Once the weather starts getting cold we get many calls from homeowners saying that they have a roof leak showing up as widespread moisture on the inside surface of the plywood and nail points inside their attic. I first encountered this problem in my early years in business when a roof we replaced kept having this same problem, sometimes forming sheets of ice all over the interior sheathing and nails. I went to the house repeatedly searching for roofing problems but there were none. It was a simple roof with a good roof pitch and all the best materials properly installed. Finally I decided there must be something else going on so we switched focus to the home’s interior and attic space. What could be causing this much moisture? I started doing some research (at the library!) and found articles about similar problems. The causes of this problem were based on dew point, humidity and condensation. The solutions were found in interior moisture control, insulation and ventilation.
A house produces a lot of moisture, in fact gallons of it every day. Cooking, showering, burning fuel, furnace humidifiers, free standing humidifiers and other household items all produce tremendous amounts of water vapor. If you had a roof leak that was concentrated in your attic space that combined all of this moisture into one spot, in your bedroom for instance, you would see a very serious problem. Drywall would be falling into your bed! However, because the gallons of water being produced in your home are in vapor form, you don’t see it. The vapor is supposed to be ducted away to the outside of your home or stay at a level that won’t cause problems. Go back to the home I worked on 20 years ago with ice on the interior surface – there was ONE specific problem we found that corrected their issue immediately – a ceiling bath fan had a 3 foot pipe pointing into their attic and was not ducted to the outside. Look at the amount of moisture that accumulates on your mirror when you take a hot shower – now blow all that moisture into your attic on a cold morning! The vapor is suspended in warm air, which carries more moisture, and is being blown into an attic with cold air which compresses that moisture out. The vapor needs to go somewhere, so it finds the coldest surfaces to cling to. Just like a glass of ice water that leaves a puddle of water on the countertop, the condensation effect in your attic deposits moisture the same way.
Dew and fog function this way: when the temperature drops, moisture in the air is forced out. The dew point is the point at which liquid water will form out of the air.
Let’s say you have a dew point of 68° F, moisture will form anywhere the air can find that is below the dew point temperature. This is why we wake up in the mornings to find our car windshields covered with water or ice. Now look at the air in your home. In the colder months, the inside air is warmer and the dew point is higher than outside air. When this warmer inside air escapes into your attic, where it is combing with outside air, if the temperature and dew point levels are lower, the higher moisture content is immediately forced out of the air escaping from the house and will form condensation on whatever is coldest, which is typically surfaces of sheathing and nails.
To solve this problem, the moisture being produced needs to be managed and kept in check. A clothes dryer removes gallons of water out of your laundry in every load so it needs to be carried away to the outside of the home. This is obvious but the same is true for bathrooms with showers and in your kitchen.
These high humidity environments need to have fans that pull the air through duct work directly to the outside of the home. The inside of the home needs to be checked thoroughly for all types of sources of moisture and then confirm that the moisture is being carried all the way outdoors.
The second thing to manage is the temperature and humidity level in the home. Many furnaces have whole house humidifiers that make your home more comfortable in the winter time by pumping vapor into the forced air system. These humidifiers, as well as stand-alone units, can quickly get out of control and overwhelm the home’s moisture balance, typically in the attic, but also on windows and other surfaces.
The third item is insulation and seals. Preventing the warm moist air from escaping freely into the cold attic space is not to be underestimated. An attic insulation level of R49 is now code while only 30 years ago it was just R19. The pull down stairs, light fixtures (especially recessed lights) and other openings in the ceiling need to be sealed off so that air does not transfer through.
The fourth thing is attic ventilation. A soffit intake area paired with ridge vents is the most common system. But this soffit to ridge ventilation pattern is largely based on hot air rising, drawing air into the soffits as the air is pushed up through the ridge vents. Therefore it can only do so much when the attic is cool and too much moisture is getting into the attic. When this occurs, an add-on item such as a power attic fan with a humidistat could be needed.
For further information regarding this issue, please refer to these articles: //www.human.cornell.edu/dea/outreach/upload/attic-condensation-2.pdf