Volume 35, Number 10, October 2000




It’s Raining in the House

understanding the nature of condensation


by Dez Farnady


During hot summer afternoons, a frosty glass of something cool is on the top of the list of desirable items. Under other circumstances, that same light coating of moisture is not so appealing. No one cares where the frosty look comes from as long as they like what is in the glass. But inside the house, it is another story.


My Glass is Leaking

The mist that appears on bathroom mirrors or steamy shower doors is usually alright. It is when it appears on skylights or bedroom windows that it becomes a mystery. People have told me their skylights were leaking and dripping, coincidentally, in the kitchen and the bathroom or over the washing machine. Someone once told me her customer’s glass was “sweating.” A few years ago I had a running battle for months with a couple who swore their skylights leaked when it wasn’t even raining. Then, there were the people with the leaky glass on their aluminum windows. They claimed it was a window and glass problem. Their new insulating wood windows were dry. Never mind that their whole house smelled of mildew.

So if that cold drink glass does not leak through its walls, where is that outside frost coming from? It is an issue that has been addressed extensively by both window and skylight manufacturers, but some people still don’t understand condensation.


Leaky Skylights

Skylights often are identified with leaky roofs and can suffer by association when leaks result from faulty flashing. Like glazing products, skylights have drainage systems and weepholes that dispose of most water penetration and rarely leak. But since hot air and steam rise in a house and condense on any cold surface available, they will attach themselves to the glass overhead first. The resulting moisture can drip from above, suggesting that it is the skylight that is leaking. Condensation gutters attempt to accommodate the moisture generated in the house by cooking, washing or bathroom steam. But if the weepholes are plugged or the unit is dead-flat and the gutters overflow, the water has to drip somewhere. In rainy weather with high internal humidity levels in warm, air-tight sealed homes, it is tough to prove that it is raining from the inside and not the outside.


Leaky Windows

Single-glazed aluminum windows are usually the coldest part of a building. If there is no skylight, the humidity will find the windows quickly. Condensation on the frame or around the perimeter of the glass does look as if the glass and frame are sweating. Fortunately for us neither material is capable of sweating. But that is where the homeowner sees the water. You can insulate the glass, change the windows to wood and maybe the sweat will go away. But rest assured that if something is not done about the humidity, the moisture will merely go someplace else inside the house. There may no longer be water on the window frame, but you certainly will be able to smell the must and mildew. The winter rains create the same problems for windows as they do for skylights; when the frames are wet, it must be the window that’s leaking.


Internal Moisture

Internal moisture accumulation usually is present as the result of excessive humidity from normal household activities, trapped in a house that is probably sealed air-tight. An air leak that permits heat loss is inefficient insulation that is not the same as ventilation. An expensive re-glaze may reduce the water on the windows and glass but it will just hide someplace else inside the house. Condensation is easy and cheap to eliminate with the proper ventilation to reduce the humidity.

wpe15.jpg (3343 bytes)   Dez Farnady is manager of architectural products for ACI Distribution in Santa Clara, CA. His column appears monthly.


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