The primary purpose of Attic Ventilation
is to remove moisture. Let’s be clear, though attic venting will remove some heat, the primary purpose for ventilation is to remove moisture, not heat. When I refer to “moisture” I am referring to water vapor. When water vapor condenses it turns into liquid water. Excess moisture in the attic can cause all kinds of problems, including rotted wood framing and sheathing and mold. Therefore we want to get rid of the moisture. Where does the moisture come from? Essentially from within the living space: cooking, showers, house plants – our bodies release moisture constantly. Water vapor is lighter than air and therefore, rises. While wallboard, paint and even kraft paper backed insulation are fair AIR barriers – they are not VAPOR barriers, they are permeable – water vapor can pass through them. Moisture will migrate into the attic, and in a properly vented attic, to the outside, even in cold weather. Older homes (unless they have been renovated) are drafty and seldom have major issues, just really high heating cost.
In a poorly vented attic, water vapor is still entering the attic, but is accumulating into a higher density. Science experiment: take a wide pan and fill it with ice (cold) and put it over your teapot blowing steam (hot vapor laden) and watch it rain as the steam condenses on the bottom of the cold pan. Now let’s take this into the attic, it’s not as fast and dramatic, but it is essentially the same. Warm moist air in the attic space condensing onto the frigid roofing members. Water goes from wet to dry – the dry framing absorbs the condensation. In a poorly insulated and poorly vented attic, do this long enough you will get rot and or mold issues. I see it all the time! Dumping bathroom and dryer exhaust into the attic or crawlspace (coming soon) really compounds and accelerates the problems. Don’t do it !! All bathroom and dryer venting MUST exhaust to the exterior. Here are some examples of what can happen by venting bathroom and dryer into the attic
Mold is beginning to form where a bathroom vent is Observe the rotting moldy area above the vent
dumping its exhaust.
This is the inside of an attic. Notice the blackened framing.
This is the exterior where the condensate
is running down the wall
Now that we understand WHY we vent, let’s talk about HOW we vent. The different types of attic ventilation, passive (convection – hot air rises) and active (motorized fans), include: soffit and ridge, gable, vent hoods, turbine vents and powered attic fans, and unfortunately combinations of any or all of the preceding. Soffit and ridge venting is the best type of venting. Hot air rises via convection. Air comes in from the soffit and as it heats up – rises and goes out through the ridge vents. Gable venting relies on outside air movement. Vent hoods and turbine vents are often used in conjunction with soffit vents and I’ve seen them with gable vents, taking the place of the ridge vent. Vent hoods and turbine vents centralize the exhaust to only where the unit is installed, leaving “dead zones” poorly vented and susceptible to moisture issues.
In the winter power fans are not operating and are the same as a really big vent hood or turbine vent. In the summer they have their own set of issues. The thought is this: cool the attic and it will cost less to condition. The reality is the attic will be under negative pressure, this means it’s sucking. Where is the air coming from? Likely the fan is exhausting way more air then can come from the venting system and it’s messing up the air flow (convection), the rest is coming from inside the home you just paid to condition. Passive venting is fine!
Science experiment: This is what I did: Warm sunny spring day, 81° outside. The underside of the attic sheathing had a surface temperature of 140°. The surface temperature of the floor 110°. The air temperature at mid height was 102°. And the air temperature at the ridge vent (I held my thermometer in the vent opening careful not to touch any surfaces) was 115°. Conclusion: The sun is applying heat to the roof surface through radiation. The roof surface is transferring heat to the sheathing through conduction. The surface of the sheathing is transferring heat to floor through radiation. The air is warming through the intermittent contact of molecules with the warmer surfaces through conduction.
If one wants to save on air conditioning air seal the attic from the living space than insulate with a high density insulation such as cellulose and confirm proper ventilation.