The temperature in my attic has been running 30-50F (112-125F) higher than outside (65-75F max) since April, which seems very high to me. The gable roof has black architectural shingles over water & ice underlayment, has a ridge vent (12.7 sq in NFA/ft x 38 ft), and 21 soffit vents (ea 5.25 in x 14.25 in; 37.25 sq in NFA). The attic area is 1064 sq ft. Doing the math, we should have about enough ventilation per the 1/150 rule (need 510.72 sq in NFA input & output; soffit vents = 782.3 sq in NFA; ridge vent = 482 sq in NFA). Furthermore, until June, we had 2″ diameter soffit vents (total 156 sq in NFA), which is less than half the input NFA that we needed, so I installed the larger soffit vents, but the attic temperature did not change. I could use some advice. Here are some questions:
1) are our attic temperatures over-hot and problematic? (maybe I’m making mountains from mole hills)
2) why did increasing the soffit vent NFA almost 4x have no impact on the attic temp? Could the ridge vent be under performing? The roofing company says they’ve installed this product on hundreds of homes with no complaints.
3) if #1 is “yes”, what would you advise as a solution? My roofer suggested removing the ridge vent and installing a power vent with themo/humid-stat.
Thanks in advance!
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I'm neither smart enough nor certified to answer your question. But I see you've gotten no hits on your original question (this one) and I've been pondering my "terrible" gambrel roof with no low-roof intake for ventilation for years and here are a few thoughts.
Hopefully someone else will jump in but I think the fastest way to get input on this is to read what you can at Green Building Advisor (search for "hot attic") or even subscribe for a month. Or look at buildingscience.com. I've read a lot in both places over the years.
I think the short answer is that that fireball in the sky is going to roast a roof and a little bit of air moving through the assembly doesn't do much to counter the roof mass being heated and the subsequent heat radiated into your attic. Seems there is a whole article in GBA about powered vents in attics; my recollection is that the conclusion is that the benefit just isn't there. Again, the heat energy in a volume of air isn't squat compared to the mass of the framing, roof deck, etc. I think I've even seen Martin Holladay just say, if you want a cooler roof, use lighter color shingles or roofing material.
It seems that the important function of a vented attic is to help control water vapor in the attic. The "shingle warranty" discussion abounds and there is a lot to read about that as it relates to a vented attic. (But don't waste much of your life there.)
Just like us, all roofs (particularly, shingled) will die sometime. You'd think a cooler attic would make them last a little longer. But as long as a roof is sitting in the sun, it's gonna get baking hot and some ambient air flowing under the roof deck isn't going to change that a whole lot.
I've also read about adding a radiant barrier under your roof deck. By my reading, they seem to get the most support from people who sell or install them; the real-world homeowner benefit doesn't seem to be much to talk about.
My particular issue with a hot attic is that I have HVAC ducts up there. Terrible. I'm air-sealing the attic floor, replacing the unsealed attic ducts and have even experimented with two layers of R-8 duct insulation. Working in an attic is the pits, even when the temperature is relatively comfortable. Only for crazy people.
Good luck with your research.
Milesahead - thanks for your suggestions. I browsed the GBA site you suggested (great site) and also found good articles about attic ventilation (incl pros & cons of powered ventilation) on Dept of Energy's Building America Solution Center (great resource). Key takeaways so far are:
- Attics get hot. 120F probably isn't a problem for the life of the roof and shingles (though probably not so great for some stuff we're storing up there)
- Powered ventilation has many downsides, especially if the intake airflow from outdoors is lower than the cfm of the fan (i.e., they'll suck air from the conditioned part of your home thru air leaks in the ceiling). But if intake/exhaust are balanced, they can help bring an attic's temperature and humidity toward equilibrium with outdoor temp & humidity.
So, I'm going to take a deep breath, monitor the temperature and humidity (using the HT1 sensor from SensorPush), and hope that the larger soffit vents are helping somewhat. And assume that the ridge vent is working as advertised, though I'm skeptical.
Good luck with insulating your HVAC ducts & hope that helps reduce the undesired heating/cooling of your condition air downstairs. Thanks again for your reply and support. I agree, working in attics sucks.
I can tell you that I originally had powered vents with the thermostats and changed them out with the regular turbines when I redid a massive attic just because I figured if I wanted the powered ones I can put them in myself later for way cheaper - a few years later and I keep a Bluetooth sensor in my attic and the temps & humidity have not changed since I removed the powered vents. They don't seem to be beneficial, or at least for my attic. Im in south Louisiana and my peak for the summer was 122degrees humidity 37%.
I honestly feel like ridge vents work the best, I've built new homes in the past and always used ridge vents and I felt like they always made more of a draft than turbines.
Also, from what little I do know, the ventilation isn't going to impact the attic temps, just keep the moisture moving