How to Prevent or Fix Window Condensation

How to Prevent or Fix Window Condensation

Brennan Windows is back again with another window issue to tackle: condensation. First, we'll define what condensation is (just so we're all thinking of the same thing), consider some techniques to prevent condensation from happening, and talk about some of the reasons why it occurs. 

Window Condensation

When you notice condensation inside your house on your windows, you automatically think its a bad thing; your visibility is lowered, the natural lighting you'd usually get is reduced, your interior finishings (rings, stains, peeling paint) might deteriorate and, above all, mold may start to form!

This kind of superficial condensation occurs when the surface temperature of a solid (glass, sash, frame) is lower than the dew point of the surrounding humid air. The moisture that is naturally present (in vapor form) has changed into liquid water on these cold surfaces, and run down the glass when the condensation is heavy or does not evaporate fast enough.

It often occurs at the edge of the glazing in windows because of conduction through the spacer and air convection within the glazing area. How do you get rid of it? Raising your temperature or decreasing the relative indoor humidity will lower or eliminate condensation. How do you prevent it from occurring in the first place though?

Prevent Condensation Techniques

There are multiple factors that contribute to the overall condensation resistance of a window. To reduce or eliminate condensation usually means you have to do multiple complementary techniques, including but not limited to techniques concerning the window itself (method of installation), interior window accessories, arrangement of heat sources (convection heaters, baseboard heating, etc), and of course, the relative humidity of the indoor air. Only with a combination of doing many of these techniques will condensation be prevented or reduced to elimination.

Manufacturers also realize the need to reduce condensation, and therefore implement several techniques to increase condensation resistance in windows. These include filling windows with convection-limiting inert gas, low-emissivity (low-E) coatings that increase the glass's temperature, insulating spacers that reduce heat conduction and using non-conducting sashes and frames.

All else equal, energy-efficient windows are least likely to have condensation, but their resistance to it depends on the indoor humidity level. Humidity levels that are more than 40%, when outside temperature is or lower than -4°F, may cause condensation even on a good energy efficient window.

Window Installation

Installing your windows properly will help minimize condensation on windows. Professional installers will know the best placement and techniques to maximize condensation reduction, but if you want to install windows yourself, here are a few guidelines to follow during installation:

  • position the window as close as possible to the interior finish
  • insulate the space between the window frame and the rough opening around the perimeter of the window
  • seal the join between the frame and rough opening on the interior side.

We highly encourage you install with a professional window installer (it doesn't even need to be us! though we'd like that more) just so a proper installation occurs, or if something does go wrong you can have them fix it quickly.

As you decide to decorate your home and windows, try to make sure curtains, blinds, or valances do not impede or inhibit the air movement near the surface of the window. Air restriction can reduce the condensation resistance and thus increase "sweating" on the window. Look at Figure 28 for an example of recommended, acceptable and unrecommended installation techniques.

Despite new improvements in energy efficiency for windows, heat loss is inevitable, but when windows are placed above heat sources, they are less prone to condensation. When you're heating up your room or house, because of its closeness to the heating system, windows will have an increased condensation resistance. You need to be careful, however, in making sure hot air isn't directly flowing onto the interior surface of the glass. Such direct hot air will make the thermal stress problems in the glazing unit intensify which could result in the glass breaking.

Controlling Humidity

The best way, of course, to eliminate condensation is to control the relative humidity inside your home. Replacing windows that are old with new, more energy efficient ones usually significantly improves the airtightness of the enclosure, and thus lowers energy bills and improves the comfort for occupants. However, natural production of humidity in the house (showers, baths, cooking activities, plants, etc) usually occur and remain unchanged, so there is a significant reduction in the rate of air exchange, resulting in humidity especially during the winter. Excessive lack of air exchange from this may cause condensation on the inside surface of new windows. Despite that, newer, more energy efficient windows overall will generally improve your reduction of condensation.

To further prevent condensation, consider these simple measure to reducing indoor humidity:

  • If your heating system is fitted with a humidifier, or if you use a portable humidifier, disconnect them.
  • Avoid hanging laundry inside to dry
  • Make sure the exhaust from the clothes dryer is vented outside
  • If you have a crawl space under your house, cover the beaten earth with .15 mm (6 mil) polyethylene. The crawl space may have to be ventilated during the summer
  • Make sure your basement (if you have one) is well drained and protected against excess moisture
  • Make sure gutters and slope of the land around the house drain water away from the house
  • Try not to produce too much humidity. Plants, laundry, showers, and cooking without lids are major sources of water vapor.
  • Avoid drying firewood in the house. A cord of wood can release more than 60 gallons of water.

If signs of excessive humidity persist, you should increase your home's ventilation. If the frequency of condensation occurs once or twice during the summer, you can reduce or eliminate the problem by briefly opening two windows on opposite walls, or by turning on the kitchen or bathroom exhaust fan. If you absolutely cannot stand how frequent condensation occurs, you should install a controlled mechanical ventilation system (Fig. 29). Systems incorporating a heat recovery unit and a relative humidity control is preferred.

March 19, 2018
July 13, 2021

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