Humidity in a small greenhouse.

[agavedave]

Does anyone have any humidity data in a smallish greenhouse for this time of year.

I am finding that the humidity frequently approaches 100% in the middle of the night. I did have a few issues with sunlight striking the sensor around 12:00-13:00 hrs. I have a better shield around it now. It was light sensitive and the data sheet does mention it in passing but no more. I first confirmed that by placing a black small plant pot upside down over the sensor.

So now I am seeing about 30% in the afternoon with very limited ventilation when it's up to 22c and then soon increases as the temperature drops overnight. This is on a dry day. When it's raining outside on a dull day it can stick at 70%, probably equalised with the outside. Leaving the double door wide open the humidity follows whatever is going on outside.

This time of year seems to be the worst where we are equalising with outside temp at 15-20C and >70% RH then letting the temperature drop to about 5C before the heater wakes up. The doors are usually closed by this time unless I'm late home. I am hoping that once outside temps stay low I should be admitting less moisture when ventilating during the day.

There seems to be no way out of this without a dehumidifier?

Regards
Dave

[gerald]

Without a dehumidifier you're always going to have to spend your time opening and closing windows, doors and vents, every day and every night, and even then you won't be able to prevent moist air or condensation from affecting your plants.

What's stopping you getting one? Mine runs 24/7, it's always dry in the greenhouse and well worth the investment in my opinion.

[iann]

Consider yourself lucky to be in East Anglia

A simple calculation will show that your numbers are reasonable. Given a dewpoint of 5C, the relative humidity obviously reaches 100% when the temperature drops to 5C, although sensors are often quite inaccurate approaching 100% humidity. At 22C, the relative humidity would then be 33%. Note that you only get this if you keep the dewpoint at 5C inside the greenhouse, which might not happen if there is a lot of condensation to shift.

It is hard to lower the dewpoint without a de-humidifier because it is simply a measure of the amount of water in the air. You can cause the dewpoint to rise though, just leave a source of water (eg. damp soil, or just live plants) in an unventilated area. The dewpoint is often quite high at this time of year, with prevailing weather from a warm Atlantic, although it is showing as 4C here at the moment, and limited sun means it is hard to both get the greenhouse very warm and strongly ventilate it. The dewpoint drops with northerly or easterly weather, and becomes generally low later in winter.

[Peter]

Probably a good reason to have a fan working full time.

[Cidermanrolls]

I'm with Gerald. Get a dehumidifier. During the day the heat might be wasted, but at night the heat the dehumidifier puts out is saved through less from the heater. You might be surprised how much water they pull out of the air.

[rodsmith]

Probably a good reason to have a fan working full time.

I have had a fan circulating air at ambient temperature on permanently for several years. We lived in Wales from 2011 to 2015 and in the winter the humidity recorded on my equipment frequently rose to 99% at night. The minimum temperature in the greenhouse in winter was about 3 deg. C. I lost a few plants to rot but fewer than 10 in the 4 years we were there.

[Terry S.]

Whether or not you would like to reduce humidity will depend very much on what you grow. I have a theory that many of the winter-growing succulents from the Western and Northern Cape Provinces of South Africa do so well in our climate because of the humidity during the winter. In the wild they depend heavily on fogs rolling in off the Atlantic and some measurements have indicated that these provide more moisture for the plants than direct precipitation does. My Cape bulbs tend to look sad when it is very cold outside and the relative humidity of the warmed air in the greenhouse is quite low. The conditions become too dehydrating for them.

And what about you growers of Chilean cacti? Don't these also depend on atmospheric humidity for the same reason as my South African plants?

[DaveW]

See:-

https://sciencing.com/humidity-mojave-desert-19526.html

https://answers.yahoo.com/question/inde ... 450AAnjPP2

https://books.google.co.uk/books?id=gDs ... ts&f=false

[Aiko]

I have a theory that many of the winter-growing succulents from the Western and Northern Cape Provinces of South Africa do so well in our climate because of the humidity during the winter.

Probably a bit more than just humidity. My winter active succulents sometimes stand in moist soil for weeks, because the soil hardly dries out after watering. I don't think I have lost one of them due to too much moisture. They just seem to be quite moisture tolerant when in active growth. Totally different compared to summer active succulents.

[agavedave]

Thanks for the responses. Just that the last month or so is the worst I have seen and also corresponded with a new RH sensor. The sensor was checked in salt chambers and was within spec at the time. Probably the unusually warm daytime temperatures that we were having bringing in more moisture and probably a bit coming off all the big pots s that were brought in during the middle of October.

Yes I did the sums as well but didn't want to believe it. I do keep a dehumidifier going in the house over winter because 'er in doors' keeps putting damp washing on radiators. If I move the DH into the greenhouse, I don't want her airing the washing in there as well.

The sensor is covered with tyvek to keep off and drips direct contact with water and does seem to recover immediately once the temperature climbs in the morning.

This is what I'm using,
http://www.farnell.com/datasheets/1685535.pdf
Regards
Dave