A properly vented attic should only reach about 105°F in this situation, thus creating a 27° temperature difference between attic air and conditioned air. An improperly vented attic could reach a temperature of 160°F, thus creating a 82° temperature difference.
In our research and testing we have found that a properly balanced attic ventilation system is doing its job if it can keep the attic temperature within 10-15 degrees of the outside ambient temperature — within 20 degrees max.
So, how hot should your attic be? Ideally, the attic temperature should be close to the outside temperature, give or take 10 to 20 degrees. If your attic temperature is getting to 130 degrees or hotter, you can expect to see damage to your home.
Note excessive heat – if it's 90 degrees outside, the attic temperature shouldn't be more than 110 degrees. More often than not, attic temperatures will be in excess of 125 degrees on a 90 degree day. This will overwork your air conditioning equipment, run up your utility bill and shorten the life of your roof.
It is absolutely possible the ventilation system on your roof is excessive. Maybe your home is older and prior ventilation was installed by a homeowner, or incorrect vent sizes were used in an older roof layout. Too much ventilation can negatively influence the attic's and home's insulation as well.
Excessive Heat in the Attic
With poor ventilation in house, the hot air concentrated at your attic's highest points stays trapped. This, in turn, can lead to uneven roof temperatures (ice dams), an overtaxed HVAC (higher energy bills), and other costly headaches.
The temperature inside of your attic can be 50 degrees hotter than the temperature outside. Your attic should never get hotter than 130 degrees in the summer. If your attic is suffering from more heat than that, then it is time to call a professional and see what you can do to fix it.
If it's 90 degrees outside, most people would set their AC to a perfectly comfortable temperature of 68-72°F.
If your attic temperature is consistently hitting 130 degrees or hotter, damage to your home and a high utility bill is very likely. You can keep a thermometer in your attic to check – it doesn't have to be an expensive one and will give you a better idea of how hot things really are.
By improving ventilation in your home's attic, installing an attic fan can help reduce moisture and even lower the temperature within your entire house.
On a 100-degree day, an attic can reach temperatures of 170 degrees or more due to the radiant heat effect.
Contact a professional to add extra vents and a roof exhaust to enhance the natural circulation of air throughout your home and reduce the temperature in your attic. For a more active way of easily reducing the heat in your attic, a solar fan is ideal.
When calculating how much attic ventilation you need, your roofer will either use the 1/150 or 1/300 rule. The one they use determines how much Net Free Ventilation Area (open area for air to pass through) you'll need for every 150 or 300 square feet of attic space.
Now, from the example above, with 75 degree air being pulled in at the air filter, what temperature should we expect to be blowing out? Well, the general consensus is somewhere between 14 to 20 degrees colder than the intake temperature. From my example, 55 to 61 degrees would be an acceptable range.
Exhaust vents are best positioned near the ridge or peak of the roof, where hot air can easily escape. This placement leverages the natural flow of hot air rising to create a continuous airflow, which reduces the workload on cooling systems and improves energy efficiency.
Just because the outdoor temperature is above 100, the Ideal temperature to set your AC unit to doesn't change — it's 78 degrees. Higher temperatures in the home are to be expected when the weather is hotter than usual, but your thermostat can still be set the same as if it were a lower temperature outside.
Your AC might not cool properly due to thermostat issues, an undersized unit, or extremely high outdoor temps—so check those first. Keep an eye on airflow—replace clogged filters every 3 months, and make sure vents and ducts aren't blocked for efficient cooling.
How often should AC run on a 90-degree day? On a 90-degree day, your AC system might need to run more frequently to maintain a comfortable indoor temperature. Typically, an AC unit should run for about 15-20 minutes per cycle.
When it comes to an attic fan's temperature setting, it's best to adjust it to anywhere from 90 to 110 degrees Fahrenheit. It's impossible to state a single number because your attic can effortlessly soar above 95 degrees (on average).
Rafter vents should be placed in your attic ceiling in between the rafters at the point where your attic ceiling meets your attic floor. Once they are in place, you can then place the batts or blankets, or blow insulation, right out to the very edge of the attic floor.
Adequate ventilation is key, and one of the first things looked at in an attic inspection. Cooper says attics must be properly ventilated so the roof and attic can “breathe.” “Signs of poor ventilation include, mold, rusted nails and condensation — typically in the winter — on sheathing,” Cooper says.
MYTH: Considering the problems caused by insufficient insulation, it's best to have as much air flowing through the attic as possible. Truth: Ventilation needs vary. Too much ventilation can cause leaks, encourage pest infestations, and leave your attic vulnerable to storm damage.
A vented attic can heat up to 130 degrees in the summer, causing your HVAC system to work much harder than necessary. Unwanted moisture can also form as the result of condensation. These problems can be solved by applying spray foam insulation to the underside of the roof deck.