We created this model as an industry standard model that shows how well a basic 3 tab Asphalt Shingle roof will vent. In the next paragraph I will get into the weeds of the test result temperatures. But first I wanted to talk about what how this was built. It was built with 2×4 framing, a basic roofing deck and normal Asphalt Shingles. This is almost a direct replica of how my home’s attic was built. My home was built in 2003 and has a soffit vent and ridge vent just like this model. In my attic once it gets over 85 degrees outside I can’t go up in the attic because it’s too hot.
Let’s take a look at the data on this model and see why it would be too hot in an attic that has venting on an 85 degree or hotter day. Feel free to follow along by downloading the spread sheet at the bottom of this page. At 12:36 PM on 9-8-2015 the peak temperature was 122 degrees Fahrenheit and the base was 112.6 degrees. The temperature was 89.5 degrees at this time. This is amazing that this is the industry standard. I’m going to keep going through the rest of the numbers for this day and then talk about what that does do you AC unit.
The highest temperature was at 3:02 PM when the peak rose to 127.2 and the base attic temperature was at 116.7. 
The outside air temperature was 91.6 at 3:02 PM. This makes a 10.5 degree difference from the peak to the base of the attic. It stayed over 110 Degrees in the base of the attic until 4:38 PM and over 110 degrees in the peak until 6:09.
The Main problem is that when you have about 5 hours during the day when your average base temperature is over 110 and the peak temperature that is over 120 the AC unit is going to have a hard time keeping up. Keep in mind that the high on this day was only 92, and on 9-8-2015 is was only over 90 degrees for 3 hours. These tests were taken in Virginia and we only had days that reached the low 90s in 2015 so if you live in the southern states or a place like Texas where I grew up, where it was not uncommon to have 60 days in a row of over 100 degrees these numbers would be higher and longer would keep that attic temperature extremely hot for double the time or more. The point is that on a 92 degrees day with a properly industry vented attic you still have temperature at the peak of 127, that is not only unacceptable it will cost you a good amount to keep you AC unit overcoming this high heat, on a moderate summer day.
If you are trying to keep your AC at 75 degrees on this day you will have to overcome about four hours of the base attic temperature of 110-116 which is an average of 37 degrees difference from what you are trying to keep you inside temperature at. That is 2 cycles of your AC unit, if we are able to get it to 1 cycle it will save you a good amount in cooling cost over the course of your summer. The Bottom line is that we want that base temperature of your attic to be close to 10 degrees or less of what the outside air temperature is, and in fact if you are ever able to get that base temperature down to the same as the outside air temperature it would be a home run. Check out Asphalt Shingle Roof with Therma Vent system.
Download the spreadsheet for Temperature Stats