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Monday 17th of March 2014
Information provided by DairyCo a division of the Agriculture and Horticulture Development Board (AHDB).
Correct building design is critical to ensure adequate ventilation. This is extremely important to maintain air quality. To make sure you have adequate ventilation it is important the building is designed to remove excess heat and water vapour, remove microorganisms,dust and gases, provide a uniform distribution of air and provide the correct air speed for stock.
Natural ventilation is the least troublesome, most efficient and least expensive system for providing an optimum environment within a building. Buildings will naturally ventilate best when they are sited at right angles to the prevailing wind direction.
Heat produced by the livestock naturally rises. If it is unable to exhaust from the building at the highest point (at the ridge), it will condense and remain within the building, causing humidity to increase. As the air cools, it will fall back onto the bedding, increasing the moisture content and creating a suitable environment for bacteria to thrive.
If the warm air is able to exhaust from the ridge of the building, this draws fresh air into the building through the side inlets, ie the 'stack effect'. However, if there are insufficient air inlets, warm air cannot escape from the building as a vacuum will not be created.
There are a number of methods to achieve adequate outlet ventilation which includes various ridge designs or a slotted roof. It is essential that there are adequate outlets in the ridge of the building. An open ridge is generally between 0.3-0.4m wide and should be unrestricted.
There should be 5cm of ridge opening for every 3.0m of building width.
Slotted roofs (where the roof sheets are inverted and fitted with a space of around 10mm between each adjacent side sheet) can be very useful, particularly if considering housing during summer months.
The inlet area, ideally split evenly across the two side walls, should as an absolute minimum be twice the outlet area and is preferably four times the outlet area.
The aim should be, where possible, to ventilate the building from the sides. Inlet areas in the gable ends are only recommended where the building is excessively wide (>25m), or where there are restrictions in the inlet areas along one or both sides of the building.
The pitch of the roof can influence how well the stack effect is established but selecting the pitch of a roof, particularly with a span building, will always be a compromise between ventilation and overall ridge height.
Roofs are normally pitched around 12.5% although examples can be seen with roof pitches of 22.5%. The building height will be significantly greater with a 22.5% pitch, this may pose issues with the planning authorities.
There are many farms installing curtain sides to the cubicle building which allow the amount of air admitted through the inlets to be varied according to prevailing weather conditions. These curtains can be lifted and raised manually or automatically and provide greater environmental control.
Consideration needs to be given to the prevailing wind direction when considering inlet ventilation. If there is insufficient weather protection, rain will drive into the building and result in wet cubicle beds. In addition, wind velocity may blow bedding off the beds and lead to lower cubicle occupancy in some areas due to the 'draught', this will result in an increased stocking rate in the rest of the building.