Oftentimes, when a farmer is designing a building they will design it based on the Department of Agriculture specification S.101. The specification has been continually improving and ensures that sheds are built to a very high standard.

It outlines different inlet and outlet ventilation requirements, based on the width of the building. However, it does not take into account the internal layout of the shed.

Stack effect

Buildings should be designed to take advantage of both natural ventilation and the stack effect.

The stack effect is where heat generated by livestock warms the air, which rises up and out of the shed at the apex.

This in turn sucks in fresh air from the inlets along the side of the building. This highlights the importance of having an outlet for air.

If there is no outlet at the apex of the shed, then stale air will be pushed out through the inlets at the side of the shed.

This will mean that fresh air and stale air will be pushing against each other which will result in a very poor flow of air into the shed.

In sheds over 15m wide there is a requirement by the Department of Agriculture to leave a continuous opening of 600mm directly below the eaves. If there is vented sheeting along the side of the building, this decreases to an opening of 450mm.

While a continuous opening is a very good source of inlet ventilation for older stock, for young calves it can lead to draughts.

While more detailed calculations can be completed to evaluate the amount of inlet and outlet area needed for ventilation, using rough figures we need an outlet area of at least 0.04m2 per calf up to 100kg, increasing to 0.1m2 for growing and adult stock. These figures are modified by stocking density of the shed.

The inlet areas for ventilation need to be twice and ideally four times the calculated outlet areas, so 0.2-0.4m2/animal.

For anyone who has built a shed, have you ever considered the actual ventilation requirements based on the stock that will be housed in that shed?

Example

Take two sheds of the same size – one is fitted with cubicles and the other is fully slatted.

Each shed has a passage running down the centre and an animal area either side of this passage of 115m2.

There are very different requirements for these sheds based on the number of livestock they can hold.

Figure 1 shows a cubicle shed where cows would have an area of 5m2/cow, common for cubicle sheds.

This would mean 23 cows could be held either side of the shed, or 46 in total.

If we take a similar-sized slatted shed used for finishing cattle and allocate space of 2.5m2/animal (Department recommended stocking rate for cattle over 275kg) it would mean that we could fit 46 animals on either side of the shed, or 92 animals in total.

This is double the amount of animals, but ventilation requirements are the same for both sheds, according to the Department specifications.

Having an opening of 600mm directly below the eaves (as the shed is more than 15m wide) gives the shed total inlet ventilation of 27.6m2 between the two sides.

If we have 92 cattle in our shed as seen in Figure 2, at an inlet requirement of 0.2m2 (double the 0.1m2 outlet area required), we require 18.4m2 of inlet area. If we increase this to 0.4m2/animal it means we require 36.8m2 of inlet area.

However, for our 46 cows in Figure 1 we require 9.2m2 of inlet ventilation (0.2m2 x 46 cows), or 18.4m2 if we increase this to 0.4m2/animal. The shed design ensures that there is more than enough inlet ventilation.

However, a continuous opening under the roof will not suit all situations, particularly on exposed sites, or calf sheds, for example, where it can lead to draughts. The only other option left to farmers is to use spaced boarding or Yorkshire boarding. As the shed is over 15m wide we are not permitted to use vented sheeting alone.

If using spaced boarding, it must be a minimum depth of 1.5m along the full length of the shed while boards must be 75mm wide with a gap of 25mm.

This equates to a void area of 25%. If a farmer uses spaced boarding along the length of the shed outlined above, at a depth of 1.6m for example, he has a total inlet area of 18.4m2 (1.6m x 23m = 36.8 x 25% = 9.2m2 on either side of the shed).

While this would be sufficient for our 46 cows in Figure 1, it would not provide sufficient inlet ventilation for the 92 cattle in the shed in Figure 2.

While both a continuous opening underneath the eaves of the shed and spaced boarding are excellent options for inlet ventilation, it all depends on what is in the shed. Ventilation should not be viewed as a one-cap-fits-all approach and maybe it is time we go back to the basics with shed design and put the focus on the animal.

Department specifications

Up to this point there has been confusion from farmers as to whether Yorkshire boarding is allowed on grant-aided buildings.

However, the Department has moved to clarify the situation, outlining that specification S.101 is “currently undergoing revision to update requirements, include new items and updates in relation to ventilation.

Space boarding will be considered during the review. However, if an applicant wishes to use it at present, they should submit details of their proposal to the Department.

The Department of Agriculture has grant-aided buildings with double-sided spaced boarding (more commonly known as Yorkshire boarding), while not specifically accounted for in specification S.101.”

The Department also stated that vented sheeting is now required to have a minimum void area of 18%, with the target ratio of 25%.

There is no date as to when this specification will be updated.