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Humidity advice

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first grow in a grow tent, that is in an uninsulated shed, in the hills north of Melbourne. Hence very cold and wet atm.

I have a heater connected to an Inkbird, so temps are good. My extraction fan is set for 15 minutes every hour, to reduce constant heating costs, and a humidifier set to 70%RH (germination stage).

Now, the problem is my humidity levels are up and down like a brides nightdress, from 90% to 50% when the fan comes on. It is averaging about 70%, will those hourly fluctuations stuff up my grow?

Thanks in advance.

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hi pom :)


there's a bit going on here mate and a couple of different ways to tackle the situation.


out of interest what type of light are you using and what is the volume of your tent ... also, could you give a bit more info about the shed - how sealed is it (or can it be) and also the volume. Could you also say whether you are extracting from the tent to the shed or straight outside?



from the info ... the first thing for me would be to look at ways of controlling high relative humidity levels due to the cold and wet temps - working on this should indirectly help a fair bit with the humidity fluctuations too.


it's worth noting that there is a fair bit of difference between seeds and clones. seeds can germinate and grow quite well from 45% up to around 75% rh and can tolerate fluctuations a bit - clones are different and do better with constant, high humidity levels. one of the things that helped me a great deal with temp/rh was a good vapour pressure deficit chart. seeds/seedlings only need between 0.4-0.8 please see the attached vpd chart, which should help you out regarding good humidity/temp ranges for all stages of your plant's growth. you'll notice that if it's 15 degrees that a relative humidity range from 65% to 40% is within good parameters. you might actually find that your humidifier won't be needed. :)


also, during the germination (and even seedling stage) you don't need much air exchange - i would reduce the extraction fan time to 1 min every 15-20 minutes or if the timer doesn't allow for it, 3-5 minutes every hour. this should help with retaining heat, which will directly help stabilise your humidity levels at a higher suitable rh% (or to be more accurate ... find a nice vpd range for growth). you can play with this, so that you find a nice sweet spot (vpd level) between heat and humidity levels. eg at 20 degrees a rh of 70-55% would be good for propagation/early vegetative growth.


depending on your light and its heat output (hid or led), you could also reduce (or change) your heater's use, by using a different timing schedule ... perhaps 20hrs on and 4 hrs off. another way to control humidity through heat is to set the lights-off time during the hottest part of the day - this will depend on how light-sealed your tent is and also when you want to visit your plants. that way during the cold nights, your light could provide the necessary heat. around your area, you'd probably find that around 4 - 8am has the lowest temps and most likely the highest rh. if so, it would be nice to make sure that the lights are on during this period and if possible, a couple hours either side would be beneficial too. for example, if you have a cmh (or other hid) lamp in a light-sealed tent and you enjoy visiting your plants in the arvo, I would set my lights to go on at 2pm and off at 10am - lights-off would be off from 10:01 am until 1:59pm. that would cover the coldest and most humid times and still allow you to visit your plants from 2pm.


this all changes of course when you you go into flower and use a 12/12 lighting schedule. depending on how difficult it is to seal your shed, how big it is and how long it retains heat ... a more long-term solution would be to extract from your tent directly to a sealed shed - not outside. it takes a little playing, but your inkbird should provide the necessary data to work out the temps inside both the shed and the tent. for the last few years, i have grown in tents in sealed air-con rooms and have had no worries with oxygen/carbon dioxide levels. if the shed is sealed and retains a bit of heat, then the heater could be used to adjust for fluctuations in temperature, similar to how I use an aircon - this would go a fair way to get rid of most of the heat and humidity issues.


hopefully it helps and isn't too far off track. :)


Edited by pug1010
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Thank you for the in-depth reply, 3footx3foot tent, 600w Viparspectra dimmerble leds, and the extractor is connected to a fan speed controller on a low setting.

The shed it is in is about 4 metres x 8 metres, uninsulated and can see the sky in big gaps around the roof line (not my shed). I am using passive intake for air exchange with the extractor. 3 Northern lights Autos, in 5 gallon fabric pots and organic soil/perlite.

I looked for a timer with settings lower than 15 minutes, but couldn't find any.

I also insulated the tent with space blankets, just as well as it was -3.5 one morning the other week.

Thank you again, for your time and advice.

Edited by pom
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hey pom ... no worries mate and thanks for the details.:)


wow ... -3.5 ... fuck me. that's a challenging environment, especially with the winter months.


okay ... had a quick look and it seems that your light uses 260w of power and being an Led, most of the energy will go into producing light with heat as a byproduct - this will come in very handy in summer, but will not produce enough heat to keep the plants warm over winter in the current shed - so please ditch my suggestions in the last post. those big gaps in the shed are great for checking out the moon and stars, but not quite so good for retaining heat. it will require a lot of heating energy to keep your soil (and air temp) warm enough to grow your plants, in your current setup.


mate is there any chance that you can move your tent into your house ... at least for this grow? i haven't grown autos, but one bigggg advantage is that you could keep your lights running on a 24 hr cycle or even 18/6 for the whole grow ... on a 24 hr cycle, light leaks (if not worried about stealth) aren't an issue and would make growing indoors in a spare corner, way easier. the heat from your light would also work well to keep the tent warm.


i think you will need expert advice from growers who have grown in a similar environment (tent in a shed in a cold environment), as with your current setup, it is beyond my skill level. :) if you can move your tent indoors though ... then i reckon you will find it far last challenging and a lot more enjoyable.


sorry i can't be of more help mate.


regarding a good timer though ... i've been using these timers for a few years now and have had no issues and they go down to 1min intervals and would be able to handle your electrical load ...



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