The main attraction of roll-up sides, as a method of ventilation, is that it is economical. There is not a significant investment and can be added to virtually any structure. Regardless of structure length, nowhere is very far from open air.
It is only by properly understanding the limitations and drawbacks that one can maximize the benefit. Just because something is inexpensive does not mean that it won’t end up being costly if it does not fulfil its purpose.
Roll-up sides should always be done in pairs (both sides). Even if you have a really high percentage of prevailing wind from one direction, there will be times when you need to ventilate into the prevailing wind.
Any time you have wind going in without opportunity of escaping,
there can be damage to the building.
The average vertical opening of roll-up sides is usually about 4’. If your greenhouse is in an extremely exposed area without any protection or wind breaks, quite often 2’ of vertical opening will be sufficient.
On the contrary, if the building is extremely sheltered, 6’ of vertical opening will be required.
One thing to remember on the height of opening is that if you have a low profile structure with a high opening, a sizeable area could get wet inside when it rains.
Regardless of the size of opening, if there is not a breeze, you will not have air movement.
It is based on this potential problem, that we recommend having end wall openings as high as possible. These openings will trigger a “chimney effect” air movement that will get rid of hot air that is trapped in the greenhouse. These openings will double as your first stage of ventilation since they would be opened when it is too cold outside to start rolling up the sides.
A drawback of roll-up sides is the amount of exposure they create for airborne weed seeds and pests. It is important to remember that when you use shade cloth to minimize this drawback, you are also significantly restricting incoming air.
The biggest drawback of roll-up sides is that there is no practical way to automate them. In other words, you have to be there to open and close them. This can be challenging on the cool, sunny days that have sporadic cloud cover. The sun goes behind a cloud and the temperature plummets and then soars when the sun reappears.
The best way to minimize this challenge is to add a small exhaust fan to the set up. This way you will have something to handle the borderline situations and then open the roll up sides once there is a more predictable need.
In the next article we will explore forced ventilation in greater detail. We also have many resources on our website, as well as videos if you want to explore the topic of roll-up sides further.