Acrylic Sheet Inside Storm Windows

 On some of our windows, we are using a single sheet of eighth inch thick clear Acrylic plastic sheet.    The sheet attaches to the window frame via spots of Velcro on the frame.  These are very easy to do, not too expensive, and probably add at least R1 of thermal resistance.  Payback is excellent -- see below.   In the summer, we will take the Acrylic panels down and store them. Be careful not to block a window that is required as an emergency exit, and don't use these Acrylic windows where there are big time heat sources nearby (e.g. wood stoves).

Here are some pictures:

This is the Acrylic panel in the process of being installed.  I did a sloppy job on the Velcro.  For the next set I'll  use the little round spots of Velcro, and get them in a color that matches the windows frame.  I'll also use panes that cover the entire window, rather than the two panes I used on this one.

With Acrylic panel installed.  The slight fuzzy look is due to the insect screen on the window.  The Acrylic panels themselves are very clear -- you can't really see them at all.

Economics:

If you live in a 7000 heating deg-day climate (mid to north US), and you have single glazed windows now, then for an example 3ft by 4ft window, you might save:

Window area = (3ft)(4ft) = 12 sqft

R value for single glazed windows  = 1

R value with Acrylic pane added  = 2

Heat Loss single glazed = (7000 deg-days)(24hr/day)(12 sqft)/ (R1) = 2,016,000 BTU per season

Heat Loss with Acrylic pane = (7000)(24)(12)/(R2) = 1,008,000 per season

Cost saving if you use propane in an 70% efficient furnace is:

Gallons saved = (2,016,000-1,008,000BTU)/((90,000BTU/gal)(0.7efic)) = 16 gal/yr saving

Around here, that's worth about \$32 per year.

The Acrylic costs about \$2 per square foot here, so this 12 sqft window would cost about \$24 to do.

So, the payback for single glazed windows is less than one heating season!!

For double glazed windows, the saving is about \$11 per year, so the payoff would be about 2 seasons.

Still a 50% return, inflation protected, tax free investment -- try and get that on the stock market :-)

And, it may qualify for a tax credit -- take a look here and see: Rebates

My state (Montana) pays 100% of the cost under the current tax credit for energy improvements program.

Larger Window Example

This is an example of using an Acrylic inside storm window just like the ones above, but on a larger window.

It also shows how it works well on windows with an irregular shape.

Because the larger Acrylic sheets are kind of large and floppy, this is a bit more work than the example above.  There are times when it would have been nice to have a 2nd pair of hands available.

The two large panes with the trianglular tops have the added pane of clear Acrylic installed in this picture.

This is the larger windows with the inside Acrylic panels installed.

The Acrylic is so clear that you really have to look twice to see that anything has been added.

These windows are about 58 inches wide.

These inside storms will stay up year round as they are on fixed (non-opening) windows.

I used fairly thin Acrylic sheet (about 1/8th inch) that I special ordered from Home Depot.  Don't remember the exact price, but it was fairly inexpensive.  The down side is that Home Depot requires that you order more than on sheet -- I had to order 4 sheets, but I have used nearly all of it.

The techniques for cutting etc are the same as decribed for the smaller window above.

I made the long cut to width on the table saw with a finish cut blade.

The angled cut across the top of the window was made with an electric sabre saw as shown below.  The Acrylic is thin and a little bit brittle, so you do have to use some care in the cutting.  Use a fine tooth blade (like a metal cutting blade), and support the work on both sides.  Go fairly slow.

The most difficult part of doing the larger windows is putting the larger pane of Acrylic in place.  Its kind of floppy, and takes a bit of patience to get it in position.   A second pair of hands might be a good deal.  With a large pane like this, I think that screws going through the Acrylic and into the window frame are better than Velcro.  I used 9 screws around the periphery to hold the pane in place.  They don't have to be very big screws, and be careful to make sure that you don't get to close to the glass panes with the new screws.

Small screws hold the Acylic in place -- this is 1 or 9 screws.

This took about 2 hours to do both windows.

Starting with double pane windows, in our climate, and with propane fuel, the saving per year is about \$46, with a 280 lb per year CO2 saving.  Not bad for a couple hours work :)

The saving for a single glazed window would be quite a bit better.

Here is the Insulation Upgrade Calculator output:

If you want to run the calculator for your own case, its here...

Gary

12/5/05

Added large window December 11, 2009