Controlling Humidity in the greenhouse

[UnKn0wN]

Hey all hope you are doing well, on my first grow and about 6 weeks in now and getting way to high humidity readings in the greenhouse over night. 

 

Just wondered if any of you had any cheap ways to control humidity? Obviously there is the option of ventilation, heating and a dehumidifier but some of these options can be pretty costly. Have any of you ever used products such a moisture absorbers? Will an everyday bedroom humidifier work well in a greenhouse?

 

 

[thatman]

Just now, UnKn0wN said:

Hey all hope you are doing well, on my first grow and about 6 weeks in now and getting way to high humidity readings in the greenhouse over night. 

 

Just wondered if any of you had any cheap ways to control humidity? Obviously there is the option of ventilation, heating and a dehumidifier but some of these options can be pretty costly. Have any of you ever used products such a moisture absorbers? Will an everyday bedroom humidifier work well in a greenhouse?

 

 

fresh air flow, open some vents

[UnKn0wN]

22 minutes ago, thatman said:

fresh air flow, open some vents

 

Have opened the window and slid the door across a bit to allow air to flow through but humidity readings still going too high. Also ensuring i water the other plants in the greenhouse in the morning to reduce the amount of moisture in the air over night

[thatman]

6 minutes ago, UnKn0wN said:

 

Have opened the window and slid the door across a bit to allow air to flow through but humidity readings still going too high. Also ensuring i water the other plants in the greenhouse in the morning to reduce the amount of moisture in the air over night

 

The air flow is still not enough, you need a circulating fan.

[Geo]

Forced ventilation is the best option if you have power in the greenhouse.

[Stuart96]

1 hour ago, UnKn0wN said:

Hey all hope you are doing well, on my first grow and about 6 weeks in now and getting way to high humidity readings in the greenhouse over night. 

 

Just wondered if any of you had any cheap ways to control humidity? Obviously there is the option of ventilation, heating and a dehumidifier but some of these options can be pretty costly. Have any of you ever used products such a moisture absorbers? Will an everyday bedroom humidifier work well in a greenhouse?

 

 

 

The sort of dehumidifier you would need in order to make any difference in a green house would be inordinately expensive to run. I have one that I sometimes use in the house and it reduces humidity just fine but when the timer shuts it down humidity quickly climbs back. 

[UnKn0wN]

1 hour ago, Stuart96 said:

 

The sort of dehumidifier you would need in order to make any difference in a green house would be inordinately expensive to run. I have one that I sometimes use in the house and it reduces humidity just fine but when the timer shuts it down humidity quickly climbs back. 

 

Ah yeah that was my initial thoughts. Guess my best best is improving air flow, will get a fan and run an extension lead out to the greenhouse and see how that goes. Did have a look at some solar fans but all have pretty shitty reviews. 

[UnKn0wN]

2 hours ago, Geo said:

Forced ventilation is the best option if you have power in the greenhouse.

Okay sweet, thanks for the input mate

[Guest]

Get a nice big box fan and a filter. Filter might die from the humidity tho 

[Military Grade]

5 hours ago, UnKn0wN said:

Have any of you ever used products such a moisture absorbers?

Yeah i've used a couple, They ain't really that effective tbh, I mean you could go to home bargins store or somewhere similar and by a crap load of them for a £1 or less each and see if strength by numbers help .

[Socksnsandals]

My greenhouse is on a frame on the soil, with plants in the soil. Today the humidity is the lowest I have ever seen it at 31% with three sets of louvres and three roof vents automatically open (25 degrees inside, 17 outside) and strong winds gusting.  In my kitchen, where the weather base station is, it is 21 degrees, 41% humidity.  Recently it has been 40-50% in the greenhouse when warm during the day and rapidly climbs to much more at night when the temperature drops and the ventilators shut.  In the autumn / winter it was in the high 80 - 90%s.  I am pleased that the risk of RSM is low but can see that mould could be an issue when the plants are in late flower so am doing autos rather than risk a photoperiod plant growing into a triffid in the good conditions during the summer and then getting hit with rot in the autumn.  Unless yours is on a solid impermeable base I would imagine that you will regularly hit humidity in the 70s - 80s in the evening and night. 

 

From looking online, it seems that because the water-carrying capacity of air rises with temperature you will always get a big surge in relative humidity (the % of the max possible humidity at that temperature) as the temperature drops, https://en.wikipedia.org/wiki/Relative_humidity#/media/File:Relative_Humidity.png so a greenhouse at 30c can hold a max of ~28g of water / kg of air, and is 50% relative huimidity when there is about 13gm/kg of water in the air but at 15c the max drops to 10gm/kg, so, unless it condenses out, the same amount of water will push the relative humidity to over 100% at 15c, hence, presumably the condensation / dew at nighttime especially in the cold microclimate immediately adjacent to colder objects which have radiated their heat away rapidly (like cars).  If we assume for simplicity that the greenhouse is an odd shape of 2x2x2m (8 cubic metres) and, ignoring effects of humidity and pressure on the density, the density of air is about 1.3kg/m3 so our theoretical greenhouse contains about 10kg of air, able to hold 130g of water at a relative humidity of 50% at 30c but, if you want to maintain a relative humidity of 50% when the temperature drops to 15c, the water content in that 10kg will need to be reduced to about 5g/kg, or 40gm so your dehumidification system, whatever it is, will need to extract 90gm of water fairly quickly.  That is a lot for any disposable pads and even with completely efficient ventilation (say you could magically have all the glass vanish, leaving just the frame) from the graph you would still expect there to be 100% humidity if the outside air was 50% saturated at 30c and then rapidly dropped to 15c (when there is 100% humidity at 10gm/kg.) 

 

So, to keep relative humidity low one either has to extract the water rapidly as the temperature drops or you have to keep the temperature high.  The obvious solution is an absorption/desiccant dehumidifier which draws humid air over a chemical desiccant on a belt or drum and then heats the desiccant, driving the water off, condensing the water and draining it outside the greenhouse.  https://en.wikipedia.org/wiki/Dehumidifier#Absorption/desiccant_dehumidification That has a dual effect, removing water from the air and heating that air (allowing it to carry more water and hence the relative humidity drops even without the water being removed).  If I have problems with high humidity when the plants are in bud then I will use one of these (and cover the naked earth in the greenhouse to prevent water evaporating from that to replenish the water I have removed from the air).  I have put an electricity supply in the greenhouse.  Of course those are not cheap to buy or to run but may be the only solution.  

 

On the other hand, plants are adapted to cope with diurnal variations in humidity and it is likely that if you have good ventilation they won't suffer during the veg stage or until they are heavy with bud.

 

I know there are lots of simplifications in the rough calculations but if I have misinterpreted the physics, doubtless there are air con experts on here who will let me know!  

 

Right, best get ready with the vape for the Craig Charles Funk 'n' Soul show.... 

Edited May 23, 2020 by Socksnsandals

[UnKn0wN]

16 minutes ago, Socksnsandals said:

My greenhouse is on a frame on the soil, with plants in the soil. Today the humidity is the lowest I have ever seen it at 31% with three sets of louvres and three roof vents automatically open (25 degrees inside, 17 outside) and strong winds gusting.  In my kitchen, where the weather base station is, it is 21 degrees, 41% humidity.  Recently it has been 40-50% in the greenhouse when warm during the day and rapidly climbs to much more at night when the temperature drops and the ventilators shut.  In the autumn / winter it was in the high 80 - 90%s.  I am pleased that the risk of RSM is low but can see that mould could be an issue when the plants are in late flower so am doing autos rather than risk a photoperiod plant growing into a triffid in the good conditions during the summer and then getting hit with rot in the autumn.  Unless yours is on a solid impermeable base I would imagine that you will regularly hit humidity in the 70s - 80s in the evening and night. 

 

From looking online, it seems that because the water-carrying capacity of air rises with temperature you will always get a big surge in relative humidity (the % of the max possible humidity at that temperature) as the temperature drops, https://en.wikipedia.org/wiki/Relative_humidity#/media/File:Relative_Humidity.png so a greenhouse at 30c can hold a max of ~28g of water / kg of air, and is 50% relative huimidity when there is about 13gm/kg of water in the air but at 15c the max drops to 10gm/kg, so, unless it condenses out, the same amount of water will push the relative humidity to over 100% at 15c, hence, presumably the condensation / dew at nighttime especially in the cold microclimate immediately adjacent to colder objects which have radiated their heat away rapidly (like cars).  If we assume for simplicity that the greenhouse is an odd shape of 2x2x2m (8 cubic metres) and, ignoring effects of humidity and pressure on the density, the density of air is about 1.3kg/m3 so our theoretical greenhouse contains about 10kg of air, able to hold 130g of water at a relative humidity of 50% at 30c but, if you want to maintain a relative humidity of 50% when the temperature drops to 15c, the water content in that 10kg will need to be reduced to about 5g/kg, or 40gm so your dehumidification system, whatever it is, will need to extract 90gm of water fairly quickly.  That is a lot for any disposable pads and even with completely efficient ventilation (say you could magically have all the glass vanish, leaving just the frame) from the graph you would still expect there to be 100% humidity if the outside air was 50% saturated at 30c and then rapidly dropped to 15c (when there is 100% humidity at 10gm/kg.) 

 

So, to keep relative humidity low one either has to extract the water rapidly as the temperature drops or you have to keep the temperature high.  The obvious solution is an absorption/desiccant dehumidifier which draws humid air over a chemical desiccant on a belt or drum and then heats the desiccant, driving the water off, condensing the water and draining it outside the greenhouse.  https://en.wikipedia.org/wiki/Dehumidifier#Absorption/desiccant_dehumidification That has a dual effect, removing water from the air and heating that air (allowing it to carry more water and hence the relative humidity drops even without the water being removed).  If I have problems with high humidity when the plants are in bud then I will use one of these (and cover the naked earth in the greenhouse to prevent water evaporating from that to replenish the water I have removed from the air).  I have put an electricity supply in the greenhouse.  Of course those are not cheap to buy or to run but may be the only solution.  

 

On the other hand, plants are adapted to cope with diurnal variations in humidity and it is likely that if you have good ventilation they won't suffer during the veg stage or until they are heavy with bud.

 

I know there are lots of simplifications in the rough calculations but if I have misinterpreted the physics, doubtless there are air con experts on here who will let me know!  

 

Right, best get ready with the vape for the Craig Charles Funk 'n' Soul show.... 

Yoo that was a highly educational reply, many thanks for the time and effort you put into that.  Study physics by any chance? 

 

My greenhouse is the same as yours, frame above the soil with plants in the soil, the chronic however is in pots.Yeah you are bang on, been hitting the high 80s in the evening so been bringing the plants inside till i have it under control. Got one plant in week 6 at the moment so obviously not a good idea to be that high over night. Gone and got a oscillating fan and set that up to improve air flow. Hopefully this will remove the layer of stagnant air and reduce the areas in which condensation forms. However as you have stated, its not air flow which reduced humidity but its relationship to temperature. Will be interesting to see how improved air flow alone affects it alone, may need to have a heater in there as well. As only my first grow, trying to see what i can do with as little money as possible, so an expensive dehumidifier is a little out of the question. May have to just put her in shed at night if worse come to worse.

 

hahah sounds banging, enjoy, love a bit of Craig Charles and got a dry herb vape for the first time the other week, been enjoying it highly. Enjoy your evening mate.

[Socksnsandals]

3 hours ago, UnKn0wN said:

My greenhouse is the same as yours, frame above the soil with plants in the soil, the chronic however is in pots.Yeah you are bang on, been hitting the high 80s in the evening.

 

hahah sounds banging, enjoy, love a bit of Craig Charles and got a dry herb vape for the first time the other week, been enjoying it highly. Enjoy your evening mate.

 

Thanks, enjoyed it, though I got trapped in a place with spiky railings about 6' high and a woman whose conservatory overlooks it was worried by the old codger with headphones clinging to a tree while trying to get over the railings and asked me what I was up to (I thought it blindingly obvious but, not wanting to alarm her and since she seemed puzzled at my antics informed her that I was just climbing over the railings and not to worry).  She was talking to someone on her radio landline handset so I scrambled over, getting a comedy rip in the bum of my gardening trousers... Craig Charles cackling away as the soundtrack, all rendered extra weird by the Lemon Juice Express auto vapour buzzing in my head.  Not the "exercise" not the chilled walk to a groovy soundtrack which I had expected and as I went home there seemed to be an unusual number of police cars - of course there weren't but the rest of the traffic has reduced so they appeared more obvious, especially to the stoned brain, the back of one of the legs which transport it about is unpleasantly revealed and rendered a bit chilly by the rent in the now-ragged trousers. 

 

Anyway, back to the thread and the question of humidity.  I'll be interested to know what effect the fan has.  Looking at my post I see a small error - at 15c our theoretical cubic greenhouse would be able to hold 50gm of water at 50%, not 40gm, so the dehumidifier would have to reduce the amount of water present by 80gm, but the principle is the same.  Earlier I looked (22:10hrs) and the greenhouse was 13c and 57% r.h. (vs. 25c and 31%) and the kitchen had barely changed at  21c and 45% -vs- 21c and 41% (possibly a small real increase rather than a measurement error as the rh of the outside air rises sharply as the temperature drops in night-time).  There has been no rain and apart from the wind having dropped and night having fallen, no  significant climatic change so I expect that there would be no reason for the real atmospheric humidity to have altered and therefore the rh should have risen solely in inverse proportion to the temperature drop and very satisfyingly, the ratio of greenhouse temperature when I initially posted to that now (25c/13c = 1.9 and if you multiply the rh when the greenhouse was 25c by 1.9 then [assuming that there is a linear relationship rather than the more subtle one which the graph implies] then the rh should be that at 25c (31%) x 1.9 = 59% which is within a smidgen of the observed rh of 57%... yessss!  That seems to bear out the hypothesising earlier and to reinforce the conclusion that the only way to keep rh down is to dehumidify or to heat the relevant space.  

 

[UnKn0wN]

1 hour ago, Socksnsandals said:

 

Thanks, enjoyed it, though I got trapped in a place with spiky railings about 6' high and a woman whose conservatory overlooks it was worried by the old codger with headphones clinging to a tree while trying to get over the railings and asked me what I was up to (I thought it blindingly obvious but, not wanting to alarm her and since she seemed puzzled at my antics informed her that I was just climbing over the railings and not to worry).  She was talking to someone on her radio landline handset so I scrambled over, getting a comedy rip in the bum of my gardening trousers... Craig Charles cackling away as the soundtrack, all rendered extra weird by the Lemon Juice Express auto vapour buzzing in my head.  Not the "exercise" not the chilled walk to a groovy soundtrack which I had expected and as I went home there seemed to be an unusual number of police cars - of course there weren't but the rest of the traffic has reduced so they appeared more obvious, especially to the stoned brain, the back of one of the legs which transport it about is unpleasantly revealed and rendered a bit chilly by the rent in the now-ragged trousers. 

 

Anyway, back to the thread and the question of humidity.  I'll be interested to know what effect the fan has.  Looking at my post I see a small error - at 15c our theoretical cubic greenhouse would be able to hold 50gm of water at 50%, not 40gm, so the dehumidifier would have to reduce the amount of water present by 80gm, but the principle is the same.  Earlier I looked (22:10hrs) and the greenhouse was 13c and 57% r.h. (vs. 25c and 31%) and the kitchen had barely changed at  21c and 45% -vs- 21c and 41% (possibly a small real increase rather than a measurement error as the rh of the outside air rises sharply as the temperature drops in night-time).  There has been no rain and apart from the wind having dropped and night having fallen, no  significant climatic change so I expect that there would be no reason for the real atmospheric humidity to have altered and therefore the rh should have risen solely in inverse proportion to the temperature drop and very satisfyingly, the ratio of greenhouse temperature when I initially posted to that now (25c/13c = 1.9 and if you multiply the rh when the greenhouse was 25c by 1.9 then [assuming that there is a linear relationship rather than the more subtle one which the graph implies] then the rh should be that at 25c (31%) x 1.9 = 59% which is within a smidgen of the observed rh of 57%... yessss!  That seems to bear out the hypothesising earlier and to reinforce the conclusion that the only way to keep rh down is to dehumidify or to heat the relevant space.  

 

I am very baked right now, this shit was a bit crazy to read haha, started off as a novel and then into atmospheric humidity. Thanks for the info dude, may have to give it another read tomorrow

[Geo]

11 hours ago, Socksnsandals said:

 

Looking at my post I see a small error - at 15c our theoretical cubic greenhouse would be able to hold 50gm of water at 50%, not 40gm,

 

I make it about 36.7g but thats after converting metric to imperial and back. The leaf/plant temperature is easy to overlook, it may be a lot lower than the greenhouse air temperature.