Temp and Humidity in polytunnel

[Eoghan]

Here in Caithness it is struggling to reach 16C today but the polytunnel can easily reach 35C. I am under the impression this is generally good but the humidity was a problem last year. This year I put in mesh panels on the doors, either end, which helps reduce the humidity. My raspberries in particular suffered from fungus last year and my glasses steamed up as soon as I entered - not this year!

What are the optimal conditions? [I will add pics of the ventilation mesh and the pull down cover]

[Eoghan]

The pictures show the "blind" up and down over the door being held in place by shockcord. I know it would be simpler to open the door but that would let in insects and a dozen hens who are dying to get inside!

[ameno]

What's optimal depends entirely on what you are growing.
If you were growing mainly heat-loving crops like melons, peppers, etc. then low 30s would be an ideal temperature.
For the likes of raspberries, though, that's rather too hot, really. Mid-20s is ideal for them, but they do absolutely fine at lower temperatures (they just mature slightly less quickly).
As for humidity, most plants don't really want it above 80%, although if temperatures in there are too hot for certain plants then higher humidity actually increases their ability to tolerate the temperatures, as it reduces water loss (although in the long run it also encourages fungal diseases).

I admit I have never lived up where you are, but personally I would have thought raspberries would be okay outside, even up there, maybe with a wind break if strong, cold winds are an issue.

[quanglewangle]

The RH reading on the meter in the picture (25%) seems very low. That's the level you get in deserts. Is it in direct sunlight?

[ameno]

The RH reading on the meter in the picture (25%) seems very low. That's the level you get in deserts. Is it in direct sunlight?

You're right, I didn't notice that. Indeed, that reading seems impossibly low for this country.
I wonder if the hygrometer is even working properly?

[Eoghan]

Humidity is back up at 60

[quanglewangle]

RH will be all over the shop in the space of a day. The real problem is that the high levels occur at night when you won't be around to ventilate.
Here is my greenhouse RH for. today. Door open since about 0900. Notice high levels during the night - this is quite usual,.

[Eoghan]

Back up to 72% at 8.15 pm using the digital hydrometer from the living room. It is a LIDL purchase and uses temp and humidity to give you the dewpoint.

[quanglewangle]

If you really want to know about optimum humidity then Google vapour pressure deficit but be warned, it gets pretty involved.

[quanglewangle]

My greenhouse RH is currently stuck around 90%.

The dehumidifier is working its socks off, extracting 4-5l / day.
Our problem here is the sea fog and mizzle, meaning the outside atmospheric RH is up around 100% so greenhouse humidity goes up if the vents open!