According to the Goldenrod site, they are supposed to be used in a container with ventilation holes top and bottom. Or a loose safe door, I suppose. This allows the expanding warmer moist air to escape.
After seeing this thread yesterday morning I did a little measuring with a digital gauge from Radio Shack. It even has a memory feature and pushbutton light and was only $20.
Outside - 66*F and 87% humidity when I started measuring. Then we had another downpour. It's rained here 9 Saturdays in a row and a lot of other days, too.
Basement - 68*F and 72% humidity.
With most of the windows in the house open all night the cool damp air ends up in the basement(The kitchen door to the basement stays open so the cat can roam.) The basement is pretty much the way it was built 87 years ago - bare concrete floor and bare solid brick walls. The only warmth during the summer is from the pilot light on the gas boiler and the hot water tank, but it's wrapped in insulation. All but a foot and a half of the basement is below grade. Think cave.
Big Liberty safe - 72*F and 58% humidity.
2 Goldenrods - 24" & 18". The door is not sealed. Try the refrigerator door gasket test - close your safe door on a dollar bill and see if you can pull it out or if it's easy to slide it around.
Having used the memory feature from time to time, I know that the humidity in the safe ranges from 50% to 60% and is always lower than the basement reading. The temp reading in the safe is always 3 or 4 degrees higher.
I initially worried about the 60% reading, but then realized that the small safe on the second floor has been exposed to our usual summers - 90+ degree days and high humidity - and a small Goldenrod has worked there for years. I have AC, I just like big windows, 10-foot ceilings and fans.
Time to go to the range
John...current outside readings - 65*F and sunny with 73% humidity.