[myself248]

I had a side-job installing active, subslab depressurization systems in the 90s, and they were quite well established at the time. The EPA has quite clear and cogent guidance on radon, paraphrased as follows:

Before going with active depressurization, start by installing a sealed sump cover, caulking the basement wall-to-floor joint, and caulking all the cracks in the basement walls, in that order. If that doesn't get the number down where you want it, drill the subslab access hole and install the ventilation piping to the outdoors, but don't install the fan in the middle. Only of those fail to get adequate results, install the fan.

It's really simple and quite cheap. I don't think we ever did a job that was over $1000. Costs of running the fans were pennies a month, and Fantech still sells the classic FR-100 fan all these years later, though there are even quieter options now.

[hinkley]

So make the interior of the house the path of most resistance, and see if it will passively vent itself, and if not bore a horizontal tunnel halfway under the house and install a fan to do the job?

[myself248]

No! There is no horizontal boring, at least not on any system I ever touched or heard of. I don't know where that idea came from.

There's typically enough gravel under the slab and around the foundation that there's plenty of soil-gas transport without doing anything more. So the install is just a simple vertical hole through the corner of the slab somewhere. Maybe you scoop out a few handfuls of soil before sticking the pipe in the hole, but there's no horizontal boring. Here's a very typical one:

https://www.flickr.com/photos/34878756@N04/9350845974

Here's another, including a side tap to a crawlspace:

https://science.gc.ca/site/science/en/blogs/science-health/e...

Not shown is the crawlspace portion of that system, which would use a horizontal perforated pipe _laying on the surface of the crawlspace floor_ and covered with a plastic membrane that's taped to the walls:

https://www.nachi.org/gallery/radon/crawlspace-radon-system-...

At no time is pipe tunneled horizontally under the house.