For most homes, attic ventilation is critical. The exception is a new construction trend involving creating a conditioned space in the attic, often using closed-cell spray foam insulation. In that case, ventilation is sometimes not part of the home design.
In the 1970s, ridge vents were developed to provide for the exhaust of air from the attic. Before this, exhaust ventilation was done through smaller vents, often called square box vents, slant backs, or turtle vents. Of course, some older homes were so loosely built that active ventilation wasn’t even needed. But, when we brought plumbing indoors and started doing things that created moisture inside our living spaces, we ended up with more moisture migrating to our attics, carried by warm air. If that moisture reaches a cool surface and isn’t vented safely to the outside, the air contracts, and the moisture is forced out of it in the form of condensation. This combination of humidity level and the temperature differential is called “dewpoint.”
Proper ventilation in an attic consists of intake vents, usually in the soffit overhangs, and exhaust vents, usually at or near the ridge of the roof. Vents will have an NFA (Net Free Air) rating for the amount of air movement that can occur through that vent. You need equal amounts of exhaust and intake vents for ventilation to function. If anything, you want slightly more intake than exhaust venting, creating some pressurization in the attic.
Building codes call for one square foot of total venting per 150 or 300 square feet of attic floor space, depending on your location. Northern climates require more venting.
Another purpose of venting is to prevent hot spots in the attic, which result in snow melting in certain places of the roof and then running down the roof, forming dangerous ice dams and icicles on the unheated overhangs of the structure.
Ice dams are controlled through proper ventilation, insulation, and sealing air leaks from the living space to the attic. The goal is to have the attic, both winter and summer, be the same temperature as the outside air, so the conditions outside your home’s ceilings are similar to those outside your home’s walls.
Why am I not always a huge fan of ridge vents?
It boils down to the fact that they often perform too well and move too much air. That may sound a little ridiculous, but if you have a ridge vent with significantly higher NFA than what is provided by your intake vents, the ridge vent will fail. It will draw in weather rather than having the exhausting air push the weather away.
In most cases, if you enter a floored attic space, you will see a spray pattern beneath each exhaust vent. This means that in some cases, rain or snow is coming in through the vent. Usually, it’s in minimal amounts, and it quickly dries before it causes a problem. But if you have a ridge vent with very high NFA and you’re not feeding it air from an adequate intake vent supply, you can bring in significant amounts of water.
Additionally, on low-pitch roofs (I usually say 5:12 or lower), wind-driven rain trying to push its way up the roof can sometimes flood out the ridge vent and end up in the attic.
Ridge vents can be a powerful air mover to help rid attic spaces of heat and moisture, but if it is not balanced by at least the same amount of intake vent, look out. You will have problems.
Having your contractor analyze your total ventilation package before changing the roofing exhaust vents is always wise. Improper balance of intake and exhaust vents can also drive moisture into your attic insulation, reducing its effectiveness.