It is nice to be missed! I apologize for not being available sooner. I receive many plant questions every day at
[email protected] that I give priority to answering.
Mary, I don't see anything in the photos of your Spathiphyllum 'Sensation' that gives me concern. Posted photos are not always accurate in color renderings, but overall your Peace Lily seems to be very healthy. The few discolorations that I see are blemishes caused by having less than perfect conditions at all times. Allowing the soil to become just a bit too dry or staying a bit too wet will cause some mild leaf edge browning and occasionally small leaf spots. Likewise, a bit too much light for part of the day and a bit less than ideal light at other times of day can create these blemishes. You can expect the gradual discoloration and die-back of some older lower leaves as new leaves are added up top. This is normal.
Misting does very little to improve humidity and Peace Lilies do just fine in very low humidity as long as they are watered properly.
Long term, there are some considerations.The location close to a north window is very good. You mentioned that it has well drained soil. I'm not sure how you determined that, but I hope it is not because you repotted it and added or replaced soil with your own potting mix. Perhaps you are referring to the outdated practice of adding "drainage material" to the bottom of the pot. Let me know if you did any repotting or soil replacement and just how you went about it because that can cause a serious problem down the road.
Watering when the top dries out can be very subjective. The presence of fungus gnats is very often a sign of A) Poor soil quality and B) Keeping the soil too wet. It is best to water just as or before the leave start to droop just a bit rather than relying on your sense of touch.
Laurie wisely inquired about fertilizing. Some leaf discoloration can be caused by nutrient deficiency. However, adding fertilizer may not resolve the problem if the soil pH is out of line. This would be the case only if you added your own soil mix or your local tap water is on the hard side. I have posted information on the Houseplants forum titled, "Fertilizing houseplants" that explains this in greater detail.
I hope I haven't overwhelmed you with information. But I know others read this, so I try to anticipate what questions they may have, as well.
