Heating racks

So I'm going to plastic and have a rack w/ flexwatt, and sterilite tubs( I think). I am using carefresh as a substrate and there is about an inch of it in the tubs the room stays about 74 degrees or less if it matters. With this much substrate and being heated directly below the tub, how much heat is lost? If i want a 85 degrees should I go about 89 or what?

I'd say it all depends on your particular setup and other factors like that. You'd have to try it out and test it and see what's right for you. For instance, my flexwatt has to be around 110 degrees during the winter to heat my hot spot to 92 degrees. For others, it'll be different, depending on how close the heat tape is to the tub, how cool the room is, how much substrate, how thick the plastic tub is, etc, etc.

Once you find out how much heat needs to be coming off of your flexwatt, write it down for future reference. If it takes 110-degrees from November through February to reach a comfortable temperature inside the enclosure, make a note of it so that you're not wasting time in the future to find the same "sweet spot." Keep this with your feed-charts and shed-charts. I keep a clipboard with my feeding schedule, and a large notebook/binder with all other information on my herps.

When your room temp goes anywhere below 75, you'll probably have to run the flexwatt at full power to get up to 92. Rather than fry your heat tape, use a small space heater in that area of the room.