A roof needs to breathe and be vented in order to last long and keep your house healthy. First you need to understand what the differences are and how ventilation works.
A good exhaust vent should draw air out of your attic. In the summer, you want to get rid of heat that heats your attic space and can even delaminate plywood. In the winter you want to get rid of moisture and keep the roof deck cool to reduce the risk of ice dams. Keep your focus on preventing problems from happening.
There are two types of vents — active and passive.
Active vents will actively draw air out of your attic. These are vents like electric or solar powered vents with fans. But they could also include wind powered vents like turbine vents.
Passive venting relies on heat rising. Ridge vents, slant back vents or pod, sometimes called “mushroom”, vents are the most common type of passive vent. Ridge vents, slant back vents or pod, sometimes called “mushroom”, vents are the most common type of passive vent.
Here’s a key element that even some experienced roofers neglect — a properly vented roof must have both intake vents and exhaust vents. Just like a person, who needs to breathe in and out, a roof needs to breathe as well.
Intake Vents
The most common mistake a roofer can make is to not have sufficient intake vents. Even active vents, which allow a roof to breathe out, will not function properly if there is no way for the roof to breathe in. Intake vents are typically in the soffit (overhang) of a house. So air comes in from the underside, and breathes out from the exhaust vent on the top.
So why would you choose one over the other and how should you install them? Both active and passive vents work, but not equally.
Over the years we’ve developed some very effective methods for ventilation and just as our climate has changed, our recommendations for ventilation have changed as well. It’s important to understand how air travels.
Understanding Air Flow key to understanding how ventilation works
Not all houses are the same and therefore, not all venting solutions should be the same. Think about how the air flows through those spaces. Could a squirrel run from near your soffit up to the top? Could that little monster run from one portion of the attic to the other? If so, that is one of your conditioned attic spaces. Vent that as its own space. Do you have another, non-contiguous area where that little squirrel can’t get to? If so, apply the squirrel test to that space as well and if you want to design a good ventilation system, will that squirrel be able to run from a lower portion of the roof (intake) and escape through an upper portion (exhaust) of the roof?
Active ventilation utilizes a power source to suck hot air out of an attic space. It can be a natural and sustainable power source, or it can be electric. You don’t typically need as many active vents as passive vents because the active vents are “actively” drawing air out of your attic. If you can keep your air cooler, the downward pressure of heat in the summertime will not be making it more difficult to cool your home.
The Industry’s Dirty Little Secret
I’d like to be able to tell you that roofing manufacturing corporations have your best interest in mind. But these same companies are pushing the plastic ridge vents as part of their comprehensive warranted system. These ridge vents are about 1 inch tall. Our average snowfall in southeastern Wisconsin is 1 to 4 inches. Logically, how much air flow do you think you’re getting with that sort of vent? Whose best interest is being served by your roofer purchasing their product in a climate with snow, that will clearly have little to no ventilation for a significant part of the year? Oh, they’ll say, we’ll give you a lifetime warranty. They know the average person only lives in their house for 7 to 10 years. Now you know it. Connect the dots.
The Truth about Ridge Vents
Some houses do not have long ridges that can accomodate a passive ridge vent. If it cannot meet the 1/300 rule that we’ve discussed in past articles, then passive may not be right for you. Houses with a short peak are often good candidates for active ventilation. Canadian standards require a 1/150 rule which would require one square foot of ventilation for every 150 square feet of roof space. This stricter standard makes it nearly impossible to ventilate many if not most roofs with a ridge vent. If you do use a ridge vent, we recommend you double check your manufacturers ventilation calculator to make sure you have sufficient footage.
Alternative High Volume Venting
A good solar vent or turbine vent can move a massive quantity of air.
Your options for active vents are either electric or solar. Both are equally effective, but there is a fairly big difference in price. That being said, the #1 cause of attic fires is electric power vents. This makes solar vents an excellent choice. Turbine vents offer yet another option that are even better for snowy climates as a turbine vent has a collar that stands about 10 inches off of the roof decking. Requiring only a gentle breeze to draw the moist air out of your attic, the turbine vent offers great benefits over most other vents for residential applications.