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How To: Build Interior Storm Windows

diy 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

measuring for storms
Measure 6 places

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

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

Cutting interior storm window

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

assemble interior storm window

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

glazing interior storm window

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

weathersealing interior storm window

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

storm window 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!

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21 thoughts on “How To: Build Interior Storm Windows

  1. One tip i have is to glue up the frame and instead of using clamps, put the frame in the window and wedge it till it dries. Much easier to get the frame square with the window.

  2. why not use shrink wrap on both sides ? What would the R values difference be vs only one sided ? From experience, put in an extra cross piece for any window segment over 36″.

  3. I have a very old Victorian home with curved wrap around porch. I have over 30 windows and my heating bill runs about $1000 a month. So I started doing the plastic but noticed it removes my paint on some of the windows. But the bill went down to around $800 during winter months. My next thought was to replace windows but I cant even imagine the cost. My neighbor is replacing his 2022 and I offered to assist so I can get a sneak peak on what I will be in for. But now after seeing your video I plan to make interior storm windows. I just came across some discounted thin plexiglass .04 thickness, about as thick as a DVD. Will this be ok to use?

  4. Hi.
    I’m confused by the 1/2″ gap on all sides. I can’t find felt that is 1/2″ thick. do you have a 1/4″ gaps on all sides (1/2″ total), and use felt that is 1/4″ thick? Thanks.

  5. Hey Scott, I love your blog. You’ve convinced me to keep all my old windows instead of replacing them. I’ve never been very impressed with most of the replacements anyway.

    I made an interior storm window very similar to what you describe here and have an observation that may be useful for a slight modification of the recipe: a different type of weatherstripping besides felt. While it does block drafts, I noticed that the felt doesn’t stop water vapor from migrating through the window opening.

    When I started out on this I found that my (exterior) storm windows were completely covered in condensation from inside the house each morning here in wintry Pennsylvania. I covered the interior of the window opening with plastic and sealed it with packing tape on the edges to see if that would make a difference. It did – the condensation was eliminated. For this new interior storm window, I removed that plastic layer and incorporated it into the frame, just as you describe in the second option of the recipe. With this, my condensation problem is back.

    Since many windows with integrated weatherstripping usually have a rubber “fin” embedded in the felt layer to block the draft and moisture, I’m going to experiment with a different type of weatherstripping, perhaps something made from an impervious material like rubber or nylon. I’ll try to let you know how that works out.

  6. Exterior storms look ugly because they sit flush and remove the indent that gives older house windows such a beautiful look (breaks of the solid mass of a single plane). That is one of the things that make older houses so much more isually appealing.

    I have been trying to make interior storm windows for our 180 y.o. house for several years. I keep failing. for various reasons (miss-measure, corner will not hold together, lexan cracks, while cutting, mis-measure, giant dog steps on the frame and breaks them to bits, mitered corners no cut tight enough, etc.). I like the idea of the felt weather-stripping – it makes the frame more forgiving is you mess up on the measurements slightly.

    Is there a good way to cut Lexan without cracking it or otherwise making a mess of it? I tried a skill saw with a paneling blade, but it tore the lexan up and that stuff is expensive. I bought a cutting tool specifically for Lexan, but it did not work at all. It binds or veers off. I tried a zipsaw, but you cannot control a zipsaw well enough to work for this.

    I would just give up and start buying them from innerglass, but we have 54 windows to do. It would cost a bloody fortune.

    I do pretty well with rough carpentry, wiring (I rewired our whole house), plumbing fixtures (and PVC piping), installing things, hardware etc. But I am terrible with fine carpentry. I lack patience. I am convinced that if I can get past getting the first one done correctly, it will be smooth sailing for the other 53 windows. or at least smoother sailing. So far each year, i try one, mess it up, get mad and find other things to do. Maybe your video wil help me get past that first properly constructed window black I seem to be having.

  7. if you use the shrink wrap you may want something more than 1×2 pine. I did this for an apartment for the bedroom window. in the past I put the shrink wrap directly to the window frame, but invariably at some point I’d burn something in the oven and needed to tear out the shrink wrap to open the window.

    Using the basic idea here, first the plastic wrap made the wood bow in at the middle of each side and twist backwards. I ended up putting a second layer in the front that got rid of the twisting but increased the bowing. also the pulling of the plastic made the insert out of alignment with the window after carefully aligning it before the shrink wrap. I ended adding weather stripping on the back side in addion to the edge to press against the frame, and stuffing packing foam between the Frame and the bowed parts of the
    window. It’s inelegant but works. in retrospect I wish I put a cross piece in the middle and used 2×3 for the frame.

    I have to say at first the custom online options looked expensive but if you start adding up the costs I saved maybe 40% over the cheapist option and I have a janky result.

    Live and learn, I’m on the fence for the other windows but with leaky single pane aluminum frame windows and the heat of summer coming, I need a plan b

  8. Great post Scott. I’ve been searching for a solution for a historic home in newport that has visually unappealing exterior storms.

    Can these be easily removed in the off season (spring through early fall) in order to open the windows?

  9. Is there a way to get ahold of some good e type glass at a reasonable rate or use an e film on glass to get a better quality interior storm window or would that be too heavy?

    1. I have the same question about glass. I’m excited to find this article after searching for a way to keep our 100 year old Craftsman windows in our little Calif Bungalow.

  10. My awareness of our plastic usage is heightened as we see climate changes impacting weather around the world. For those wanting to use environmentally friendly products, polycarbonate plexiglass takes hundreds of years to decompose. However, I read that if the plexiglass had EcoPure® as an organic additive, it can cause the plastic to biodegrade* through a series of chemical and biological processes in a landfill disposal environment. Check out the plexiglass that you use.

    1. If exterior storms aren’t approved by your district or undesirable for aesthetic reasons. Sometimes on old houses with Eastern casing installing exterior storms is difficult or in other cases where access to the outside is difficult like tall commercial buildings or condos.

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