They probably didn't spray foam around the window when they installed it. I would remove the trim and check it out. You will not get condensation on a surface unless it is colder than the air surrounding it. " Is our house humidifier set too high?" What is the humidity in the house? Should be between 30% and 60%.

Thanks, Larry. Our humidity is on the high side. I'll have to check out behind the trim in the spring. In the meantime, what can be done about the mushroom? I've read that bleach is not the most effective. Is there something available at a home center that will kill the fungus?

"Is there something available at a home center that will kill the fungus?" They should have something.

If you are using some industrial strength cleaners or bleach to kill the mushrooms, please remember to open the doors in the home to let some air in. some fumes can be quite hazardous to human and pet health.

Anything like bleach will kill it. Try to avoid opening it up in the future.

I think Vinegar and tea tree oil also can be used to get rid from any kind of fungus or mushrooms. It really works for this problem.

Herculite glass doors

Humidity will cause condensation and mold growth, as will having window coverings such as blinds, shades or curtains closed the majority of the time.

Below are recommended humidity levels, and may not be applicable for every household. Differences in glass types (LoE vs. clear) will allow for variances in humidity levels. Window condensation is a good indicator as to the maximum allowable humidity level. If your windows begin to sweat, the humidity in your home is too high.

Outdoor Temperature Indoor Humidity at 70° F
-20 degrees F or below not over 15%
-20 degrees F to -10 degrees F not over 20%
-10 degrees F to 0 degrees F not over 25%
0 degrees F to +10 degrees F not over 30%
+10 degrees F to +35 degrees F not over 35%

I just found a beige, round umbrella mushroom on my attic ceiling window frame...looks delicious, but I know that mushrooms can be extremely poisonnous...too bad!