Roof Vents - For the health of your roof

Ventilation and Insulation - Make Your Home Breathe Easy

When it comes to the longevity and sustainability of a roofing system, the primary concern is the ventilation of the roof. It is so important, that no manufacturer will warranty your roof without it.

So, what is roof ventilation and how does it work?

Roof ventilation is a combination of intake and exhaust through the attic area to the end that the temperature under the roof deck is as close to the outside temperature as possible. Basically, the goal of ventilation is to reach equilibrium between the outside and attic air temperatures.  Typically, the cold air comes in through the intake ventilation and carries out the warmer air through the exhaust.You may be thinking this already, but what, if anything, does insulation have to do with this? Yes, quite a bit actually. Insulation works in tandem with ventilation to help reach this temperature equilibrium. Without proper insulation the best ventilation will be hindered and vise versa.First, let’s discuss where we are tying to get with the ventilation. The International Residential Code (IRC) has specified that (unless certain conditions are met) the Net Free Area (NFA) of ventilation has to equal 1 sq. ft. per 150 sq. ft. of attic space. Doing some easy math then, if you own a 1500 sq. ft. ranch, you would need 10 sq. ft. of NFA of ventilation.

There are two types of ventilation: Intake and Exhaust. And a proper ventilation system will balance the two. Taking our example above, if we need 10sq. feet, a properly balanced ventilation system would have 5sq. ft. of NFA intake and 5 sq. ft. of NFA exhaust.

Now, let’s break down intake vs. exhaust.

Exhaust ventilation.

This is probably what you think of first when you think of ventilation. Most all of us have seen the vents on the tops of roofs. There are really three main types of exhaust ventilation in use today:

Ridge Vent
Pot Vents
Gable Vents

Ridge vent is the most common today, and has taken the place of Pot Vents as the primary means of ventilation. A standard ridge vent has about 18 sq in. per lineal foot of NFA. A pot vent has about 100sq in. each. This means you would have to place a pot vent every 5.5’ in order to get the same NFA as you would from a continuous ridge vent.

Going back to our example above, to reach the goal of 5sq ft. NFA of exhaust, you would need 40 LF of ridge vent or 8 pot vents.

Gable vents are really not helpful in many ways. First, if you have two on the house in our example, you are only getting about 25% of the proper exhaust ventilation. Second, the ventilation is only at the ends of the house. This means that there will be a lot of stagnant air in the middle of the home unless there is a substantial crosswind. Third, if ridge vent is installed, the gable vents can short circuit the ventilation so that intake ventilation is reduced. This is why we often seal off a gable vent from the inside when we are installing intake and exhaust ventilation.

Intake ventilation.

There is really only one main player here, and that is soffit ventilation. If this is not obtainable, there is another option, and we will get to that shortly.

Soffit ventilation

Soffit ventilation is far and away the primary means of intake ventilation. All new homes and many older homes have vented vinyl or aluminum soffits that allow air to move through it. A standard aluminum soffit will allow about 19.6 sq. in. of NFA per lineal foot. You would need 37 lineal feet of vented soffit to attain the goal in our example.

Edge vent

When a home has no soffits or the soffits are blocked off, there is a work around that will still allow you to attain the goals of a balanced ventilation system. That option is called edge vent. Edge vent allows for 9” of NFA (half of ridge vent) per lineal foot. You would need 80lf. of edge vent to accomplish the needs in our example. Edge vent is installed either right at the edge of the roof or back up the roofline at the edge of the exterior wall. The placement of the installation is determined by the framing and insulation of the home.

Pulling it all together.

With properly installed ventilation system and sufficient insulation with baffles that are installed to allow for unrestricted airflow, you now have the opportunity for a warrantied roof system that will increase the longevity of your shingles and eliminate ice build up.

M1501.1 Outdoor discharge The air removed by every mechanical exhaust system shall be discharged to the outdoors in accordance with Section M1506.2. Air shall not be exhausted into an attic, soffit, ridge vent or crawl space. Exception: Whole-house ventilation-type attic fans that discharge into the attic space of dwelling units having private attics shall be permitted.

Michigan Residential Building Code


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