Make winserts – the spline method

This method uses a mitered joint frame and spline for attaching the film.

The frames are a little more complicated to make than butt-joint frames, but the film can be replaced very easily.

Interior window panel tools and materials


  • tape measure
  • hair dryer
  • razor blade or utility knife
  • scissors
  • power drill with drill bit, countersink, and philips head attachments
  • staple gun for attaching pull tabs
  • miter saw (chop saw)
  • table saw
  • two corner clamps (optional)
  • screen spline tool


  • strips of ¾” thick pine or other soft wood, free of black knots. Strips are usually 1 1/2” wide, but can be wider for larger windows.
  • wood for cross-braces every 3 ft. for 1 1/2” frames, or 4ft. for 1 3/4” frames
  • water-based sealer and stain and/or paint
  • 2 1/2” wood screws – coarse thread
  • 100 gauge graphics-grade shrink film (a thicker, more durable version of the heat-shrink film found in do-it-yourself window kits, though these will work)
  • pull-tabs made from cloth strips or re-enforced tape
  • 1/2″ thick x 3/4″ wide open-cell foam tape
  • 0.155 (5/32”) thick screen spline
  • wood glue

For each interior window panel, you will need:

Roughly enough plastic to cover each side of the panel

Enough screen spline to go around the face of the frame twice

Enough foam tape to go around the outside of the frame once

Two strips of wood almost as tall as the window, and two strips almost as wide as the window, and strips for cross-bracing if one dimension is longer than 36”.

Step 1: Measuring: Measure the interior width (from one side jamb to the other) and the interior height (from the top jamb to the sill) of the window frame.

Click here to see a short video on measuring your window.

If you see any separation between the moldings and the jamb, especially in older homes, take several measurements (i.e. top, middle and bottom, or left, center, and right). Because a tape measure curves at the corners and throws the measurements off, use a card to help you get an exact measurement. Add the card’s measurement to the number on the tape measure where it intersects the card. Even easier is to measure to one side and make a mark at 10″, then measure the other side to the mark and add 10″.

If the window isn’t quite square, the 1/2″ foam tape around the window panel will make up for some of the discrepancies. If the measurements are within 1/8″ of each other, use the median measurement.

If you have several windows that are close but not exactly the same size (i.e. within 1/8″ of each other), use the median measurements there as well, so that you can cut and make frames of the same size rather than fussing with very slightly different individual sizes.

Once you’ve measured the window, calculate the size that the frame needs to be by subtracting 3/4″ from each dimension. This will allow for 3/8″ of room for the slightly compressed foam tape around each edge of the finished window panel.

e.g. For a window with interior measurements of 24″ x 36″, the finished window panel frame should be 23 1/4″ x 35 1/4″. Note that some people have made panel frames only 5/8” smaller than the window opening (23 3/8″ x 35 3/8″) – this gives a more snug fit, and is better for smaller straight-framed windows.

Step 2: Making the frame: Before you start to cut the frame pieces, you will need to buy or cut wood strips that are 3/4″ thick and 1 1/2″ wide. You can rip one or more 1× 4, 1X 6, or 1X8 etc. pine boards into the right sized strips, using a table saw with a finish carbide blade. Wider wood strips are recommended for larger windows, but use longer screws.

Cutting the frame strips – Avoid using any large twists, knots, or curves in the wood. You should try to get the straightest pieces possible.

Using a miter saw or chop saw on the 45º setting, cut the frame strips so the longer dimension equals the frame measurements taken in Step 1.  Make two frame strips for the sides of the frame and two for the top and bottom. Cross-bracing (if needed) can be made from slightly thinner strips. These do not need grooves.

Create the groove – Use a 1/8” wide blade on your table saw. You can make a groove towards one edge (as in the photo) or set the fence so that the groove will be in the middle of the wood strip (then the strips will be reversible). The blade should extend 3/16” above the face of the table, so that the groove is 3/16” deep.

section through winsert frame

Run both the top face and the bottom face of the frame strips over the table saw blade smoothly and evenly, in order to make a nice clean groove. Sand down the sharp edges of the groove.

Sand the frame strips – You can use a sander or do it by hand. If you ripped the 1 1/2″ wide frame strips from a wider pine board, be sure to sand the rough sawn edges smooth.

Assembling the frame – Dry fit the frame pieces. Using a 5/32″ drill bit, drill four “clearance” holes at each corner perpendicular to the miter joint (at an angle to the wood).

If you have cross-braces, then make a clearance hole for these as well, usually at the mid-point of each side-piece. Be careful to avoid the groove. Go nice and straight and level. Use a counter-sink to allow the screw heads to be flush with the frame.

Separate the frame pieces and spread a dab of wood glue on the corners.  Refit the frames pieces. Corner clamps help to hold the mitered corners together firmly.

Screw the frame together using a screw gun or a hand screwdriver. Tighten the screws so that they are flush with the frame.

NOTE: There are also other ways of assembling the frame: you could use corner biscuits, a nail gun with brads, etc.

Check the squareness of the frame by measuring the diagonals to make sure they are the same length; adjust as necessary.

Try the frame in the window before putting the film on.

 Step 3: Stain or paint the frame – Stain or paint the frame to desired finish color.  If painting, paint all sides including the outside edge (that will be covered later with weatherstripping).  If staining, follow by applying water seal to the outside edge of the frame. This will help the weatherstripping tape adhere better. Use water-based sanding sealer.

Step 4: Attaching the plastic film – The plastic film tends to pick up dust.  Any dust on the inside faces of the plastic film will be impossible to remove without removing and replacing the film.


Cut a sheet of plastic shrink film so that there is about 3″ excess on all 4 sides of the frame, then lay it over the frame and center it.

Cut four pieces of screen spline a bit longer than the lengths of the four grooves.

The goal is to stretch and attach the film evenly all the way around the frame so that it will shrink evenly and tautly when you heat-shrink it. When gripping the film to stretch it, grip the 3″ excess strip and pull it gently.

Do the stretching in two phases. In the first phase, stretch and “pin” the film to the middle of the four sides with the screen spline in the groove so that it creates a kind of diamond pattern on the plastic.

In the second phase, stand at the corner of frame and grip 3″ width of the film, pulling toward you slightly.  Using the screen spline tool, push the spline into the groove moving from the center of the frame strip to the corner on both sides adjacent to the corner.  The screen spline stretches so you may need to trim off some excess.  Repeat on all four corners.

Tiny creases near the frame edge will disappear when heat shrinking. Larger creases can be undone by removing the spline and re-fitting it.To shrink the film, use the hair dryer set on “high” and keep the hot air moving, about 3″ from the plastic, and starting in the center of the panel. This should only take two or three minutes, depending on the size of the window panel. Continue until all the wrinkles are out of the plastic and it is quite taut across the frame.Trim off the excess plastic with a utility knife just to the outside of the screen spline.REPEAT FOR THE OTHER SIDE OF THE WINDOW PANEL.

Step 5: Add pull tabs – The two small pull tabs at the bottom of the window panel are designed to make it easier to remove the window panel from the window. Cut two tabs for each window panel, about 1″ x 2 1/2″ each. These can be made from cloth or re-enforced tape. Staple them to the bottom edge of the frame. Pound the staples flush to the frame with a hammer. If using tape, wrap it ½” up the back side of the frame, across the edge, and stick it back on itself to form the tab.

Step 6: Attach foam tape – Apply the 3/4″ foam tape all the way around the outside edge of the window frame. This will compress slightly when the panel is installed and will make a good fit within the window frame.

Use your fingers as a guide to align the tape with the frame. Pull the backing strip clear as you apply the tape. Do NOT stretch the foam when applying it. When you get to a corner, you can cut it and start it again on the next edge, overlapping it to make a nice square corner.

Step 7: Window panel installation and care – Install the window panel by pushing the top edge up to the top of the window frame, then pushing the bottom in gently.  If you have difficultly pushing the panel into the opening, trying slipping a sheet of wax paper between the window opening and the window panel before pushing the panel in.  Once the panel is in window opening, slip out the wax paper.

If moisture builds up on the glass or window frame when the window panel is installed, remove the panel and wipe the moisture away so that mold does not form. To prevent moisture in the future, you can smear a thin layer of clear caulk on one surface of the foam, let it dry overnight, and re-install the panel. This will create a moisture barrier on the foam.

You can leave your winserts in all year round and they will also be effective at keeping the summer heat out. To store winserts, stack them carefully and protect them behind a solid surface if possible, to reduce the risk of the plastic being punctured. With careful handling, window panels will last for many winters. Small tears in the plastic can be repaired with clear tape.   After several years you many need to replace the film and/or weather stripping.


You are now the proud owner of a well crafted window insert, or “winsert”, which you can use year after year.

Now spread the word! …and give us your feedback!


2 thoughts on “Make winserts – the spline method”

  1. I want to build some winserts using the spline method but when I click the “section through winsert frame” link it takes me through some kind of portal instead of showing me how the grooves are cut in the frame. Can you tell me where i can find this information?


    1. Thanks for spotting this incorrect link – we re-set our website a little while ago but didn’t catch this older link. I’ve fixed it now and you should be able to see the diagram of the spline section.

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