RH and temperature in basement...


Hi there folks,

my future grow site is in a basement.

I measured a base RH of 60% but we recently had fog, it's winter and I measured a max 75% RH during that time.

Now for clones, mums and veg this should be fine from what I understand.

But for flowering this is definitely too humid.
Will I be able to keep RH levels at acceptable levels for flowering just through the heat from the 600W HPS and air movement (filter/scrubber)?
Or do you guys think a dehumidifier is needed for the flowering tent?

I am also planning to have my "dry tent" with its own scrubber and filter in the same room.
60-75% RH is definitely too high for drying and from what I could gather it is at the level where I start risking mold if I don't combat it.
Again the question: Scrubber/fan enough to get RH down to acceptable level?
Any way to know that in advance or do I just have to test?

If I need a dehumidifier for both the flowering tent and the dry tent, I would just buy a bigger one and lower the RH of the entire room in the basement.
Then I use the pre-warmed exhaust air from the flowering tent as the intake air for the veg tent and put a plate/bowl of water in there to keep RH for clones, mums and veg a little higher.

Do you guys think I should install a heater in the room? Right now in winter, temperatures are around 10 °C. They could drop even lower and also be at that level anytime from October-April...
The tents will also stand on a tiled floor that will radiate cold from the floor in these months (in the summer all that cold stuff of course will be good for the plants... basement ;) ).

The way I see it, a heater for the room the tents are in would help provide a basic temperature in the winter so it doesn't get too cold (which would slow down veg considerably since there will only be fluorescents in there, flower probably can handle the low temps with the HPS running and heating the place up).
It would also be likely to drop overall RH in the room considerably.

What do you guys think?


New member

Listen the best piece of advise i can give you is dont listen to every thing you here. I grown in basements my whole life and only one or two times did i ever have a problem with humidity and if i did i just got a dehumidifier keep it simple and you will have good success. If you listen to everyones problems you will be unsure of everything. it's not that dificult of a thing. keep your air moving keep your nut's ph around 5.7 and you will do fine


New member
I think a dehumidifier is a must have and I had to just get a humidifier also.
Both are problem solvers and I got mold once and that was enough so I got a decent dehumidifier.
I use fans also and keep the leaves wiggling.
During certain times of the year I don't need it but sometimes it pulls 25 gallons a day.
When I started using a tent the RH seemed to higher than an open room.
I think 40-50 RH is the acceptable ranges for bloom.
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New member
High humidity is a good thing if you can get the temps up. VPD or vapor pressure deficit is one of the most overlooked aspects of environmental measurements & controls. Keep your temps & humidities in the sweet spots & watch the plants grow like bonkers.

Article is required reading if you've never heard of VPD.
Vapor Pressure Deficit - The Hidden Force on your Plants | Just For Growers

Some of my best yields & crops have been in a very humid basement with a kerosene heater adding heat & co2. No budrot or powdery, just huge buds (from 400w lighting :D )


Hi Patrick,

once again thanks for the quality feedback, you are really helping me out here at several construction sites.
Appreciate and won't forget!

It makes me happy to see just 4 growers cited here. I stumbled upon their youtube videos and really dug their style/liked how fast and compacted and to the point they were. Really liked that source.

Hadn't seen their article on VPD (I didn't read through everything on their homepage yet). Started reading it and am still cementing what I read (re-reading/follow up reading on certain aspects of the article etc.).

Good help Bro, thanks!