I’ve seen a lot of folks lean hard on TU-35 (especially doubled up), but I wouldn’t call it bulletproof. It’s solid, sure, but even the best underlayments have their weak spots—especially if you’re in a high heat or high fire zone. There are thicker synthetics out there, like TU-43 or some of the newer self-adhered options, but honestly, “thicker” doesn’t always mean “better.” Sometimes it just means more material to trap heat or moisture if your ventilation isn’t dialed in.
About those O’Hagin vents—yeah, the math can get pretty wild when you try to hit code with low-profile tile vents. Eighteen is a lot, and cutting through trusses for cross-ventilation is a headache most roofers would rather avoid (and I don’t blame them). I’ve seen jobs where someone tried to shortcut that and ended up with dead air pockets and condensation issues down the line. Not fun.
As for higher-flow dormers, there are a couple brands that claim better NFA (net free area), but once you add ember-resistant screening, you’re back to square one with airflow. The fire mesh is non-negotiable in your zone though—seen too many embers sneak through standard vents during fire season.
Solar-powered vents sound good on paper, but like you said, servicing them is a pain if you can’t get at them from outside. Plus, they tend to fail after a few years in harsh sun—motors seize up or panels crack. I’ve replaced more than a few for folks who thought they were set-and-forget.
The powered fan on the gable vent idea… I get where you’re coming from, but reversing airflow like that can mess with natural convection and sometimes pull conditioned air out of the house instead of just ventilating the attic. Plus, any powered solution means more maintenance and another thing to fail when you least want it.
Honestly, sometimes the best move is just maximizing passive ventilation with what fits your roof design and code—maybe fewer holes overall but placed smarter. And yeah, every option feels like a compromise between longevity, fire safety, and not turning your attic into an oven. Haven’t found a perfect answer yet myself...
I get the hesitation with powered vents and doubling up on TU-35, but I’m not totally sold on the “fewer holes, smarter placement” approach either. When I re-roofed last year (clay tile, SoCal, low slope), we tried to minimize penetrations, but still ended up with hot spots and some minor condensation in winter. Maybe it’s just my attic layout, but spacing vents farther apart didn’t seem to help as much as I’d hoped. I’m starting to think a hybrid approach—mix of passive and a couple well-placed powered units—might be worth the extra maintenance, especially if you’re already running electrical up there. Anyone else notice that thicker synthetics like TU-43 actually made the attic hotter? That was a surprise for me.
Anyone else notice that thicker synthetics like TU-43 actually made the attic hotter? That was a surprise for me.
I had the same thing happen with TU-43 on a composite shingle roof (Central Valley, CA, not super different from SoCal weather-wise). I thought the thicker underlayment would help with heat, but it seemed to trap it instead. My attic was noticeably warmer last summer compared to when I had the old felt. Maybe it’s just less breathable, or maybe the heat just can’t escape as easily with the extra thickness. I’m not sure if it’s supposed to be that way or if I just installed it wrong, but I was definitely surprised.
About vent placement, I get the logic behind fewer penetrations, but in my case, it was a trade-off. I tried to be “smart” with vent placement, but I still ended up with weird hot zones, especially above the garage. I’ve got a low slope too (3:12), so maybe that’s part of it. I haven’t tried powered vents yet, but I’m tempted to. The idea of mixing passive and powered makes sense to me, especially if you’ve already got electrical up there. Maintenance is a pain, but if it helps with those hot spots and condensation, it might be worth it.
One thing I noticed—when I swapped out a few of the older box vents for ridge vents, it helped a bit with general airflow but didn’t totally solve the heat issue. Maybe the attic layout just doesn’t allow for good cross-ventilation, or maybe the underlayment is just too good at blocking airflow. Kind of feels like there’s no perfect answer, just a lot of small improvements.
I’ve also wondered if the color or type of tile impacts attic temps more than we think. My neighbor has lighter tiles and says his attic stays cooler, but I’m not sure how much is the tile versus the underlayment or venting. It’s a lot of variables to juggle.
I haven't really spent a lot of time in the crawl space to see IF there is any condensation issues, but the walkable portion of the attic (opposite side of the home), seems to be fine without any issues (no vents on that side).
Attached, is the original builder's drawing of my roof.
One interesting observation, is the manufacturer of TU-35, rates 2 layers of that stuff at 20 years, TU-43 extends it to 30 years. A lot of the local roofers like using Royal Westlake tile seal for their go to when it comes to peel and stick (rated at 260* degrees).
I totally get the curiosity about underlayments. When I bought my place, the inspector pointed out the original felt underlayment was barely hanging in there after 18 years. I ended up going with a peel-and-stick (not Royal Westlake, but similar) and, honestly, it’s held up way better through our crazy summer heat. I do wonder if two layers of TU-35 would’ve done the trick, but I didn’t want to risk it since I’ve had enough roof drama for one lifetime.
