Ventilation is a key requirement in all agricultural buildings whether for cattle, crops, implements or horses. It is measured in terms of the number of times the volume of air in the building is changed per hour and is more of an art than a science.
The main driver behind passive ventilation in enclosed buildings is the "drawing" effect of wind blowing across the roof. As the air rises over the ridge it generates an area where the air pressure is lower than that inside the building. This pressure differential pulls air high up in apex space of the building out through any apertures at or near the ridgeline which in turn draws pulls air upwards from inside the building to replace it. It can be a powerful but smooth force even at low wind speeds and as any window cleaner will tell you there are very few days so windless that there is no breeze once you are a few yards off the ground.
This same mechanism keeps aeroplanes in the sky, makes your cars carburettor work and forces the smoke from the fire up the chimney and not in your house.
So this free natural machine is working almost all of the time - all you need to do to harness it is to ensure there are the right number and size of exhaust apertures at the ridge and inlet apertures at the perimeter to allow fresh air into the space to replace that exhausted. A bigger building or a higher ventilation rate can be met by providing more, or bigger, exhaust and inlet apertures to suit.
If your building is well sited with the ridge perpendicular to the prevailing breeze and clear of other buildings and trees this natural phenomonen will provide all the ventilation you need.
If you look at the illustration below you will see that there is also a low pressure area created in the lee of the building. This is weaker but also pulls air out from inside encouraging an airflow across the building towards the lee elevation.
Even when the wind is blowing directly along the ridge the low pressure in the lee of the gable provides some ventilating draw so if this is your prevailing wind direction it is a good idea to provide exhaust apertures in the gable peak using louvres or space boarding.
How to provide exhaust apertures
Open Ridge
The simplest way to create an exhaust space at the ridge is to simply omit the capping leaving a slot of the required width. Generally this is between 4" and 12" depending on the span of the building. Sometimes upstands are fitted each side to provide some protection from snow and driving rain.
Open Protected Ridge
Is a development of the open ridge which uses upstands to secure a flat sheet over the exposed slot excluding rain but allowing air out. These do the job but are a bit involved and prone to damage.
Ventilating Ridge Cappings
Provide good exhaust capacity and good rain protection but fine snow can still penetrate. They are very flexible in that they can be mixed with closed ridge cappings to get the right balance. If powdery snow is likely to be an issue then a longer sheet can be used in the top tier of sheets so there is more weather protection under the cowl.
Raised ridge
The ridge capping is held off the roof with a suitable spacer system allowing the air to exhaust between the roof cladding and the ridge. This offers excellent weather protection and high levels of ventilation. There are a number of variations of this which incorporate clear panels to provide extra daylight.
How to provide inlet apertures
As a rough rule of thumb you need to provide twice the area of inlet as the ridge outlet area. In smaller buildings with a low ventilation requirement the space under the corrugations at the eaves will be sufficient but if you need more the options are listed below.
Air permeable cladding
A time honoured technique whose simplest form is "Yorkshire" or "space" boarding. One inch gaps between vertically laid 6" wide boards allow air to pass through but reduces draughts. Its cheap and its easy to do but it does let wind driven snow and rain enter the building under bad weather conditions.
There are modern alternatives in the form of louvre punched and perforated steel cladding which are very smart looking but the actual open area for air to pass can be lower than the timber original and often needs additional air inlets to achieve higher ventilation rates. They are very useful for introducing air flow in gable cladding.
Panel Soffits
Where the building utilises precast concrete panels fixed inside the columns there remains a useful horizontal soffit space between the bottom of the cladding and the top of the panel which provides plenty of inlet area. This approach generally satisfies the inlet requirement for small to medium buildings with a high ventilation requirement.
Eaves soffit
A slight overhang it provided at the eaves of the building, and a seperate top sheeting rail provided so that a soffit space is created between the top of the wall cladding and the underside of the roof. This space is protected by the overhung roof edge. This works well for larger cattle buildings with a high ventilation requiremnet.
The golden rule is to have twice as much inlet area as exhaust area for good airflow. If you restrict the incoming air you will also reduce the outgoing air stifling the ventilation flow.