Does anyone have any info/thoughts on the effectiveness of crawlspace vapor barriers over the soil in a vented crawlspace in the bay area, and how well it reduces indoor humidity?
IMO, I think there's a lot of snake oil here.
It's very important in humid climates.
It may be less important in dry climates, you'd want to consult a dry climate expert like Mike MacFarland or Rick Chitwood. Where you are has a lot of microclimates, so if it's not important for dry climates keep in mind it will probably be quite variable from one project to the next.
I imagine if there is a "wet season" where the soil gets saturated it'd be a good idea.
Yes, I agree that it is usually less of an issue in a dry climate. One of the problems I've seen in "dry" climates is if there is any vegetation, watering is required. Even though this can become more of an issue with slab on grade, the crawlspace can also be a water source into the building's interior. Needless to say, it does rain as well. Most downspouts in dry climates terminate at the foundation, keeping that area of the building nice and wet during rain events. When the rains (or watering) occur it is cheap protection covering the soil and preventing higher levels of moisture from entering the home.
Now you can remove the unnecessary venting, especially if ductwork is present.
For a good paper on crawlspaces see Craig DeWitt's Nov. 2003 ASHRAE Journal "Crawlspace Myths".
It depends where you are in the Bay Area. If on high ground with dry soil there isn't much effect. But in my old house in Alameda where water came up out of the ground when it rained and the soil surface was permanently wet then the effect was significant (no more condensation on windows, for example). So it isn't so much a climate effect, its a soil moisture issue.
Climate is definitely a factor, especially in the many varied climates of the SF area. Ground water needs to be controlled even with a complete vapor barrier. Radon mat systems work to remove excess moisture under the vapor barrier, but will not easily remove the ponding that may appear during heavy rains. This requires a sump as well.
Even in dryer crawlspaces I'd still recommend a vapor barrier, sealing and insulating. Though cost is always a factor to consider, especially the insulation in the moderate climates of the Bay Area. Those dry crawlspaces are typically wet sometime during the year. Even with venting the poly will help reduce the moisture load of the space during those "wet" times.
Looks like your answer is "always recommend" as you have no idea or control of what may happen in the future with weather or soil. This summer my lawn NEVER turned brown, and some people indicated their dehumidifiers were not keeping up and mold was forming for the first time ever.
Why is it so crushing to have people not follow every recommendation we make? I used to obsess on trying to recommend only what I thought people would buy, now I make LOTS of recommendations that people delay or disregard. If you recommend crawlspace vapor barrier and they decline, is this a big deal? NO! On the other hand, by recommending things you protect yourself should something happen later.
Recommend the vapor barrier. Should the lack of vapor barrier contribute to a problem that responsibility does NOT lie with you.