Inside Storm Window From Multi-Wall Polycarbonate

The set of pictures below shows how to add inside storm windows made from multi-wall polycarbonate glazing. 


These storm windows should cut thermal losses through double glazed windows about in half, while still admitting daylight and allowing a (somewhat distorted) view out.


While these storm windows could be left in place permanently, we have mounted them with Velcro tabs, and intend to install them at the beginning of each heating season.


Here are the pros and cons of these polycarbonate (Lexan) panel storm windows as I see them.


The pros:

The Cons:
























Two samples of Polycarbonate glazing.  Twinwall  clear in front, and 5 wall white tinted in back.
The glazing comes in clear or various tints.



Building and Installing

Please read this before you proceed.


We have vinyl window frames that provide a nice flat surface to mount the storm windows against.  They look like the picture below.  If your frames are different, you may have to invent a new mounting technique.  If you come up with a method for other frame configurations, please send it in.





















Vinyl frame provides an inch wide mounting surface

parallel to the glass, and about an inch from the glass.

This is used to seal the storm window panels against.



Measuring and Cutting:

Note that the polycarbonate sheets have a UV resistant coating on one side, and this side must face the sun.     Make sure that when you do the markup on unsymmetrical windows that you mark it up such that the UV resistant face ends up facing the sun.   The protective film is marked to show which side has the UV coating.


Also note that the ribs in the polycarbonate sheet should be vertical when the window is installed -- not horizontal.


The polycarbonate sheets come with a plastic film protective layer on each side.  Leave this in place as long as possible.


Measure your window dimensions.  Subtract about 1/8 inch from each edge, and layout the dimensions on the polycarbonate sheet with marker.   The 1/8 inch allows for some waviness in the window frames, and allows for thermal expansion of the polycarbonate.  I cut the polycarbonate with an electric jig saw, which works fine.  Other wood working saws would probably also work fine.  I used a fine tooth blade -- see picture.

Its a good idea to pull back the protective film on the UV coated side and mark the panel "OUTSIDE" with a marker, so you don't loose track of which side goes out when the film is removed.





Cutting the sheets with a fine tooth jig saw blade.










Do a trial fit to see if you need to do any trimming.  You can trim the panels with a plane or other woodworking tools.  There should be a little wiggle room for thermal expansion.


Adding Velcro and Felt:

Update Oct, 2007:  The method shown below that uses Velcro and felt strips works as described, but I have had trouble with the adhesive on the back of the Velcro tabs deteriorating to the point where it no longer holds the window in place effectively.  


This year, I added 3 to 4 small, pan head screws to each window to replace the Velcro.  These screws go just a small distance into the Vinyl window frames, and are small enough that the holes are not visible when then windows are taken down for the summer.   The polycarbonate material is so light that just this small number of screws is fine to hold the storm window in place.  Make sure that the screws do not go far enough to interfere with the window glass.


If I were starting the project over, I would dispense with both the Velcro and the felt strips that are described below.  This makes the job easier, looks a little better, is still easy to install and take down, and still holds the window securely in place.


If the screws described above are not a possibility for your frames, you can still use the Velcro and felt as described below.  You might be able to find a brand of Velcro that holds up better -- please let me know if you do.


Another alternative would be to use small wood strips that match your window framing material to hold the polycarbonate in place.  The wood strips can be secured to the wood or sheet rock that frames the window.


I am more sold on this multi-wall polycarbonate for inside storm windows each year.  They hold up well, insulate very well, and are easy to install and remove (or just leave in place).


Gary Oct 6, 2007



Update Nov, 2008: In putting the inside storms up this year, I noticed some slight stains or smudges on the inside of the polycarbonate panes (that is, between the layers of polycarbonate).   Apparently some material gets down into the corrugations.  This is very slight, and not really a problem, but to keep it from developing further, I added aluminum tape over the top edge of the polycarbonate panes (see pictures).  This is actually what most of the installation instructions recommend, but I did not do it initially because I did not think it would be needed for inside use.  I only added the tape on the top of the panes, but you could add it top and bottom.

The aluminum tape will also prevent air from thermosyphoning through the space between the pane walls, which would reduce the effectiveness of the storms.


I really do like these inside storms -- very easy to put up and take down, or just leave them in place.   The stiffness of the dual or triple wall construction makes them easier to handle than the single thickness Acrylic panels, not to mention more thermally effective.  An easy way to cut your window heat loss in half (or a lot more if you have older tech windows).


Click on pictures for full size

Applying and trimming the aluminum tape.
Pull the backing off, stick in place, and
trim the overhanging edge with a razor
blade knife.  You can use the leftover part
for the next edge.

Finished aluminum tape.
Does not show at all.

We plan to install the storm windows each heating season and remove them from for the summer.  To allow for easy in/out, we installed a few Velcro tabs around the periphery of the panel.   The Velcro spaces the panel about a sixteenth of an inch out from the frame surface -- to fill this gap we used felt strips in a color that matched the window frames.

The Velcro that we used has an adhesive back, which we used to attach the hook side to the polycarbonate and the loop side to the window frame.


Once you have the window cut to size, you will want to clear out the cutting chips that tend to get in the window ribs.  If you have a compressor, just blow the chips out.  Otherwise, some tapping and blowing will get the job done.  I guess one could also run water from a hose down the ribs to clear them and then let them dry.


Lay the window down on cardboard with the outside face up. 


Pull back the protective film from the edges. 


Cut a few (usually about 6 to 10 depending on the window ) Velcro tabs about 1 inch by 2 inches each.  Its best to stick the hook and loop components of the Velcro together before cutting them, so you cut both at the same time.  The color of Velcro should match the color of your window frames -- else it will show.


Peel back the film to expose the Velcro on the hook side, and apply the patches of Velcro to the polycarbonate at a few places around the periphery of the sheet.  Leave the loop side of the Velcro patches stuck to the hook pieces, and don't remove the film over the loop side adhesive until just before you are ready to put the panel in place.  This makes it much easier to install the windows -- you don't have to worry about aligning the Velcro patches on the frames to the Velcro patches on the polycarbonate panels.


Cut some strips of fairly heavy felt about 3/4 to 1 inch wide -- these will be glued in between the Velcro tabs you just put in.  Use felt that is about the same thickness as the stuck together thickness of the Velcro hook plus loop pieces.  The felt strips prevent air from circulating between the polycarbonate sheet and the window frame.


Lay a small bead of silicone down along the edge of the polycarbonate panels between the Velcro tabs to glue the felt to the panel.   Its best to use a Silicone caulk that is  approved for use against polycarbonate (Lexan).   Clear  is good.


Press the felt strips down into the Silicone.  Let the Silicone cure.


Click the pictures to enlarge.















Cutting the Velcro tabs -- cut hook and loop sides at same time,

and leave them stuck together.















The panel with all the Velcro tabs stuck in place.






















Cutting the felt strips with a razor knife.















The panel with the felt strips glued in place with a bead of Silicone caulk.

Note that the felt follows curved edges easily.
















Install something that will allow you to pull the window back out after they are installed.  We used two pan head screws.

The two screws allow you to pull the window

back out after its installed.


Note: Ron suggested using brass dresser drawer pulls instead of the pan head screws.  This would look better and work better.



Installing the Window:

The window panels are pretty light weight and easy to install.


Clean up the windows frame area, and (maybe) wash the inside surface of the glass.


Remove the protective film from both sides of the polycarbonate panel.

The panels tend to pick up static electricity, which in turn tends to pick up just about everything.  Wiping the panels down with an anti-static cloth helps.


Peel off the film covering the top of the Velcro tabs.


Place the panel in the window frame opening and press until the adhesive on the Velcro tabs sticks to the window frame.












Make sure the polycarbonate is clean -- an anti-static cloth is helpful.















The panels are lightweight and easy to put in place.

They can be left in place permanently, or installed each Fall.
























Have a beer.























Great for odd shaped windows

The upper window shown is 5 ft wide by 8 ft high, and still installs easily.