Lower temps/higher humidity or higher temps/lower humidity

Hello Jez1973 You want to balance temperature and humidity for optimal growth. This link has some info on VPD and shows a chart to follow as a guideline. https://www.dimluxlighting.com/knowledge/blog/vapor-pressure-deficit-the-ultimate-guide-to-vpd/ Good luck and Happy Growing

This is what you can do about it: 1. Get higher humidity 2. Make your delator(carbon filter) run faster 3. Inflate fresh air 4. Inflate Air conditioning 5. Inflate Co2 with like co2 pots or tanks, your plant can now stand more head, but 39c is still to high 6. Make sure your lamps are not to close to the plants, because of the temperatures, you can get leaf burns faster Try to play with the options above, the more options you can do, the better temperature you will get :) Goodluck :)

Higher Temps - Higher Humidity Lower Temps - Lower Humidity VPD - Increase Temp = Increased VPD Decrease Temp = Decrease VPD Increase Humidity = Decrease VPD Decrease Humidity = Increased VPD High Temperatures needs higher RH% EGXAMPLE - When it's too hot the VPD will be to high [thus] lower VPD by adding moisture. Flowering should have slightly higher VPD VPD requirements for Cannabis: Clones / Seedlings 0.6 – 0.9 kPa Vegetative Stage 0.9 – 1.2 kPa Flower 1.2 – 1.6 kPa

I'd go for the higher temps and lower humidity... you have to worry about bud rot with high humidity and that's a killer.... You didn't get to finish the question but 40% humidity is great for flowering... You're going to have issues when it comes to drying, too, you know... Why are you choosing to grow indoors? Whatever the reason, good luck!

I have the same sort of temps in summer where I grow and regularly get up around 30-32 in the tent with no real negative impact. I would keep it going along at 40% humidity coz bud rot would be worse case scenario