What kind of odor? (Most typical are sewer-gas, musty/moldy, and old urine sort of smells) Is it localized or all over? What kind of flooring?

Have you cleaned the sink overflow passages?
Sometimes some clorox and water does the trick, other times you actually need to use a bottle brush.

1) What kind of odor?
I'd call it sewer gas, but one plumber said it was not like raw sewage but more like "grey water funk." It doesn't smell like mildew or urine, but I suppose there might be some kind of mold that smells that way. However, I don't see any discoloration in the space beneath the corner tub where the odor is the worst.

2)Is it localized or all over?
Usually I can smell it as soon as I open the door into the bathroom. I closed the water closet door, and that made no difference. It's worse near the corner garden tub, and much worse when the access panel along the side of the tub is removed.

3) What kind of flooring?
Ceramic tile over Ditra over concrete slab

4) Have you cleaned the sink overflow passages?
I poured bleach into the sink overflow openings and tried to pour some behind the chrome cover for the bathtub overflow. I have not tried to use a bottle brush in the sink openings.

FWIW, the skies cleared today and the odor is not as bad. Maybe that's just coincidence, maybe not. My husband used the shower this afternoon, and I ran the exhaust fan for about an hour afterwards with the bedroom windows open since it was around 70 today.

Thanks for your help!

are they sure vent stacks are ok, sounds like might be clogged above, not letting venting out.

another thing I ran itno was tolet wax ring had minor leak, not enough to show water but enough to give off smell

Hi, I was reading one of these forums, the guy had a mold smell in his basement and isolated it by taping plastic sheeting over different sections. Maybe that would work for you tape the tub and panel up with plastic and if the smell goes away at least you know what you have to work on.
Good Luck Woodbutcher

All 3 plumbers said they checked the vent stacks. One just looked w/ a flashlight; the others sealed the stacks on the roof w/ duct tape and did a peppermint oil test, as well as did a visual inspection from both the attic and the roof. I suppose they could have missed something, but I don't know how else they could check.

Because there's no odor in the enclosed water closet, the plumbers don't think that the wax ring on the toilet is the culprit.

When asked, one plumber said it wouldn't hurt to run a bead of silicone caulk around the joints in the pipe beneath the garden tub. I'm going to ask dh to try that, but it probably will be next weekend before he can do it.

Taping plastic sheeting to try to isolate the problem sounds like a good idea. Would it be okay to tape over the tub's overflow cover & drain as well as around the access panel?

Finally, when the plumbers removed the access panel, I could see the slab foundation had an opening about a foot square where the tub drain pipe went down to join the main drain. It looked like loose grey insulation filled the hole (along with some construction debris.) I don't know if there's any sort of plastic barrier, but I sure hope so. This area has major radon problems (& we do have a radon vent inside a wall in the center of the house.) While radon is odorless, now I'm wondering about other gases seeping up through that opening.

Its a long shot but if you are under the International Residential Code you could have an auxiliary vent for that bathroom terminated in the attic space directly above and the sewer gases could be settling down through the stud wall or through ceiling openings.

If you have a vent terminated in the attic space have it extended through the roof.

I also would use plastic and "tent" over things to isolate the smell.

But it does sound like a leak under that tub. I would fill the tub, run it, etc. and get in there with a flashlight, hand mirror, etc. and even listen for drops of water after you shut everything off. Also, if you can reach your hand far enough down, feel the bottom of the p-trap, see if it's damp at all (or if that insulation is damp). It could be that some water is getting past the drain connection between the tub and the drain.

Also, do you have separate tub and shower? I would stop using the tub for several days, keep access panel open to "air out", etc. and then see if it comes back when you use it again.

".....not like raw sewage but more like "grey water funk." It doesn't smell like mildew or urine,..."

"It's worse near the corner garden tub, and much worse when the access panel along the side of the tub is removed."

Excellent! You've got it identified and isolated...something the plumbers probably couldn't do during a short visit. Time to bring one of them back.

Simply isolating odor sources can be a real pain because of the way they perfuse (is that the right word?) throughout a given area. Since you've got it isolated and identified as non-sewage, I think you're on your way. "Grey water funk" is a really great way to describe it. I'd look for persistent saturation in that open area under the tub. Can you use plastic sheet and tape or something like that to seal that area temporarily and see if it makes a difference? Can you scoop up some material from that area and smell it independently?