By Greg Stopher | Stopher Construction
Is there an issue with my attic? Out of sight out of mind, right? I have gotten a few calls lately, well more than a few calls about moisture in the attic. Water condensing in the attic on the roof decking, nails with frost on them, roof decking with frost, and when the temperature warms it actually raining in the attic.
What causes this? Is it a roof leak? How can it be stopped? There can be a couple of causes for the moisture in your attic, and it all could start in the crawlspace. Really! the crawlspace can cause issues in the attic. Even if a crawlspace seems dry and the dirt in the crawlspace seems dry, the dryness is caused by evaporation. If the crawlspace is not properly sealed with an airtight vapor barrier or no vapor barrier at all, the moisture in the earth below from the dry dirt is evaporating into the air as vapor. That is exactly why the dirt in the crawlspace is dry, evaporation.
The home acts like a big evaporation chimney and this is how it works: cool moist air from the crawlspace travels to the warmer air of the house and the warm moisture laden air travels up through the home through leaks in the attic where the moist air finds cool attic air. The moist air hits its dew point on the roof decking and freezes or condenses on the roof deck. The roof deck is warmed easily by sun or warming temperatures and causes a little micro climate rain storm in the attic. Damp roof decking will eventually turn to a blacken charred look. The black charred look is actually a type of fungus and this fungus is slowly eating the roof decking. The fungus or mold can be any type of the 400,000 plus types known and would have to be sampled and sent to a lab to determine the kind, but the fact of the matter is that there is a problem with moisture.
Moisture from life in the house contributes even more to the attic moisture problems. Cooking, pets, bathrooms, and just breathing plays an even bigger part to moisture in the attic than the crawlspace. Think about how much water is turning into vapor when boiling a pot of water for pasta. I cook a little bit when I have time and at least 2 or 3 cups of water are evaporated into the air every time a pot of water is boiled off to make pasta, maybe more. If the cooktop vent is not being used or is undersized and not vented properly, that moisture laden air is escaping into the attic. A pet and a family of four just living in the home is releasing 5 to 10 liters of water vapor into the air a day, some studies say as much as 14 liters a day for a family of four. All of this moisture is also escaping into the attic. Showers, baths, dishwashing, and plants, all contribute tremendously to the water vapor/moisture laden air that finds its way into the attic if its not caught at the source and vented to the exterior of the home.
Lastly, leaky ductwork in the attic releases moist air into the attic even if it is caught at the source. Leaky ductwork is like the highway express to moisture problems in the attic. Moisture caught at the source from cooking and showering which should be vented to the exterior of the building is finding a direct route through leaky ducting to the attic, a super highway!
Attic ventilation plays its part as well. How do you get air to flow from the vents on the eaves of the home to the gable or ridge vent in the dead of winter when the air is not moving and there is no breeze outside. Code requires a certain amount of attic ventilation per square foot of attic but I have to ask, how does the air actually flow if there is no air flow outside? Does the water vapor that finds its way up through the crawlspace from the interior see an opening in the attic via vents and tell all the other little water vapors to get the hell out of here and travel on its own to freedom? I don’t think so. It finds its dew point and starts a rainstorm in the attic.
How do we stop this water vapor from entering the attic? Well, we need to seal between the house and the attic. Foam or chalk all the penetrations from home to the attic to stop the air flow. Insulation alone does not stop the problem, in fact it accelerates the vapor to water transformation. The key is air seal, air seal, air seal. Then tackle the moisture within the home with proper ventilation and heat. The heat will come a little easier now that it is not all lost with the water vapor into the attic and the ventilation will work more efficiently with the sealed ducting.
Now that everything is sealed up, check the moisture content of the attic roof decking to see if the air sealing was successful. If the roof decking moisture content falls below 12% then the attic roof decking moisture content has fallen below a moisture content that will allow mold or fungal growth. Remedy!
Greg Stopher has over 16 years of experience in the construction field and earned a degree in Construction Technologies from the University of Alaska – Southeast. His company, Stopher Construction, LLC, is a general contracting company specializing in remodeling, custom finishes, additions and new home construction projects. He can be reached at 907-321-2350.