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Is Window Condensation Bad?

CT Replacement Windows: Materials to Minimize Condensation

It wasn’t too long ago that you were basking in the summer sun, enjoying a refreshing glass of lemonade, and noticed that both you and your drink were sweating.  Now that summer is over, you might notice that your windows look like they are sweating.  Glass can’t actually sweat because it is not permeable.  The water that appears on your drink comes from water vapor in the air, just like the water that appears on the inside of your windows.

Understanding Condensation:

  • Air contains water vapor, which is water in the gaseous phase.
  • Relative humidity is a measure of the amount of water vapor in the air divided by the maximum amount of water vapor the air could hold at that same temperature and pressure
  • Dew point is the temperature at which water vapor in the air condenses (changes from a gas to a liquid)
  • When air becomes saturated, water vapor will condense, but it needs a surface on which to condense, which could be a window, a blade of grass, or a particle of dust in the air
  • Warm air can ‘hold’ more water vapor than cold air.  If you cool a parcel of air, the relative humidity will increase, possibly to the dew point, and then the water will exit the gas phase and condense to liquid

Condensation on Windows

In the summer, your house is like a glass of lemonade; you may have noticed condensation on the outside of your windows.  As the weather turns chilly, the condensation will move inside, along with your flip-flops and patio furniture.  This does not necessarily mean that your house is too humid.  In fact, allowing the humidity inside your home to fall below 20% can aggravate your sinuses and make your family more susceptible to the virus that causes the cold.  The optimal range for indoor humidity is between 35% and 50%.  Sunrise windows provides additional information on how indoor humidity affects your home and your health.

On a cold day when your heat is on, the temperature of the air just inside your windows will be cooler than the air in the rest of the house because windows don’t provide as much insulation as the thicker walls that surround them.  As the air nearest the window cools, its relative humidity increases.  If the air cools to the dew point, water vapor will condense on the window.  Think back to that glass of lemonade.  Connecticut can be very humid in the summer.  As that glass of lemonade cools the air closest to it, the amount of water vapor in the air stays the same, but the relative humidity increases as the temperature drops until it reaches the dew point and condenses on the glass.

Replacement Windows and Insulating Capabilities

Condensation does not appear on your walls because your walls are better insulators. The insulation value of a wall varies based on its material, thickness, and density. The ‘R factor’ is the term you will see most widely used to describe a material’s insulating properties: the higher the R factor, the better the insulation.  To put these values into perspective, an un-insulated concrete wall might have an R factor of 15, while the R factor of glass ranges from 0.85-4.25:


  • Single pane Glass: .85
  • Clear Insulated Glass; 7/8″ overall thickness: 2.08
  • Hard-Coat (pyrolitic) Low-E insulated glass: 2.45
  • Hard-Coat (pyrolitic) Low-E insulated glass filled with argon gas: 2.75
  • Soft-Coat (sputter) Low-E insulated glass: 3.50
  • Soft-Coat (sputter) Low-E insulated glass filled with argon gas: 4.25

Figures courtesy of Cardinal’

R factor of other materials

Choosing replacement windows with a high R factor will reduce the amount of condensation that appears on your windows, and better insulators can help save money on home heating costs. Soft-Coat Low-E insulated glass filled with argon gas has the highest R factor in the industry. Low-E (Low-Emissive) glass emits less heat than regular glass by reflecting heat rather than absorbing it.  That means heat stays on the same side of the glass from which it originated: in the winter, infrared heat radiation from inside your home is reflected back inside.  In the summer, infrared heat radiation from the sun is reflected back outside.

Sunrise replacement windows are the most energy efficient replacement windows in Connecticut.  Replacement windows are always custom manufactured to prevent unnecessary energy loss.

Window Frames and Insulation:

It doesn’t matter how well your windows insulate if their frames are not good insulators, too.  Frames that insulate well should have low thermal conductivity and a high R factor.  Most residential window frames are made of vinyl, wood, or aluminum.

Wood: A standard 4′ wood frame has an R factor of 5.0.  Wood is a good insulator, but because it is a natural material, it is not as size stable as manufactured materials.  Just like floorboards and furniture, wooden window frames shrink and expand with fluctuations in temperature and humidity.  Though we wouldn’t recommend it, if you choose wooden window frames, make sure they have been treated properly before installation.  The insulating properties of wood will not matter if outside and inside air are allowed to mix freely.

Aluminum: The thermal conductivity of aluminum is over 1,000 times greater than that of Vinyl (PVC) or wood.  Aluminum frames contain a less conductive material between the inner and outer frames to mitigate aluminum’s inherent conductivity and reduce condensation.  This material is called a thermal break.  The R factor of a metal frame depends on the thickness and density of the material used for the thermal break.

Vinyl PVC: The R factor of Vinyl PVC frames depends on what the frame is filled with. Sunrise vinyl windows are constructed from energy efficient PVC, and offer polyurethane foam-filled extrusions that provide higher R-values than other PVC windows for truly effective insulation. Sunrise windows use only environmentally friendly polyurethane foams.

Sunset Magazine’s “guide to green windows” is an excellent resource for updating or replacing windows with energy efficiency in mind; however, their information is focused for those residing out west, so, as always, consult a local specialist about your needs in Connecticut.

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