Attic Ventilation: Surviving Chicago Summer

attic ventilation oak brookSummer heat in Chicago means higher electric bills. Cutting your AC off and wearing less clothes may sound like a plan for right now, but overall, thinking long-term and more globally is what you should be doing.   

One of the biggest heat sources in our home in Chicago is the attic.  The attic load is much more than the walls and windows. Homeowners tend to spend lots of money on new energy efficient windows and not invest in more attic insulation. Insulation will help reduce the amount of heat that comes down from the hot attic. If you go up in your attic (your favorite place in the summer I am sure) and you can see the ceiling rafters above the insulation, then you need more.

But insulation only keeps the attic heat from transferring into our homes, it will also help if we can get the attic cooler.  An attic in the afternoon in Chicago summer can be 120 to 140 degrees and some even hotter.  Many people think that the way is to add powered attic fans to draw more air through the attic. In most cases this causes more problems than it helps. These powerful exhaust fans actually pull the conditioned air out of the house and into the attic. Yes, this cools the attic down, but it actually costs more because you are losing your conditioned air from the house to the attic.  The air you lose to the attic is then drawn into the house from outside which causes your cooling bill to go up.

Another good option for keeping your attic cooler in the summer is Radiant Barrier. This is a coating material that is sprayed on the underside of your roof to reduce the heat that is radiated into your attic.  

The “mushroom” vents that you see on many homes that spin with the wind to draw air out of the attic are not really made for the Chicago area.  Once the wind gets over about 8 miles per hour the spinning actually hinders the airflow and decreases the amount of hot attic air that can escape. We don’t have that many days that have lower than 8 mph winds in the Windy City… The other problem with these is the bearings go bad and they get very noisy.

First of all, let’s figure out how much venting do you need for your roof? The rule is 1 s.f. for every 300 s.f. of attic floor space if you have a vapor barrier. Most good primers are considered vapor barriers but, if there are holes in the ceilings for can lights and other fixtures, you need more venting. Then you need 1 s.f. for every 150 s.f. This is typical for much older homes. Only use the area of the house covered by the roof. If you have a two story home, only use the area of the 2nd floor.

Let’s say you have a 1500 s.f. house. (1500 / 300 = 5) You need 5 s.f. of vents on your roof. Not five roof vents. Here’s why. Roof vents are rated by NFA (Net Free Area) usually in square inches. One square foot is 144 square inches. So for our house we need 5 x 144= 720 sq. in. Now I’ll explain your vent options.

The typical “mushroom” vent is 51 NFA (Net Free Area) this means we need 14.11 mushroom vents. When was the last time you saw a house with 14 mushroom vents? Never, so you got to use a different approach.

Rolled vent or cobra vent nailed to the roof has 14.1 square inches / foot NFA, this means we need 51 linear feet of ridge vent. On a typical hip roof you won’t have anywhere near 51 linear feet of ridge.

Plastic ridge vents offer 18 square inches / foot NFA, a little better but we need 40 linear feet of ridge vent.

This shows you that you should be using mushroom vents and probably more than you have on the roof. Also, make sure you have enough soffit vents to allow for free airflow.

The idea is to use convection currents to vent the roof. Warm air rises out of the vent, cooler air enters the soffit vents. You do NOT want more vent on top than you have on the bottom, soffit venting. If you do, the “make-up” air will be drawn out of your home. It will draw the precious conditioned air right into the attic. And when that air is sucked up, you get more warm moist air pulled into your house from bad windows, outlets etc.

Another thing to check for is to make sure your vents are not restricted.  I have seen many times where the soffit vents are completely plugged up either with insulation that was blown in the attic, or from dirt and rust from the airflow from outside.  You need to check yours to make sure they are open and will allow the maximum amount of air to pass through them.

Actually the winter time is the best time to do anything in the attic, that is when it is cooler and almost bearable to be up there. The problem is that we have a tendency to not think about these things until we need them.  A little thinking ahead can make the job a lot more bearable…

So for now, you might just have to suck it up, but it’s a good idea to put it in your Google Calendar (or whatever app you use) a reminder, let’s say, for February 1st, 2017.  Something like “It’s time to work on the attic ventilation. Your future self is going to thank your current self next summer.”