Venting your Greenhouse
The highs and lows of greenhouse ventilation
The highs and lows of greenhouse ventilation
Unless you are a cucumber which thrives on high heat and humidity levels it is essential that your greenhouse is equipped with sufficient ventilation. With the notable exception of the cucumber and certain other tropical and rainforest species most plants, like people, thrive on fresh air. Dense humidity is fatal to many plants and stale stagnant air can cause plants to rot so ventilation in the greenhouse is a must to benefit strong healthy foliage.
The percentage of vents in the roof, remembering that hot air rises, should not be less than 16 – 20% of the total floor area. so in an 8 by 10 greenhouse some 12 to 16 square foot should vented. On a cultivar greenhouse the roof vents are found on both sides of the ridge and run the full length of the structure at the highest point. They exceed 20% of the total floor area. These being frameless offer the maximum light too so you get the best of both worlds. Ridge vents alone will not give your plants the ventilation they need so these should be used in conjunction with low level vents. Ordinary greenhouses have an occasional vent at eaves level but our extraordinary greenhouses have full length aluminium vents at the lowest point. When these open even the smallest amount the cool air is drawn in at the lowest point which is evenly distributed around the greenhouse before being expelled at the ridge through the roof vents.
Running along the length of each side at the lowest point you’ll find Cultivar’s unique full length aluminium vents, operated on automatic openers. These in conjunction with the ridge vents provide a superior level of greenhouse ventilation but don’t just take our word for it, here’s the results from an independent assessment.
Cultivar commissioned RPS, a multi-disciplined international group with a detailed understanding of mechanical engineering and fluid dynamics to carry out an assessment on our unique low level vents to ascertain the effectiveness of these as compared to the traditional eaves vents. To make it more interesting we assumed the model with the eaves ventilation had the vents running the full length of the greenhouse even though traditional greenhouses usually only rely on occasional vents. Computational Fluid Dynamics (CFD) simulations combined with dynamic thermal calculations were used to indicate the velocity of air movement and pattern of air movement throughout the greenhouse. A Summer Date of 1st August, based upon the CIBSE Building Energy Code of Manchester, UK, was set for this exercise and a typical glasshouse of measuring 2.4 x 3 metres was used as a model.
In both instances the internal average peak temperature was some 4 degrees Celsius higher in the greenhouse than the external ambient temperature. There was a minimal difference between the two ventilation systems in the average peak internal temperature. The heat gain was modest as a consequence of high levels of air change. In the case of the Cultivar’s unique plinth level ventilation the air change was 34 times in an hour preventing heat build up.
Scientifically proven to have superior air distribution it was proven that cultivar’s unique low level ventilation gives superior distribution of air throughout the greenhouse with no stagnant areas. There was no indication of hot or cold pockets of air with air flowing between 0.2 and 0.4 m/s. The model with the eaves ventilation had a much more uneven flow of air.
The low level ventilation provides a more even distribution of air and a high level of change
You can of course have both eaves and low level vents should you have specialist growing requirements for plants such as alpines.
The automatic vent openers that come as standard with a cultivar greenhouse are a simple wax filled cylinder mechanism that expands when warm, pushes out a piston which in turn forces the vents open, no electric is required. This means if you’re not there on a warm day your plants will get the ventilation they need.
Whilst continuous humid conditions are not desirable in excessive summer heat dry conditions can be harmful too. Obviously frequent watering is essential but ‘damping down’ by spraying the floor and benches with water will help provide good levels of humidity. If you are unable to offer your greenhouse fairly constant attention perhaps an automated watering system should be considered.
Heat build can be corrected with automatic openers or by opening your vents, some plants such as alpines require constant extensive ventilation and in this instance we can provide additional ventilation. On the glass to ground and paneled models there are, as standard, low level vents but extra manual vents can be added too just below the eaves.
The even circulation of air through the low level and roof vents is rewarded with healthy plants. Opening the door will help in the more extreme cases, although hot summer days don’t seem to last. Enthusiasts might also consider a re-circulating fan which may double up as a greenhouse heater for winter use. This will provide your plants with air movement, a popular method used by commercial growers for guaranteed result. If you have specialist growing needs then feel free to talk to us about the most suitable greenhouse ventilation prior to making your decision. In the instance that you would like two different growing environments then a partition can be added so you can keep your cucumbers happy in one section without affecting the plants in the rest of the greenhouse.