How to caulk your windows: A step-by-step guide

Key points

• Caulking drafty windows can increase energy efficiency and save you money.

• Acrylic latex caulk is generally the cheapest, but siliconized latex caulk is more durable.

• Foggy glass and warped panes can be a sign that it’s time to replace your windows instead.

Maybe some of your rooms feel drafty, making you wonder whether your home’s heating or cooling is working effectively — or maybe you’ve noticed visible signs of caulk peeling off or cracking. It could be time to recaulk your home’s windows.

Some windows have caulk along all of the seams, inside and out. As your home is exposed to the elements and temperature shifts, caulk may crack, become unstuck or lose its flexibility.

We can help you decide when to caulk your windows, how to do it yourself and when you should throw in the towel and replace your windows instead. It doesn’t take much time, or many tools or supplies to seal windows yourself.

Why should you caulk your windows?

Caulking windows helps keep out moisture and drafts while retaining heated or conditioned air. It protects your home, saves you on utility expenses and gives window joints a finished, professional look.

Heat transfer through windows is responsible for 25%–30% of home heating and cooling energy use, according to the Department of Energy (DOE), which recommends caulking windows to help combat this.

Sealing gaps can help prevent costly repairs caused by moisture buildup, according to Jay Sanders, a licensed contractor and owner of Castle Dream Construction, a home remodeling company in Baltimore.

How do you prepare for caulking?

Effective preparation for caulking your windows will help you achieve attractive, long-lasting results and prevent extra trips to the hardware store.

Detect the leaks

Close the windows and doors snugly. Move a lit candle or some incense around the joints of the window. If the smoke is pulled out or blown in, you have a leak. I like to use blue chalk to mark each leak I find to help focus my inspection.

Check every window in the same fashion. This will help you determine how much caulk to buy. If you buy a case of 12, the cost per tube might be lower. Also, caulk typically has a one-year shelf life, so this is a good time to determine your caulking needs for the immediate future.

Obtain necessary tools

The following tools will help with your caulking project:

• Caulk
• Caulk finishing tool (desirable)
• Caulk gun (with a trigger)
• Caulk removal tool, putty knife, painter’s multitool or utility knife
• Caulk softener (if needed)
• Newspaper, cardboard or drop cloths
• Painter’s tape (optional)
• Soft sponges and cloth rags
• Wire brush (if needed)

How do you choose the right type of caulk?

You can choose from several types of caulk, so select one that suits your project and environment. Here are the primary caulk types for home improvement projects:

Be sure to read the instructions and recommendations of any caulk you consider buying to ensure it is compatible with your window materials and to determine which cleanup process to use. Carefully read the safety precautions and directions for use.

You will want a half-cartridge of caulk per window, according to the DOE. You’ll likely spend less per tube if you buy a 12-pack, which underscores why you should check all your windows before shopping. You will want to caulk the inside and outside of any drafty window.

How do you apply the caulk?

Applying your caulk to a clean, dry surface will yield the longest-lasting, most professional-looking results.

Step 1: Remove the old caulk

Use a putty knife or similar tool to remove all the old caulk. If the old caulk resists too much, soften it with a warm wet towel (for water-based caulks) or a rag soaked in mineral spirits or a caulk loosener. If you need to soften the caulk, let the surface dry before moving to the next step.

Pay close attention to any areas you marked with chalk to determine if any rot contributes to the leak. If there is rot, you may want to consult an expert to ensure caulk is an appropriate solution.

David Goss, owner of David Goss Construction, advised getting an estimate for new windows if there is significant rot in single-paned, wood window frames. This is especially relevant if you want to upgrade to double-paned windows.

Step 2: Prepare the surface

Wipe off any loose dirt or caulk particles. You might need a wire brush for concrete or other hard surfaces. Wipe with a clean cloth and keep the surface dry. At this point, you can use painter’s tape to mask the joints; this will help you apply a straight, even edge. Apply a strip of tape along each side of the joint so they are a quarter of an inch apart.

Step 3: Load the caulk gun

Remove the nozzle’s tip. Most tubes have cut lines to help you make the size cut you need for an appropriate bead of caulk. It is best to start with a small opening — about one-quarter of an inch — and increase it later if needed. Cut your tube at a 45-degree angle to help ensure a smooth application. Use a nail or similar object to pierce the inner seal.

Insert the cartridge into the caulking gun and draw a test bead on scrap paper or cardboard. Run a couple of test beads to become familiar with your tool. Use one hand to support the gun near the tip and the other to pull the trigger. Keep your arms bent and wrists firm, and try to relax as you pull the bead of caulk.

Use even pressure on the trigger to help pull a consistent bead. Start your application away from your body, pulling the gun closer as you continue the bead. Make any necessary adjustments to the tip size before you move to the next step.

Step 3: Caulk the window joints

Position the caulking gun in the corner of the window farthest from you and apply a smooth, steady line in the joint. It’s helpful to make a couple of practice passes without pulling the trigger on the gun. One continuous bead will look professional and help you achieve a good seal, and a practice pass will help you become used to the motion.

If you’re not able to run one continuous bead for each joint, there’s no need to stress. Just run a bead to the middle of the joint, start a new bead at the other corner of the joint and meet the two joints in the middle. You’ll address the meeting point in the smoothing step. Apply caulk to all the nonmoveable window frame joints.

For vertical joints, start at the top corner and run the bead from top to bottom. Bend at the knees or elbows — not at the wrists — to keep a uniform bead on the sides of the windows.

For horizontal joints, start on your nondominant side and move toward your dominant side. Complete both vertical joints followed by the top horizontal and bottom horizontal joints.

Step 4: Smooth the caulk

Smooth the bead of caulk to create an attractive even seal. I like to use a caulking tool to help ensure a consistent-looking bead, even if I caulk some windows on different days. Alternatively, you can use your moistened finger, a popsicle stick or a plastic utensil.

You will have the best results by smoothing the entire length of the joint in one pass. However, if excess caulk oozes out as you work, you can make a few shallow passes to reduce the excess and end with one long continuous pass.

Check your work to see if any areas need more caulk. The caulk should stick to both sides of the joint. If you find areas that need a touch-up, apply a bit more caulk and resmooth the bead. If you used painter’s tape, carefully remove it starting at the top corner, pulling it away at a 45-degree angle.

Step 5: Dry the caulk

Caulk needs to dry before it can be painted. Follow the instructions for the specific caulk you used.

Where should you not apply caulk?

To avoid compromising window functionality, avoid applying caulk to certain window areas.

• Above the window frame are drip edges for snow or rain to run off the window. Avoid sealing this area to prevent trapping moisture.
• Avoid caulking any moveable parts. To improve those seals, use the appropriate weather stripping. 
• Weep holes at the bottom edges of outdoor windows allow moisture collected behind the frame or in the channels to trickle out. Avoid blocking these holes to prevent trapping moisture.

Caulking vs. window replacement

Caulking drafty windows can make your home more comfortable and save you money on heating and cooling costs. After you check for leaks, but before you begin caulking, determine if your windows are sound and appropriate for your climate.

“Windows lose their ability and efficiency when they have been used more than 20 years,” said Sanders. He added that you should watch for foggy glass and warped frames.

You can always call an expert for advice — especially if you’re unsure how old your windows are or whether they are made of the best material for your climate.

Many window repair or replacement companies will perform on-site free estimates; this is a great time to ask questions and learn about your home. A visit from a professional window installer can help you compare the cost of repairing a drafty window against the cost of replacing it.

Professional vs. DIY caulking

Caulking windows is not complicated or time-consuming, and it doesn’t require expensive, specialized equipment. It can be a good DIY project if you are willing to prepare. Caulk is inexpensive, and each average-size window will only take a couple of hours to seal. If your windows are modern and in sound condition, caulking your windows might be all you need to repair air leaks.

Weather stripping and solar film can also improve efficiency; however, these repair choices are insufficient for uninsulated or unsound windows.

If you have unsound windows, you should call a professional contractor. If your windows have rot or mold, leaks not in the joints, or damaged frames or panes, you’ll want help from a professional. Caulk is not the solution for these issues. Additionally, use a professional to repair windows that require ladders or scaffolds to reach and repair.

What’s next?

If you’ve finished caulking your windows, congratulations. If your windows are not energy-efficient or solid enough for caulking to fix their costly air leaks, consult a professional window installer about replacement windows.