Warm climate attic ventilation
I'm struggling a little to understand and improve my attic ventilation. I want to avoid mechanical methods (electric/solar fan, "whirlybird", etc) and just use the natural buoyancy of hot air rising to do the work.
I have about 1,800 SF of attic floor space which means I need 12 SF of total net free air (NFA) ventilation. This is using the 150 SF of attic floor space to 1 SF of total venting as my guide which seems to be consistently referenced as the standard. I also have a fairly low pitched roof meaning less total attic airspace, but I’m not sure how that factors in.
Question 1: I see some recommendations that 50-80% should be high mount ventilation (ridge vent or dormers high towards the peek) and the rest low mount (soffit or louvers at the base of the rafters which is what I have.) Other recommendations say 60% low and the rest high. Which is correct? Does it matter? Should there be more top vents or more bottom vents or should it be equal? I’m trying to understand the science around the correct answer and get the maximum air flow effect. For example, does more venting at the top create a vacuum effect causing more air to be drawn in the bottom and out the top or does that vacuum effect restrict the amount of air transfer there would be with more lower venting?
Question 2: Most of my research seems to address cold climates where snow buildup and ice dams are a problem. Given those climates, it seems a certain level of heat (above the outside temp) is needed in the attic to avoid these issues so there is a maximum amount of venting you’d want. I’m in a warm climate in California. No snow and the coldest it gets in the winter is 45-50 degrees F and we maybe get 2-3 days of temps approaching freezing on an off year. Given this warmer climate, why would there be ANY limit to the amount of venting I’d want? Why wouldn’t I want my attic temp to equal the outside temp? In summer, it means my attic insulation (and A/C) would have to work less and there would be less heat impact to my attic structure and shingles – I’ve measured temps over 130 degrees F in my attic 20-30 degrees above the outside temp. In the winter it would be the opposite. My insulation would have to work harder (along with my heater), however, I’m typically only trying to raise the temp 5-15 degrees F in the winter anyway. Also, the variance in attic temp to outside temp in winter is a lot less normally so the actual impact I'd experience would be less. Overall it seems to be more efficient in my case to improve my summer performance with additional venting (getting the attic temp as close to outside temp as possible) while offsetting the summer benefits with the winter impact.
Any thoughts? Am I missing something here?
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