I fumed lumber all the time when I lived in Maine. My shop would drop down to about 40 - 45 at night and I had no problems with the fuming. You can give it extra time, if you think you want to get it as dark as possible. Usually, in about 24-30 hours the wood will darken the most. After that is a waste of time. I do all of my fuming in the shop. I doesn't require a great amount of sealing to keep the ammonia in the chamber.