What's the most effective way to further reduce attic temperatures in hot weather?

Question

I just bought a house 6 months ago. Early on I discovered the not only was the attic poorly insulated, but it was not properly ventilated. The 1000 sq ft attic had 1 blocked soffit vents in each corner and 6 round exhaust vents near the peak of the roof.

Fast forward to the present and I've air sealed the attic, installed baffles and installed an additional 8 soffit vents to provide a total of 672 sq in of net free area intake. This may sound like overkill even if following the 1:150 rule, but it better balances the 678 sq in net free area exhaust that was already installed. The reason for so much exhaust venting is to accommodate a whole house fan (which works great).

In spite of the extra ventilation, the attic still hits 133F on a day in the upper 80's. With our recent heat waves of 90F+ and 100F+, the attic is reaching 155F at the hottest point of the day. This obviously affects my ability to efficiently cool the home and I'll even start to be able to smell "attic" in a few second story rooms.

I'd like to be able to keep the attic cooler. I'm not ready to re-roof the house yet, so installing a continuous ridge vent isn't possible at this point. I'm currently thinking that retrofitting a one or two of the round roof exhaust vents with a solar powered fan would be the next step. Thinking about something like this solar-powered gable vent so as to not have to cut into the roof.

Any pros/cons to this, or other ideas I'm not considering? Thanks!

Update: I've continued to monitor my temperature more throughout the latest weather changes (more clouds & haze from smoke). The big contributor to the overall temperature does seem to be the sun on my dark roof. A cloudy or hazy day will have cooler attic temperatures even if the outside thermometer temp is the same.

Also, I made a few additional adjustments in the attic to increase internal airflow. I added a second layer of baffles so I can increase my overall insulation depth. Additionally, I busted my butt to put baffles in some bays that were blocked by toilet exhaust pipes, bathroom fans, etc. These additional changes also improved the temperature without adding extra vents.

I think I will see the largest improvement from a reflective roof technology, but it will be a few years before I can do that. In the meantime, I am still considering a fan to help with airflow or possibly a radiant barrier. I'm afraid in my dusty climate a radiant barrier wouldn't be effective for very long though.

Would still love some input from people who have used solar powered attic fans to force some extra airflow. My whole house fan does this wonderfully in evenings when I run it, so I feel like something that will perform similarly during the day would be great. I am not increasing the number of vents since I definitely already have more than required for my square footage.

Answer 1

Some of your calculations may be off. The 1:150 rule is for attics that do NOT have “cross”ventilation. Cross ventilation is defined as a minimum of 50% and not more than 80% of the vents in the upper portion of the attic and a minimum of 3’ above the eave or cornice vents. (If your ventilation falls in these parameters, then you only need 1:300 ventilation for your attic.) (See Code ICC R806.2)

Therefore, you need a minimum of 1000 sf / 150 = 6.66 sf or 6.66 sf x 144 = 960 sq. in.

If you currently have 672 sq. in. you may need to add ventilation or confirm you added ventilation to qualify as “cross” ventilation. So, to reduce the heat buildup in your attic, I’d follow Code guidelines and the DOE guidelines.

Let’s say you meet the Code requirement for ventilation, so now try the following: 1) white (or light colored) roofs reflect heat, 2) add additional insulation between the living space and the attic will help isolate the attic from the conditioned rooms, 3) adding whole attic ventilation to the attic can only reduce the temperature in the attic to the outside temperature. That is to say, as you bring in more outside air, it will only reduce the attic temperature to the outside air temperature. However, if it’s 90-100 degrees outside and 155 in your attic, this may be effective. (I live where there is a lot of wind, so we never use whole attic ventilation.)