Doug,
I remember a story told during a continuing ed course back in the 90's. A mitigator returned to a crawlspace he had sealed the previous day to find the plastic had inflated to the point he couldn't get into the crawlspace. This may have been an exaggeration and, if true, the plastic would have to be pretty light weight but understandable when you consider the forces involved.

wpb-radon.com/pdf/Membrane%20Adhesive.pdf — Randy Weestrand

wpb-radon.com/pdf/Membrane%20Adhesive.pdf — Randy Weestrand

wpb-radon.com/pdf/Membrane%20Adhesive.pdf — Randy Weestrand

I have been using urethane for permanent crawlspace membrane sealing and silicone for sump lids since my first mitigation job in 1989. I have visited membranes sealed down over 20 years ago that are still sealed as tight as the day they were installed. I did have a job where a homeowner purchased the membrane from me but then used urethane construction adhesive instead of urethane caulk. Within days the urethane adhesive became ridged and no longer adhered to the membrane. He resealed it with polyurethane caulk and had success.

Perhaps that could explain Bob Wood's experience. Does anyone have information on the difference between polyurethane construction adhesive and polyurethane caulk (that is meant for concrete crack sealing)?

We had a similar situation. We used a crawlspace sump pit from RCI, which is smaller than a standard sump pit. We had enlarge the hole somewhat to accommodate it. We replaced the pedestal pump with a submersable and installed a Dranger drain in the lid so water could drain down through the lid.
We also used the refurbished pit for our suction point.

I reversed the fans on a difficult home with positive results on lowering the radon level but changed them back because of the fear of freezing plumbing under the floor during the winter.
It is rare but I have added suction to the block wall on more than one house when the subslab system didn't reduce the level below 4. It is important that the top row of blocks has been poured full of concrete or sealed in some other way.

I used depressurisation on a very difficult house in South Dakota. It did reduce the radon level but because of our cold winters it could not be used as a permanent solution.

Can't open the attachment. What is the date?

Dick has good advice. We have set up a radon fan for the exhaust with a thermostat that turns it off if the temperature approaches freezing. It may be difficult to find a thermostat that goes below 40 degrees. There are manufacturers of crawl space ventilation fans that come equipped with temperature (and humidity) controls. You may be able to override the humidity control as you want the fan to keep running nomatter how dry it is.
If I was confident that a lot of outside air was not going to enter the crawl space you would not need the thermostat.

I looked at the referenced "Health Physics Society" and couldn't find the statement that radon was not a risk until it reached "27 billion picocuries". Late in the article they seem to hint that 8 pCi/l should be the number. Any idea where 27 billion comes from?