How to Fix Gaps Between Windows and Frames

If you want to learn how to fix gaps between your windows and frames, this article will help. Luckily, you can normally do so with little effort and only a little DIY know-how. In fact, in most cases, all you need is a caulk gun and expanding foam. 

  • Use caulk where gaps are smaller than 3/4 of an inch.
  • Buy expanding foam for gaps that are larger than 3/4 of an inch.
  • Use oil-based caulk on the exterior of the window.
  • Tape off the windowsill to prevent expanding foam from ruining the paint or UPVC.

If you want to know the full process, keep reading. 

How to Fix Gap Between Window and Frame

If you have a gap between the window and the window frame when the window is closed, you’ll want to take the window out and re-install it squared. The only reason your windows won’t close fully is that the frame is warped or bent out of shape. 

However, most people googling how to fix the gap between the window and the frame mean between the window frame and the wall or the window trim and the frame. 

What You Need:

  • Caulk and caulk gun. Look for an oil-based caulk for outdoors meeting ASTM C920-18 standards.
  • Expanding foam (for large gaps)
  • Masking tape 
  • Box knife 
  • Paint 
  • Disposable gloves 
  • Optional: Backing rods 

Tape off your windowsills 

If you’re using expanding foam to fill gaps in window frames, it’s important to use masking tape to cover nearby surfaces. Expanding foam can expand a lot more than you think. And, if it connects with UPVC or even painted aluminum window frames, it can ruin the paint job. Taking a few minutes to put masking tape and possibly newspaper around the site could save you a considerable amount of work later. 

Fill large gaps between the window frame and the wall with expanding foam 

Take a can of expanding foam and use it to fill the gaps around the window frame. Here, you normally want to shake the can and insert the nozzle as far into the wall as you can. In most cases, it’s important to follow any specific instructions on the can. In most cases, a single can is more than enough foam. However, if you want to seal multiple windows or have an especially large gap around a large window or a glass door, you might want more than one can. 

Here, you also want to try to use minimally expanding foam. The idea of using expanding foam is to create a moisture-resistant and insulating barrier inside your wall. The less the foam expands, the better the result will be.

Make sure you allow the expanding foam to dry for at least 24 hours before moving on to the next step. This is important because expanding foam continues to expand for this period. Then use a box knife to cut off any protrusions. 

Caulking the window

Use an oil-based caulk for outdoor use and carefully caulk around the windows. Make sure you’re wearing disposable gloves. Use a long, slow bead and try to apply the caulk evenly. Then, use your fingers or a soft tissue to smooth the caulk out into the seam. If you’re covering a larger area of expanding foam, you should caulk the full surface. This creates a waterproof layer. And, if you smooth it out, it’s a good base for paint. 

Allow the caulk to dry following the recommendations on the tube you buy. For the best results, you should caulk on the inside and the outside. 

Adding paint 

If you’ve filled a thin gap between your window frame and the wall, you probably don’t have to worry about the paint. That’s especially true if you picked caulk that’s close to the same color as your window frame. On the other hand, if you’ve filled a large gap between the window and frame, you almost definitely want to paint over it. Luckily, caulk is easy to paint. 

Here, the best option is to buy a paint repair kit for your window frames. That ensures the filled gap is the same color as the frame. However, you might also want to match the paint to the wall. This can be extremely difficult if you have brickwork or vinyl siding. However, opting to match the color as closely as possible is usually good enough. 

Optional: Backing rods

It may be a good idea to purchase and install backing rods into the gap between the window and the wall. This is especially important if the frame isn’t getting enough support. For example, if you have a brick wall and the frame isn’t fully secured. Backing rods provide support by adding a solid layer between the wall and the frame. 

Related Questions 

If you still have questions about filling gaps around your window, these related questions might help. 

What can I use to fill gaps around windows? 

Whether you have gaps between the frame and the wall, between the casing and the glass, or between the frame and the trim, you can use caulk. If you have a larger gap, you’ll want to use minimally expanding foam first. Here, it’s important not to use wood filler. The wood filler should only be used to fill small holes and cracks, not gaps. 

Why are there gaps between window frames and brickwork? 

If you’re installing double glazing windows into a brick building, you probably noticed there are gaps. Those gaps exist to prevent the brick and the windows from cracking as the brick shifts. In fact, it’s standard practice to build windows with a 1/8th of an inch gap between the brick and the frame. This should be filled with a backing rod and caulk or another filler material meeting the ASTM C920-18 specification. 

If you don’t have those gaps, any shifts in the brick could cause the window to crack. 

Why do gaps appear between the window and the fame

If you have a gap between the window and the frame it rests in, you likely have an issue with the frame or with the hinges. Here, you should inspect the hinges first. If there’s any sign of protrusion or if the window is pushed out from there, you might have issues with the hinges. Here, issues are only likely if you’ve recently changed the windows. 

Additionally, you might be facing issues with a frame that has been cracked or bent out of shape. You can check if the window frame is still square using a simple ruler square or a square box. If it doesn’t fit fully into all of the corners, your window might have shifted. The only real fix is to take the window out and reinstall it or replace it. 

If you have a gap between the frame and the glass, your window is defective. Therefore, you likely want to replace it. 


Filling gaps between windows and walls is relatively easy. However, you might need more than 24 hours to complete the job. If you have large gaps between the walls and the frame and want to paint, you’ll have to work on the fix over about three days with good weather. That’s because you’ll want to let expanding foam and caulk fully cure before adding the next layer. On the other hand, if you only have a small gap that you want to fill, a simple bead of caulk on each side of the window will do the job.

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