nMEEKS
Horticulturist
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Now to discuss some of the things I think about when I debate my drying temps and humidity. First things first, let me establish that it would be the most ideal to dry to buds at the lowest temperature possible, as close to 32 degrees or its chilling threshold, if it is sensitive, as possible. This is because by reducing the temperature of the room (and eventually the product) you will be drastically reducing the respiration rate of the buds. As I have talked about before, respiration without photosynthesis means you are depleting your sugar/carbohydrate reserves. Obviously now that you have cut the stems off of the root system, there will be no new sugar generated even though respiration continues. To maximize the amount of sugar in your final dried buds, you want to decrease the amount of sugar the buds burn through during the drying process by keeping temperature lower (lower temp = lower respiration rate by an average of 2-3x per 18 degree drop)
Now that I have established why you would ideally want to run close to freezing temperatures to maximize final sugar content of your dried buds, let me talk about why most people, including myself, run their temps as high as 65 degrees. Besides the obvious increase to cooling costs that would arise from maintain fridge-like temperatures, there is the issue of humidity control at lower temperatures to maintain a high enough VPD to dry out the product.
VPD, or vapor pressure deficit is something I have talked about once before, and is a very important concept to understand, especially for post harvest handling. It is a much better measure of the pulling force on the plant than humidity is. Relative Humidity is a measure of the amount of water in the air over the amount of water the air at that temp could hold. The water holding capacity of the air increases as temperature increases, so it takes more water in the air at a higher temperature to equal the same %RH. Here is a chart that displays calculated VPD values for different set temperatures and humidities (the green range is ideal for plant growth, and does not relate to my discussion of drying).
As you can see in the chart, the lower the temperature is, the lower to humidity must be to equal the same VPD. Without a high enough VPD, the plants will lose water too slowly and mold/mildew problems will arise in your drying room. As Big Cheese said, a common drying room setting is 65 degrees and 40-45% RH, which on the chart corresponds to a VPD of 1.15-1.30, and since we know this VPD works to dry buds fast enough to prevent disease problems, let's use it as a baseline. From the chart, it appears we could get the same VPD range if we set our drying room to 60-62 degrees and 35% RH, or at 70-72 degrees with 50% RH . . . but with temperatures lower than 60 degrees it is very difficult to get a high enough VPD to dry the product fast enough without achieving a 10-20% RH, which for most people is not possible.
If the concept of VPD being more important than humidity still isn't making sense, this is an analogy that made it easy to understand for me (numbers are for illustration purposes only, not actual VPDs):
Imagine two buckets. The first bucket is a 5 gallon home depot bucket and represents a room at 70 degrees. The second bucket is a 1 gallon painters bucket and represents a room at 40 degrees. The 'humidity' in both rooms is 50%RH and will be represented by filling both buckets to their half way marks. The VPD for each room is represented by the empty space left in the bucket.
At 50% RH, the half full 5 gallon bucket representing a 70 degree room still has 2.5 gallons of empty space (2.5gal representation of VPD). At the same 50% RH, the half full 1 gallon bucket representing a 40 degree room only has 0.5 gallons of empty space remaining (0.5gal representation of VPD). You can see that even with the same 50%RH, the VPD in the 70 degree room was 5x higher than at 40 degrees!
So to wrap up this answer, the best temps are the lower temps to reduce burning more sugar than you have to through respiration, but deciding what temp you want to dry at will depend on what VPD you are shooting for, and then how low you can maintain your humidity to equal your desired VPD at a set temperature.
I hope this helps you out, I like understanding the reasoning behind what I'm doing, but that doesn't make rule-of-thumb answers like the one BigC gave any less valuable!
-Meeks