VPD

0.85 is a great VPD for veg 1.30 is great for Bloom VPD is the humidity pressure against the leaf surface vs. the moisture pressure within the leaf itself. The difference between the 2 pressures gives you the deficit. Which controls the rate of transpiration. Low VPD numbers translated into simple to understand terms would be, well, to achieve a low VPD ideal for veg you would need a relative humidity of around 65% to 70%, and a temperature of around 72F to 75F. That will = around 1.00 to 0.90ish VPD. High VPD example for Bloom would be around 1.30 VPD. That will look like around 50% Humidity, and about 76F to 79F Achieving VPD is hard, and hard to maintain. You really have to play around with your temps and humidity to get a balance achieved. If your temps are set to 78F and your humidity is set to 50%, and your light is heating the space to close to the set point of 78F, and the exhaust fan is tripping on every minute, you'll never get your VPD dialed in because the humidity will constantly be out of range due to the exhaust frequency due to the temp control. A temp of 80F and a Humidity of 30% will give you a sky high VPD of over 2.00. Thats not good. Your plants will be sucking up water like mad, as well as nutrients. So much so that you can actually cause your plants to experience nutrient burn because they are taking in more nutrients than they can process. But once you achieve a balance that you want with VPD, man do your plants show their appreciation!

VPD is Vapor pressure deficit...it's how much vapor is left in your grow room basically....It's also how your plants draw moisture from there soil and transpire it into the air through stomata. It's not something you can have explained to you in one sitting unfortunately but here's a good rule of thumb. For seedlings to mid veg, keep your temps and humidity about the same. (Mid to upper 70's with an rH of the same) In mid to late veg, upper 70's with an rH of 65% Transition to early Flower, mid 70's with an rH of 55-60% than gradually drop your rH as you get closer to harvest but never let it get below 40% You can find some VPD Calculators online with charts that will help you out immensely after you get the hang of the conversions. Good luck brother...once you master VPD, you're a beast

like others have said, to cold at night and to high of RH, you want to keep your extractor fan on to keep rh lower. ideal is aorund 50-55% in flower 60-70 in veg. If you can't fix the heat issue switch your on/off cycle of lights, So they are on at night heating the tent preventing it going down to 13c, and off during the day when its normally warmer so they hover aorund 18-21. Water as needed and by how much the plant is drinking not a set amount as some plants will not drink as much as others, Be aware of feed concentration so your not underfeeding or overfeeding. Good Luck!

13.5 is troubles, anything below 17 is troubles. Now vpd, some charts are smart and has couple options for veg and for flower. Its relation between temps and humidity. Vpd chart shows optimal at certain points. Looking at vpd, half of my grows should be bust in flower, i sometimes finish with 70 rh. So for me personally its good tool to understand, but if i cant get to best numbers, i dont sweat it and do my best by conditions i have.