plastic sheet) that used a magnetic strip around the perimeter to attach to the window trim..

3M Corporation had just released a window film product that was intended to be adhered to the window trim every autumn and removed in the spring. The film was not reusable. After applying the film, you warmed it with a hair dryer to make it shrink tight and cause the wrinkles to disappear. It was (and still is) a good product.

But I wanted to avoid the expense and hassle of applying film to windows every year.  So in 1990, when I bought my first house, I tried making a simple frame of 1x2 lumber, custom-cut for each window jamb opening.  I stained and varnished the frame, and then applied the window film to the wood. With a few other touches, I had a successful window cover that held back the drafts. 

This window cover, made in February 2000, is barely noticeable, even when the curtains are removed.

This frame is quite large, about 4' wide by 5' tall.  To prevent the sides from bowing inward, I added the horizontal cross piece. This piece is positioned to lie directly in front of the junction of the upper and lower window sashes, so it does not interfere with the view.

This frame was made from 1x2 red oak. But I have made other window covers from 1x2 clear pine and even 1x2 "whitewood", which is usually spruce, a low-grade wood prone to warping.

I made each frame about 1/8 inch shorter and narrower than the window jamb opening. This left about a 1/16 inch gap all around the frame. There is no point in trying to make a perfect fit.

The corners were mitered and joined with glue and brad nails. The first frames I made a decade ago were assembled with a single wood screw at each corner, which helped hold the joint together while the wood glue dried. Another good  way to join the boards together is to use miniature biscuits, but this requires a special biscuit-slot-cutting power tool.

After the glue dried I applied a coat of urethane varnish. This is necessary because a non-porous surface is needed for the double-sided adhesive tape. Semi-gloss paint could also be used, but first the bare wood must be primed.

The Gap:

After I made my first window cover ten years ago, there was still a small gap all around the frame. I experimented with different gasket materials. Foam weather-strip tape didn't work... it would roll off the wood as I pushed the tight-fitting frame into the window opening. Then I tried placing the foam strip inside a rolled-over strip of plain polyethylene plastic. That was too bulky. So I tried just a long scrap of polyethylene plastic, folded over and stapled to the back of the frame. That gasket design worked great and cost almost nothing! I have made at least ten window covers with this simple gasket and find that it does an excellent job of sealing the frame to the window jambs. And when the cover is installed in the window, you can't really see the gasket. 

A scrap of plastic, folded over and stapled to the back of the frame, makes the gasket.

There is one piece of gasket for each side. At the corners I simply overlap the gasket strips. I let the loop of plastic extend out about 3/4 inch, so it folds over the narrow dimension of the wood when the frame is pushed into the window opening.

This loop handle helps to remove the tight-fitting window cover, otherwise there is nothing to grab onto. The handles are just plastic strapping from some packaging that was going to be thrown away. (It pays to be a pack-rat!) I just stapled them to the back of the frame and folded them over.

To keep the window cover from blowing into the room, I screwed some small scraps of wood to the window jambs. I'll admit these cleats look sort of cheesy, but you can't see them when the curtains are in place. I could have spent more time making nicer looking blocks.

I labeled the back side so it would be obvious which cover belonged to which window. I also labeled the top end because the cover may not fit upside down. Storage of these covers during the summer season could be a problem. The plastic film is very fragile and cannot tolerate much abuse.  I recommend an out-of-the way place where children won't be able to reach these. I used to have a space behind the furnace where these covers would not get damaged.

One time my big 'ol Labrador Retriever put her paws up on a window sill and poked a claw through the window cover. I patched the hole with clear packaging tape and never bothered with any further repairs.

You Know They Work When...

These window covers can be so air-tight that they must be pushed into the window opening s-l-o-w-l-y to allow the trapped air to escape.

When the wind blows, the plastic film may bulge inward a little. During strong winds I have seen this large window cover bulge inward by over 2 inches. I can tell the direction of the wind by which room's window covers are bulging inward.

How About The Money?

The wood for these frames can cost a little or a lot, depending on the type and quality used. The plastic shrink-film may cost as much as $10 for a large window like this. Some large sheets might be enough to cover two or three windows. I can't imagine spending more than $25 for a window cover like this, unless exotic woods were used. I have built smaller covers for around $7 each. For basement windows I have used polyethylene for the window film, which renders the window impossible to see through. Combined with ordinary lumber, that is a really cheap solution, under 2 bucks. 

Copyright © 2001, 2005  HammerZone.com

Written January 7, 2001
Revised January 8, 2005