How To Insulate Windows

person insulating windows

The Spruce / Michele Lee

Overview
  • Total Time: 1 - 2 days
  • Skill Level: Intermediate

When extreme cold or hot temperatures bear down, your home's interior may suffer if the structure's thermal envelope is not properly doing its job. Cold and heat can enter your home through myriad avenues. But before you look at large expanses like walls, ceiling, and flooring, consider smaller intrusion points that are the more likely culprits: windows. Extreme temperatures can press into your home both through and around windows, even with quality windows that are in good shape. Not only is fenestration (how your windows are configured) a major area of thermal loss, but it is also one of the easier areas to repair.

Insulating windows, a simple and inexpensive project, can reap huge benefits for your comfort level and your monthly energy bill. While insulation cannot transform a bad window into a high-performance window, it can make a marginal difference to get you through a season or two. Newer, top-quality windows may only require insulating draperies, but likely not the comprehensive array of insulating steps suggested below.

When to Insulate Your Windows

Ideally, you should insulate your windows before cold weather sets in. On the other hand, because it is difficult to assess the state of windows' insulating abilities when temperatures inside and outside are nearly equal, you may wish to wait until it is time to start cycling on the heat or air conditioner.

Most of these following window insulating steps are inexpensive. Shrink-fit window film insulation, caulking, weatherstripping, and spray foam are so inexpensive that you can recoup their costs in energy savings over one or two seasons. Of all of the steps, installing energy-efficient window treatments are the most expensive, costing up to 25% more than ordinary, non-thermal window curtains.

What You'll Need

Equipment / Tools

  • Paint scraper
  • Caulk gun
  • Paint stirrer
  • Putty knife
  • Screwdriver (optional)
  • Thermal curtains
  • Draft stopper or draft snake
  • Needle and thread or sewing machine (optional)

Materials

  • Window film insulation kit
  • Caulk
  • Weatherstripping
  • Spray foam
  • Filler for draft stopper (optional)
  • Socks or fabric (optional)

Instructions

items for window insulation

The Spruce / Michele Lee

  1. Apply Window Film Insulation

    Prep your window for the film by thoroughly cleaning and drying the window. Use a paint scraper to scrape off bits of paint and grime from the glass. Squeegee the window clean. Dry it with a lint-free cloth. Follow the manufacturer's directions to add window film.

    window film insulation around window

    The Spruce / Michele Lee

  2. Caulk Around Gaps and Cracks

    Choose the correct caulk for the interior and exterior of your windows for maximum insulation. Do not caulk over the tiny round or rectangular exterior weep holes that are there for drainage. You will probably use about a 1/2 of a cartridge of caulk for one side of a double-hung window.

    First remove any old and cracked caulk with a putty knife or screwdriver. Make sure the area is bone dry before caulking. Cut the tip of the caulk tube at a 45-degree angle, insert the tube in the caulk gun, and pull the trigger to apply the material into the crack. Caulk in one motion rather than with starts and stops to create a continuous bead of insulation. If the caulk seems to spill out of the crack, use a putty knife to push it back in. Let the caulk cure for 24 hours.

    Practice Caulking First

    If you are new to caulking, practice the technique on a paper towel before insulating your windows. It will help you develop the pulling motion you need to dispense the caulk into the crack as it comes out of the tube.

    adding caulk to the window

    The Spruce / Michele Lee

  3. Apply Weatherstripping Around the Window

    Unroll the weatherstripping and begin forcing it into place around the window using your fingers. If you have to force in the weatherstripping, use a blunt object like a paint stirrer; a sharp item like a putty knife may slice the weatherstripping, but it also may be able to effectively press the material into narrow spaces.

    adding weather stripping to windows

    The Spruce / Michele Lee

  4. Insert Spray Foam in Openings

    Older homes need more insulation around the interior windows and expanding spray foam can be potentially easier to use than inserting fiberglass batting by hand. Remove the trim from around your window by placing a putty knife behind the molding to gently pull it off and expose the big gaps around the frame. Insert the nozzle on the spray foam can deep into the crevice (it's probably a few inches deep) before you spray. It will begin expanding right away, but it may take hours before the foam fully expands. Once the foam has fully dried and expanded, replace the molding around the window.

    spray foam

    The Spruce / Michele Lee

  5. Hang Energy Efficient Window Treatments (Thermal Curtains)

    It makes a difference in how you hang drapes when they are energy-efficient window treatments. Drapes should be hung as close to the window as possible and sweep the windowsill or floor. For maximum effectiveness to reduce heat exchange, install the drapes as far up to the ceiling as possible. Seal the drapes at both sides to further minimize heat exchange. Reduce heat exchange by using magnetic or loop tape to seal the sides of the drapes to the wall and overlap panels in the center on the sides and you may reduce heat loss up to 25%. When cellular shades are tightly fit against the window, they, too can reduce heat loss through windows by 40% and reduce unwanted solar heat by up to 80%.

    hanging thermal curtains
    The Spruce / Michele Lee
  6. Place Horizontal Draft Stoppers

    Draft stoppers can be found premade in home goods or home improvement stores. Make your own draft stopper with long socks or by sewing fabric into long tubes the length of a window (or door) you would like to block. Fill the sock or fabric tube with rice, popcorn, or dried beans. Close the end of the tube after filling by hand- or machine-sewing.

    Close your window and lay the draft stopper tightly across where the sill and the bottom of the window meet. This seal can block cold or hot air from seeping in or out through gaps and crevices.

    If you have a double-hung window, you can also lay a draft stopper on the top rail across the sash lock when the window is closed to eliminate even more drafts.

    adding a draft snake

    The Spruce / Michele Lee

Working With Window Insulating Materials

It can help to understand how and why an insulating product or material works around a window. Make sure you understand the particularities of the item you plan to use before you begin insulating your windows.

Window Film Insulation

Transparent window film works best to reduce solar heat from passing through glass in summer months and some films retain room heat in the winter months. Films come in large sections that are cut to the window size during installation. The film attaches to the outside of the interior window frame with double-sided tape. Lightly blowing warm air over the film with a hairdryer tightens the film. Acting much like the argon or krypton gas that fills the space between double-glazed window panes, the dead air pocket created between the film and the window hinders air and thermal infiltration. Window film insulation is different from reflective window film (or low emissivity film). The reflective film sticks directly on the glass while window film insulation must remain away from the glass.

Caulking

Caulk is inexpensive and easy to apply. All caulk periodically needs to be reapplied, especially if the window area is subject to expanding and contracting. So, expect that you will likely need to strip away and re-caulk these areas after just a year or two.

There are three types of caulk for different surfaces. Use water-based latex caulk for cracks in the window sash or around the window that are 1/4 inch wide or less. Silicone-based caulk is best for metal and glass surfaces. Paint-quality caulk is used for sections that you intend to later paint.

Weatherstripping

Weatherstripping is simple to apply and will leave no residue or mess when removed. When applied to movable window parts, the window cannot be opened or closed. If you do wish to open the window, the stripping must be removed, then applied once again. Often, this is an acceptable solution since windows tend to remain shut during the winter anyway.

Gaps around stationary parts in or around windows can be filled with EPDM, foam, or felt weatherstripping. The gap between the sash and the window frame can also be temporarily filled with weatherstripping. Don't forget to weatherstrip areas where cold air easily seeps in, such as your garage.

Spray Foam

Only existing large and accessible gaps around the window frame should be filled with expandable spray foam. Aided by a long nozzle, spray foam can reach areas that you cannot reach by inserting fiberglass insulation by hand. Spray foam is difficult to control and can quickly expand out of the wall cavity and onto the wall or floor, so be sure to purchase low-expansion foam for use as window insulation.

Energy Efficient Window Treatments

Insulating window treatments that provide extra insulation tend to come in two forms: thick side-drawn draperies or vertically-drawn cellular shades. Insulating draperies and shades only work when closed.

Draperies: The draperies are far thicker than ordinary ones and have tie-backs to hold them against the wall, further blocking air infiltration. During hot months, draperies with white plastic backings can substantially bring down the home's heat.

Cellular shades: The pleated-type shades that are vertically opened and closed may look like mini-blinds at first glance when they are open. Yet when they are down and closed, you can see that their cellular construction forms air pockets to help maintain indoor temperatures.

Draft Stoppers

Often used to block the gaps under doors, fabric draft stoppers, or draft snakes can also block the gap between the bottom of the window sash and the window frame. Draft snakes have very few downsides since they are so inexpensive and easy to make or buy. Just understand that they block only one of four potential draft points along the perimeter of a window sash. They are good for soundproofing, as well.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Energy efficient window attachments. Office of Energy Efficiency and Renewable Energy, U.S. Department of Engery.