How effective are side vents on greenhouses?

[bario1]

Hi All,

I'm considering buying a small greenhouse (6'x4') for my garden. It would come with a self opening roof vent as standard, but one of the optional extras is a side vent (manually operated i think). How useful would this be for increasing airflow and reducing overheating? Would it mean I could leave the door shut when i went on holiday in summer?

Any thoughts appreciated!

[BertieFox]

I have been growing in small and larger greenhouses for around 30 years now and have never had side vents. You can always leave doors open or partially ajar, and a top vent allows the heat to escape, and air will always find its way in around the base or doorways.
I am sure side vents are an improvement but if you have to pay a lot more to get them, I would say don't bother.
I see your point about summer holidays, but you can leave doors open with wire mesh in the gap (if you are worried about intruding animals) and human intruders will get in if they want, vents or no, closed doors or no.

[Damian Jay]

... You can always leave doors open or partially ajar, and a top vent allows the heat to escape, and air will always find its way in around the base or doorways.
I am sure side vents are an improvement but if you have to pay a lot more to get them, I would say don't bother.
..... you can leave doors open with wire mesh in the gap (if you are worried about intruding animals) and human intruders will get in if they want, vents or no, closed doors or no.

Thanks BertieFox, that's helped me keep a bit more cash IN my wallet :-)

[Kristen]

Assuming it is a louvre? (although not sure that that matters particularly) the principle is that you open the side louvre and the roof vents and convection, due to temperature difference top-to-bottom, will cause air to rise out of the top vents and draw in cooler air through the louvre.

This is only sufficiently effective at moderate temperatures - so through to late Spring, and again in Autumn. For really hot days you will, probably, need to open the door.

The difference is that opening the door creates a draught, which increases the transpiration stress on the plants - the water in the leaves evaporates more quickly, so the plant has to draw up more water, than otherwise, from the roots. When you open the door it is open top-to-bottom, so there isn't the same heat gradient within the greenhouse hence the draught rather than convection cooling.

So in principle the louvre convection cooling is more "gentle" than opening the door.

[Bigmallly]

If you decide to fit a louvre, put it on the bottom of the GH on the opposite side to the roof vent. Automatic pistons can also be fitted to louvres.