As a follow up: If I could find a product that was a combination 60 minute timer /single pole switch that could replace one double decora rocker switch that would solve everything but that's a lot to ask in that amount of space.

You can certainly wire a time in series with a switch as you describe. The only question I had was if this circuit is 20 amp, thus requiring 12 gauge wire.

To make this more user-friendly, why not change the existing double rocker switch back to a single switch, and put a switch/timer for the fan in a different location. To me it doesn't make sense to control a single fan from two locations.

I've never seen a timer that will fit in the same space as another switch as you're asking. Your other option is to relocate the timer (or the light switch) to another spot on the wall somewhere.

I don't know what your intent is but BROAN makes a fan that incorporates both humidity sensors and timer into the fan.

When the fan switch is on, if you flip it off for less than a second, it will run for 20 minutes and then cut off, until there's a humidity buildup an then it will run until that abates.

Currently from the double decora there is 12/2 running to the fan. I'll install a new timer switch on another wall in the bath and run 12/2 from the timer(switch loop) somewhere in the attic along the 12/2 from the decora where I'll use a j-box and make the appropriate ties namely:
The 12/2 FROM the decora once cut I'll tie the whites together...neutral is now intact to fan.
From the 12/2 to the timer(switch loop) I'll relabel the white as black and tie this "white now labeled black" to the black FROM the decora.
The black FROM the timer will be tied to the black TO the fan.

SO in essence the (bottom rocker switch on the double decora) is used as the line to the new timer.

ron: the current fan is only a few months old so I do not want to go thru the expense of a new one but thatnks for that option on the humidity sensing fan. Did not know one was made like that.

I'd actually run a 12/3 with the description you have. Unless you want one of those annoying ticking timers forever, run HOT and NEUTRAL down to the timer and then the switched feed back.

Frankly, I might consider just wire-nutting the connection that went to the original switch together and replace the double switch with a single. Why have two switches in series?

ron: I was considering one of the mechanical timers by Intermatic but may take your advice and get a digital one requiring the neutral.

Also good suggestion on just wire-nutting the the connection at the current double rocker decora. I knew having two switches in series was not typical but could not think of an easy solution without grabbing power from somewhere else.

Thanks

I've got a couple of those multibutton intermatics that give you 15-30-45-60 minutes of time. One's on the receptacle for my wife's straightening iron which she's afraid she'll forget to turn off. The other is on the lights for my wine cellar (room is super insulated so leaving the lights on can really get the cooling unit going hard).

I saw that multibutton one at the store. Does it require the neutral?

Yes it does.

Lutron has a timer/dimmer.

Here is a link that might be useful: timer/dimmer