How To: Build Interior Storm Windows

Ever wondered if there was a DIY way to resolve drafty windows in the winter? Well, that’s exactly what a storm window is designed to do. A couple years ago I created a video about how to build DIY Storm Windows for the exterior of your old windows, but I figured the time was right to show you how to build an even easier version for the interior.

Wondering what kind of storm window is right for your situation? Go read my in-depth post Guide to Storm Windows to learn about all the options before you commit to building or buying one.

Just like exterior storm windows there are commercially available interior storm windows made by companies like Indow or Innerglass. I’m a big fan of these products, but I know that if you’re reading this you’re a pretty handy person who’s not afraid to get your hands dirty to save some serious money on your heating bill.

Making your own interior storm windows is extremely simple and requires only basic carpentry skills. You can watch along with the whole process in the accompanying video below to see it all in action.

In this post, I’ll walk you thru how to measure and put together your own interior storm window so you can stop shivering this winter. Each storm window only costs me about $80 in materials (depending on the size) so the payback in energy savings is huge and immediate rather than falling for the replacement window myth. Let’s get started!

Measuring For Interior Storm Windows

Your interior storm windows need to be installed somewhere in your window well. You’ll need 3/4″ of unobstructed flat space around your window. If you have a newer house that is likely a recessed section of drywall, and for old houses the best place is usually on top of the interior stops. If you have blinds or shades within the window well you’ll have to remove or reposition them onto the casing or somewhere out of the way. You may also have some hardware that is in the way of your storm and needs to relocated.

Measure the opening in six places just like the drawing here to account for out of square or bowed windows. If you’ve got an old house like me the chances of having a perfectly square window are virtually zero. The tighter the fit the more energy savings you’ll get, so measure twice and cut once, right?

It’s helpful to print or make a drawing like the image I have and write your measurements directly on that so you don’t make and mistakes in transferring measurements later which is one of the easiest mistakes to make. Once you have all your measurements subtract 1/2″ from all your measurements (you’ll find out why in just a bit). If you have significantly out of square windows then you’ll need to build your storms to the longest measurements and scribe the shorter areas after the frame is constructed. We’ll talk about that later.

Got your measurements and you’ve double checked them? Cool, let’s start building something!

Building the Frames

You’ll need a handful of tools and supplies for this project that are easy to find and relatively inexpensive. Also be sure to pick up a copy of my DIY Interior Storm Woodworking Plans before getting started.

Tools & Supplies

• Mitre Saw or mitre box
• Drill & Driver Combo
• 1/2″ Adhesive Felt Roll
• 1 sheet of Plexi-glass or Polycarbonate
• Random Orbit Sander or Sandpaper
• Wood Screws
• Flush Window Clips
• Circular Saw

I wanted to keep things as simple as possible for these interior storm windows so we’ll be using 1 x 2 select pine boards. No need for a table saw to rip them down or make any changes. You can usually find these at any home store like Home Depot or Lowes. These are clean pine with no knots or other imperfections so they will look great even if you decide to leave them unpainted, or you can prime and paint for a more finished appearance.

Don’t be deceived by the name since these are nominal sized boards. The actual dimension for each board is 3/4″ x 1 1/2″, and this will make the frame for your interior storm window. You’ll be making a rectangle with mitered corners for the frame and joining them together with pocket hole screws, dowels, or like I do in the video corrugated fasteners. It’s a very simple design that doesn’t require complex tools

Step 1 Cut to Length

Cut all four sides of your frame at a 45° angle on the ends to where the longest point of the mitre is equal to the longest length you measured (don’t forget to subtract the 1/2″). Lay everything out on an assembly table or the floor to make sure it fits properly and the joints are tight. After the frame is assembled we’ll be trimming off the excess to to accommodate for out of square windows.

Step 2 Assembly

In the video, I used a corrugated fastener gun to install three 3/8″ fasteners in each corner joint. These hold the frame securely in place and are way faster than any other method I have found. I don’t like these fasteners for exterior work but for building cabinet doors or interior storm windows they work perfectly.

You can also assemble your frame using pocket hole screws like I used in my previous post about building an exterior version of this storm window in DIY Storm Windows or any other fasteners you prefer like, mortise and tenon, dominos, dowels, biscuits or something else entirely.

Step 3 Fit and Trim

Give your frame a test fit in the window to see how it fits. If you have some areas that don’t fit with the expected 1/2″ gap due to the window being bowed or out of square then they’ll need to be sanded down or ripped down using a circular saw for larger modifications. Once you have a good fit with a 1/2″ gap all around give it a final sanding.

This would be the time to prime and paint. You can even stain and varnish it if that will blend in better with your window trim. Whatever you decide just make it look good since you’ll be staring at it all winter long.

Step 4 Glazing

Now that you have a frame you need something to keep the weather out and you have a couple options. You can use a more expensive and long lasting option like 1/8″ thick plexi-glass which I used, in which case you would just lay the frame down on the sheet of plexi and mark the outline of the frame on the plexi with a sharpie.

Once you have the outline marked out I trimmed out the plexi 1″ smaller than the outline to keep the plexi setback from the edge of the frameThen fasten the plexi to the backside of the frame using 3/4″ wood screws around the perimeter every foot or so.

The less expensive option is to use the Window Insulation Kit and install it on the backside of the frame. This kit comes with plastic sheeting and clear adhesive tape and is very simple to use. You install the double-sided tape around the perimeter of the backside of the frame of your interior storm window and then lay the plastic down tightly and press it in place.

Cut off the excess plastic with a razor knife or scissors and then using a hair dryer heat up the plastic which shrinks it to get a nice tight fit and takes any wrinkles out of it. When you’re finished you should have the plastic as tight as a drum and free of any wrinkles.

Step 5 Weather Sealing

Once you have a good fit on your frame you’ll want to apply the self adhesive felt along the perimeter of the interior storm frame. I prefer felt because that allows the frame to slide easily in and out of place, but you can use any type of foam or rubber gasket if you prefer. Putting in a couple of small weatherstrip nails strategically can help keep the felt from pulling off over the years. Test your fit again, noting that it should be pretty snug at this point. Make any additional adjustments.

Step 6 Installation

The moment of truth has finally arrived! Depending on how tight the fit is you may be able to simply press fit the storm into place with no fasteners needed which is ultimately the goal for that tight weather seal you want.

For extra security to keep it from falling out or to accommodate a slightly off fit you can use use the Flush Window Clips. Take four of these clips and install two on each side about three inches from the top and bottom by screwing them into the casing. Screw them in tight enough to hold their position, but not too tight that they can’t swivel into and out of place. If you have a very deep window well and a tight fit then you may not even need these clips to hold things in place.

Fit your storm window in place and swivel the clips to hold it securely in place. Step back and enjoy your new energy efficient interior storm window!

If you have these basic tools you should be able to build these storms relatively quickly and inexpensively. If you don’t have these tools then the good news is that this set of tools is very basic and can be used for a multitude of projects other than building interior storm windows. Good luck and happy DIYing on this fun, energy saving project!

Let me hear your thoughts in the comments below. Was this easy, too hard, what was the difference in you energy bills? I want to hear from you!