I remember the first membrane my partner installed, I had left him alone, to do the work, on the last day, I got a panic call from, him, saying the membrane is rising off the floor- what do I do- I laughed and said: Perfect, means your seal is perfect- plug in the fan.
By the time, I got off the phone, the membrane was tight to the ground.
Maybe not a direct answer to your question, but it’s all about air pressure and which one, is stronger.
We never leave poly exposed, we only use Fire/ smoke rated products.

I have not had direct experience with that happening to a passive radon barrier but I do have lots of experience with sealed poly and pressure difference in buildings and air tight buildings. This could totally happen. I see two ways to stop it. 1) ballast on top of the poly, which has pros and cons to consider, or 2) activate the system like Rob implied. My house is so tight (entirely insulated with spray foam) that the water in the toilet goes up and down with the wind and even with slamming of the door to outside. This is a delta P of up to an inch of w.c. it would take a tiny fraction of that to flap poly.

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.

I have seen photos of ballooning barriers from areas with karst geology. Not a problem where I am.

Thanks, your experience matches others, as well as my own. The problem can occur where a passive RRNC system is to be installed. Appendix F does not require the poly to be sealed to walls. If a conscientious builder decides to go a little further and seal the poly but still stay with passive system one could have ballooning issues. If on the other hand if poly was not sealed and merely brought to edges as stated in App F then if it had to be activated it would be difficult to seal to walls without the use of extension strips, replacing poly or not bothering to seal at all.

I agree with all comments that adding the fan from the beginning solves a lot of problems, but it is almost as if one would recommend NOT sealing poly if a passive sub-membrane is to be used.

Doug

Installed a membrane once that promptly inflated like a blow up pillow when the wind came up. Once the fan was on it was fine, but the fan must move more CFM than is entering under the membrane.

I remember the first membrane my partner installed, called me in a panic as it was “blowing up” no actually it’s being sucked up, because you have a perfect seal.
Yes membrane systems typically move more air, but simply measure the variations in house pressure, with fan on / off - no pressure change =no leaks!

if you install a poly but don't seal it, is it there for decoration? it is not a radon barrier, it is no a moisture barrier and it is not a air barrier unless it is sealed!

Some builders in our area install unsealed plastic in the crawlspace and then cover it with gravel. Often there is draintile under the plastic leading to a sump pit. We have had great success in reducing the radon by connecting our system to the sump pit or drain tile. The only sealing we do is around the sump.
The gravel on top pushes the plastic against the foundation and creates an lmperfect but adequate seal.