Saturday, October 21, 2017

Ice, Rain & The Pacific Northwest

March 2, 2010 by  
Filed under Inspection News

Portland Home Inspectors see it all. Living in the Pacific Northwest our homes are subject to a wife variety of weather. We don’t often think about it, but the places we live take quite a beating. Keeping up on your homes aches and pains is important to do early on, before a real problem festers.

Heavy wind during snowstorms or rain storms can carry snow or rain through attic vents at times. This is normal. Often, the moisture will evaporate before you notice it. However, if there are water stains on the insulation or ceilings, you should have a roofer move or modify the vent.

Placing a pan below the vent is a good option for an occasional small leak. See the the Figure at the end of this paragraph. The water in the pan will evaporate without damage to your home. Don’t try to get by using a pan for a large leak, though—it could lead to a large problem.

Ice Blocking the Gutters

In winter, ice can build up in the gutters, forming a condition called an ice dam. See Figure below right.

The process is triggered by excessive heat in the attic. The heat warms the roof deck, causing rooftop snow to melt. The slushy melted snow flows down the roof and into the gutters. Since gutters aren’t warmed by the escaping heat, they remain cold, and the slush refreezes there. As the process continues, the ice gets thicker, forming a dam.

Eventually, water ponds behind the ice (the same way water pools behind a river dam), and this water can leak through an asphalt shingle roof. Roof shingles are designed to shed water but will not resist ponding water. The leaks will occur just above the ice dams, penetrating the overhangs.

Your best defense against ice dams is to keep the attic cool with good ventilation and adequate insulation. The attic should have about R-40 (about 15”) of insulation. Close all air leaks into the attic, and insulate and seal all access doors.

Check ventilation openings. There should be about one square foot of ventilation per 150 square feet of attic floor space. Half the ventilation openings should be high in the attic and half should be in the overhangs. For homes with a vapor barrier below the insulation, the ventilation ratio is 1 per 300.

If ice dams persist even when there are no obvious problems with attic insulation and ventilation, you may need the help of a professional insulation contractor. Ventilation can be tricky with complicated roof designs. Air leaks from the heated space to the attic are a common cause of attic problems, but often they are hard to find.

Moisture on the Windows

Often, moisture (“steam”) condenses on windows in the fall with the start of the winter heating season. As long as moisture condenses only occasionally and disappears after several weeks, you don’t need to do anything.

Condensation requires a cool surface and moisture in the air. Inside your home, when the temperature of the glass drops below the dew point of the inside air, invisible water vapor in the air condenses on the cool glass. More condensation occurs when there is more water vapor in the air and/or when glass surfaces become colder.

Over the summer, moisture slowly accumulates in furniture, walls, woodwork, cloth and other surfaces. In the fall, as the exterior temperature drops for the first time, some of this moisture condenses on cold window glass. Most moisture leaks out of your home as your furnace runs and vent fans are used. Eventually, all the materials in your home dry out, and moisture stops condensing on the windows. This normally takes a few weeks.

If condensation continues to form on windows after several weeks, your home may have excessive moisture. Most moisture problems can be solved by limiting sources of moisture and improving ventilation.

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