Thanks to one an all for your replies. They were all helpful in elevating my understanding of thermal mass, what it can do and what it cannot.
The following is a reprint of my response to another hoophouse forum here on permies.
I have completed adding a second layer of plastic on my cattle panel hoop house. Added a total of 190 gallons of water (milk jugs and 3 55 gal drums.) all painted black. Finished it off with pallet counters covered in terra-cotta tiles. (See pictures)
At the end of a miserable cold and rainy week here in the Pacific NW with an average daily outdoor temp of 45 degrees F., the average daily interior temp was 51.8!
Even better, the interior temp At sunrise was always at least two or three degrees higher than outside. This is all I was really after so I consider this experiment to be a success.
Not sure how much of this improvement was due to the extra plastic (insulation) and how much due to the added water (thermal mass), because I added both at the same time, but I think I can state pretty conclusively that double walled hoop houses with significant amounts of thermal mass can create a better growing environment in winter in the Maritime Northwest, without the use of fossil fuels, than occurs outdoors.
Now I just have to tackle that pesky "lack of sunlight in winter" problem! 😄
PS I will be posting full results on this hoophouse along with my experiments making biochar at my blog: tinkersblessing.com