Closeup on a homeowner's hands applying weather stripping, a type of window seal, to the bottom of an open exterior window

The windows in your home make a huge difference in the quality of life that you and your family enjoy.

They allow natural light into your living areas, which is very important for many reasons, including health benefits such as lowering blood pressure, reducing stress and anxiety, and helping to avoid seasonal depression.

Quality, well-positioned windows can also reduce the need for artificial lighting in the home, saving you money and making for a more pleasant interior space.

However, windows are also a weak point in your home. They can be a source of energy loss, allowing heat to escape during the winter and letting heat into the house during the summer, causing your HVAC system to work harder and increasing running costs.

These energy losses will likely worsen over time as windows age and wear out. A common issue that can arise as windows get older is that the window seals around the windows become less effective and allow drafts to enter the home.

The rest of this article will look at the different reasons your windows might allow moisture or drafts to enter your home. Once we’ve established the likely cause of the problem, we’ll look at some easy DIY fixes that can help you to solve the problem without needing to call in professionals.

How Does Window Sealing Improve Energy Efficiency?

Window seals help keep moisture outside and prevent drafts. As they age, their effectiveness reduces, causing air penetration and increasing your HVAC running costs. Replacing the seals around your windows can fix the problem, but sometimes the best solution is to replace the entire window.

There are lots of details that are critical to getting the installation and maintenance of windows right. For example, failure to install the flashing correctly can allow water to enter the building, causing moisture problems and allowing rot to form.

For more information about the energy efficiency of windows, you might be interested in our article about the energy efficiency of hurricane windows, which you can read here.

Window Technologies

Modern windows make use of specialist technologies that help to improve their performance in terms of energy efficiency.

Every component is designed with energy efficiency in mind, and you can compare the performance of different windows by looking at the NFRC label.

NFRC Label

The NFRC is the National Fenestration Rating Council, which is the body that tests and certifies windows. The NFRC label provides information on the energy efficiency of each window. 

It is broken down into the following values, including:

  • U-Factor: Measure of the window’s effectiveness at keeping heat inside a room.
  • Solar Heat Gain Coefficient: The window’s ability to minimize unwanted solar heat gain
  • Visible transmittance: How well the window allows light into your house
  • Air leakage: How much air will be allowed into the room through the window
  • Condensation rating: An optional rating that describes how well the window resists the formation of condensation.

The seals around each window are critical to its overall performance and must be in good condition for the window to perform at the level stated on the NFRC label.

Over time, window seals will become worn, and their performance will drop off, reducing the level of comfort and increasing your HVAC bills.

Insulated Glass Units (IGU)

Most windows these days include two panes of glass separated by a gap that helps to make the windows more thermally efficient.

These “sandwiches” of glass with a gap between them are known as insulated glass units (IGUs) and offer much better thermal insulation than a single pane.

In the old days, you’d only get a single pane of glass in windows, which provided poor insulation and often resulted in condensation inside the glass.

Triple-glazed windows are pretty standard in Scandinavia but haven’t caught on to the same extent in the US, where most homes have double-glazed units installed.

The panes of glass are held in place by a flexible sealant that maintains the gap between the glass panes and serves as a seal to keep moisture and ambient air out.

Cross section of a triple-glazed window cut away to show its functionality

Usually, the gap between the panes has all the air removed to create a vacuum, which is sometimes filled with inert gas, such as argon or krypton.

Krypton and argon are noble gases that are colorless, invisible, and odorless at room temperature and pressure. They are non-toxic and, therefore, entirely safe for use in windows, even if a leak develops.

Argon is cheaper to use than krypton, but krypton performs better and is preferred for use in thinner spaces.

Krypton has a much lower thermal conductivity than air. Its thermal conductivity is 0.00949 W/(m·K), which is 64% less than air’s, with thermal conductivity of 0.025 W/(m·K). This low thermal conductivity helps minimize heat transfer across the window, keeping your home warm in the winter and cool in the summer.

Window Frames and Sash

The design and choice of materials used in the manufacture of window frames will significantly impact the windows’ energy efficiency, especially the U-Factor, which describes the rate of heat loss through the window.

Aluminum frames, or frames constructed from other metals, will have relatively poor performance in terms of insulation because metal is such a good conductor of heat.

Looking out from inside a home through a hurricane window
A pair of hurricane windows with metal frames installed in one of our Net-Zero homes

To keep heat transmission to a minimum, these types of windows should include a thermal break in the form of an insulating strip of plastic installed between the outside and inside of the sash and frame.

Other materials used in window construction include wood, composite, vinyl, and fiberglass. Wood is a relatively good thermal insulator, and manufacturers can fill fiberglass and vinyl frames with insulation, which boosts their thermal performance.

The edges around window frames are sealed with caulking to keep out moisture and drafts, which is normally made from silicone.

Window Weather Stripping

Weather stripping is a seal that runs around the frame of each window to keep out drafts, moisture, insects, and airborne particles. It fills the gap between the window jamb and the sash and forms an airtight seal.

Weather stripping comes in several designs and is made from a flexible material that will form a good seal when the window is closed. Commonly used materials include silicone, vinyl, and rubber. 

The weather stripping can be attached to the window using metal mounting strips or inserted into the frame along milled grooves.

Broken Window Seal: How to Spot a Failed Seal

If you’ve lived in a relatively old house, as I have done in the past, you are probably accustomed to drafts. If you have a broken window seal, your first warning signs will come during windy weather. 

You’ll be sitting in your living room enjoying your favorite TV program when, out of the corner of your eye, you’ll notice the curtains twitching. Having ruled out the family cat as the cause, you’ll quickly realize the reason is the wind blowing in through the window.

But the windows are closed, and the vents are shut, so where is the draft coming from?

Chances are that the seals around your windows have become worn and are no longer doing their job correctly, allowing gaps to form around your windows through which air can blow. This deterioration results in drafts and significant energy losses, driving up your HVAC running costs.

window caulk at the bottom of a window's interior that has started separating from the seam

If the window seal around the IGU itself is broken, the main issue you’ll notice is that your window will fog up, and you’ll be unable to wipe it clear because the condensation is on the inside of the unit.

Types Of Window Seals

Windows have several seals in their construction, all of which need to work correctly for the window to do its job.

The main types of window seals include:

  • The seal around the insulated glass unit (IGU)
  • The silicone seal around the window frame
  • The weatherstripping on the window (e.g., the seal between the jamb and the sash)

Let’s look at each of these and how homeowners can fix them if they fail.

How To Fix a Failed Window Seal

Some of the window seals can be fixed by a competent DIYer should they fail, but others require a professional. In some cases, even a professional fix might not be worthwhile because you’d be better off upgrading the window for a longer-term solution.

Broken Insulated Glass Unit (IGU) Seal

A broken IGU window seal will often appear in one of the following ways:

  • Fogginess in the windowpane that cannot be wiped off because condensation has formed inside the gap between the panes of glass.
  • Distorted glass – sometimes, the glass can appear distorted, indicating a broken seal, allowing the gas filling the gap between the panes to leak out.
Closeup on the foggy up glass of an exterior home window


  • Partial or complete replacement of the IGU – the best option if your window is still under warranty.
  • Hire a specialist de-fogging company – a technician will drill a small hole in the glass, remove the moisture from the gap and introduce an anti-fogging agent to the IGU. This solution can make the window look better but doesn’t fix the seal, so the problem could come back.
  • Ignore it – if the fogged window is in a discreet part of the house, you could simply ignore the problem because the energy efficiency impact of a broken seal on a single IGU might not be significant in the context of the whole house.
  • A DIY fix is not recommended, even for homeowners with gusto.

Cracked Caulking

The caulking around your window frames is there to prevent water and air ingress. Cracks or holes in the caulking will lead to your window seals underperforming.


  • Drafts entering around the edges of the window frame
  • Moisture getting in around the window frame


  • Remove the cracked caulking around the window and replace it with a new sealant
  • DIY fix is straightforward
Closeup on a homeowner applying window caulk to a seam at the base of a window's interior using a caulk gun

Damaged or Worn Weather Stripping


  • Drafts enter the room through the gap between the window jamb and sash
  • Water gets in through the window between the jamb and sash


  • Remove damaged or worn weather stripping from around the window and replace it with new weather stripping
  • DIY fix is straightforward

Final Thoughts

Modern windows provide excellent energy efficiency thanks to the technologies employed in their manufacture.

Insulated glass units, insulated frames, and airtight seals combine to make these window units highly effective at keeping the elements at bay and maintaining the comfort of your home.

The window seals are a critical part of your windows, and if they become compromised, they will severely impact the window’s performance, costing you money and compromising your comfort.

If the caulking around your window frames or the weather stripping is damaged, you can fix this yourself with the right tools and basic DIY skills.

However, if the IGU window seal is compromised, you are better off calling a professional who will either suggest a fix or replace the IGU completely.

If you enjoyed this article, check out our article entitled, “Eco-Friendly Windows: 12 Ways to Improve Eco-Friendliness”, which goes into detail about how to make your windows more environmentally friendly.

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