Passive cooling

Hello everyone,
I'm new here and I didn't find a thread that addressed this so pardon me if it's already been discussed. This is partially a cob house "problem" but it's more of a drafting question so I figured RMH'ers would be a better place to ask. My concern comes from the guy in MO who is having mold issues and venting is one of the "possibles" of his problem. It gets very hot and humid in summers here and fairly cold in winter, we just had a spell of teens and zero nights, summers into the 103's for weeks. I saw a guy in CO uses 4" PVC pipes buried about footer depth (2ft?) in order to cool hot outside air as it passes down the pipe and into his house at around floor level then rises through ceiling vents and exits out roof vents, basically a passive draft system. My questions are:
1. Similar to an RMH would different size intake pipes/vents or number of them entering the house, and I would assume the same or equal size/number as exhaust vents be needed? And assuming a 900sq. ft. house with 8ft. ceilings, how to figure out the best turnover ratio of new air, ie. with an 8" pipe it would take how long to vent all old air to new?
2. How long of a pipe say 4" would it take to actually cool to soil temperature? Would 103 degree incoming air cool down after traveling 40' in a 4" pipe? What determines how fast the air will flow down the pipes?

Thanks in advance, Gary