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The Power of Fresh Clean Air

The three main factors influencing performance at housing are Nutrition, Health and Environment. Good ventilation plays a key role in two of these pillars: health and environment. Martina Harrington, Teagasc Beef Specialist has some information and advice.

We have spoken recently about how a housed animal is the most expensive animal on the farm and how its performance cannot be hampered. The three main factors influencing performance at housing are Nutrition, Health and Environment. Good ventilation plays a key role in two of these pillars: health and environment.

What is Good Ventilation?

Good ventilation happens when enough clean air moves through the shed to remove gasses, odours, dust and bacteria. It should also remove the moisture and heat generated by the animals. To be effective it needs to work on calm days.

Fresh air is actually a disinfectant, if a virus is coughed up in a building, it will last for 20 hours. However, if the same virus is coughed up outside in fresh air, it will last for about 20 minutes. Air actually deactivates the virus, so we need to make the maximum use of fresh air in sheds.

Do I have good Ventilation?

To know if you have an issue, you need to observe the conditions in the shed before housing and at housing. When you enter a shed and look up you should not see cobwebs, condensation, blackened timber or rust damage on the walls/roof. The shed should not be warm and stuffy or smell of ammonia. If it does, it means there is not enough air circulating through the shed for the number of stock housed in it. You could use a smoke machine to see what the air flow is like. This is most useful when animals are housed as you see exactly where the airflow is. You should use it under different weather conditions to see the effect of wind etc.

Methods of ventilation

In Ireland we mainly use natural ventilation in livestock housing which depends on:

  1. The stack effect
  2. Wind
  3. A combination of both

Note: Natural ventilation is dependent on the inlets and outlets in the shed being big enough for the number and size of the stock being housed. Wind is only effective if the outside air speed is more than 3m/sec.

What is the stack effect?

The stack effect is driven by the heat produced by the animals and by the roof slope. The animals produce heat and warm the air. The warm air rises following the slope of the roof, escaping through the outlet (highest point of the house) and is replaced by clean fresh air via the inlet.

If there is no outlet available or the outlet is too small, the air will cool down and come back down on the animals.  This air will be moist and most likely carry harmful bugs which have been proven to cause ill health and respiratory problems.

If the inlets are too small the amount of fresh air coming in will be limited. So the rate of ventilation is influenced by the size of the inlet and outlets. However it is also influenced by the roof pitch or the height difference between the inlet and the outlet, the height of the eaves and the proximity or interference from other buildings. Old slatted houses typically have roof slopes of 10 degrees so stale air tends to stay within the building especially on calm days.

A couple of Rules of Thumb:-

There should be 0.1m2 of outlet for every animal housed in a shed. E.g. 100 animals require at least 10m2 of an outlet. The inlet area should be at least twice the area of the outlet. For the 100 cattle you need 20mof inlet.

If you have a 5 span shed, 23m long housing 100 cattle, the outlet in the roof should be 10/23 = 435mm wide.
The openings under both eaves should be at least 435mm and this will give you double the outlet and thus effective airflow provided the roof has the recommended roof pitch of 15 degrees.

The Department of Agriculture, Food and the Marine (DAFM) provides minimum specifications for new buildings in Specification S101. An extract from S101 is given below and this should always be followed in new buildings, whether grant aided or not.

Outlet Ventilation

Outlet ventilation must be provided along the full length of the roof apex; 450mm wide for a house up to 15m wide; 600mm wide for a house up to 24m wide; and 750mm wide for larger houses.

A ridge cap over the outlet is not recommended, but when provided it must stand unobstructed and fully clear of the roof by 275mm, 350mm, or 425mm, respectively, for the three widths of houses noted above.

Angled up-stands placed on the roof on both sides of the ridge outlet improve the ventilation and prevent most rain access. They are a strongly recommended alternative to ridge capping.

Under such up-stands, the roof-sheet cannot extend 50mm on each side to prevent rainwater dripping from the up-stand.

There are more elaborate versions of this outlet available that are very effective in that as wind hits the up-stands or baffles it sucks air out of the building.

Where spaced sheeting with a gap of at least 20mm is installed over the entire roof, a central ridge outlet, though recommended, is not mandatory.

Inlet Ventilation

Inlet ventilation must be provided directly under the eaves for the full length of each side of the house. An unobstructed depth of 450mm must be provided in houses up to 15m wide; 600mm deep in houses up to 24m wide and 750mm deep for larger houses.

Alternatively, to reduce wind-speed and rain, options such as pre-painted steel sheets with ventilation slots (vented sheeting) over their surface can be used for inlet ventilation. They must be positioned immediately below eaves for the full length of the house and have a minimum depth of 1.5m.

If the shed is >15m wide there must also be an unobstructed opening under the eaves. For full details click here

If you suspect you have an issue what can you do?

Using the calculation above or the DAFM guidelines check what area you have for outlet and inlet.

You cannot have too much inlet area but you must ensure it is not causing draughts or allowing excess rain into the shed.

If you are not meeting the inlet or outlet areas there are some options below you can adopt in your shed to solve either problem.


  • Angle out the side cladding, at the bottom. Moving out the side cladding to leave a clear opening between the side wall of the shed and the side cladding, just below the top of the side wall, will provide a good inlet. Air is deflected upwards from the side wall as it enters. This can be done easily enough with box section steel. The box section will make it is secure and will allow the gap width to be maximized.
  • Where sheds have a wall built right up (no side cladding), the easiest solution is to knock off a few lines of blocks to provide a continuous opening (provided they are not supporting the roof!). Monitor how it works to see if any more needs to be done
  • Replace side sheeting with space boarding, Yorkshire boarding, vented sheeting, or a windbreaker. Note: Check the porosity of vented sheeting, it should be at least 20% - some are less than 10%
  • Remove any trees, hedges or other obstacles blocking wind from the side of the shed and clean vented sheeting etc. at least every year.


  • Raise the ridge cap if possible.
  • Spaced sheeting in the roof is excellent, but really not practical unless all the sheeting is being replaced.
  • Raising sheets is a practical method to turn the roof into a “breathing roof”. One or two lines of sheeting per bay can be raised above the plane of the roof by about 100 mm to 150 mm and ideally with an overlap of about 100 mm to 150 mm, at each side, to prevent in-blown rain. The raised sheets run up along the slope of the roof (which is usually across the width of the shed) and possibly up and over a round roofed shed, as well, if one is present. The size of the outlet can be calculated by multiplying the total length of all the openings by the raised height and comparing them to the guidelines.

There are many more options and you can discuss these with your adviser or vet.

Draughts are just as bad, if not worse, than poor air movement and stuffy conditions. Doors should not be used to provide extra inlet or outlet area. Doors may be opened in mild/warm weather to supplement ventilation as long as they don’t cause draughts. Doors used to supplement inadequate ventilation will lead to draughts. In calf sheds it’s important to lie down at the level of the calf and see if there are draughts, if present you will have to use bales, rubber on doors or a canopy over the calves etc. to solve the issue.

Mechanical ventilation with fans can be used, as a last resort, to pump in fresh air and improve air movement in existing problem sheds. They need to be installed and used correctly to avoid creating draughts at animal level. Although they are expensive to install the running costs are low and they have worked very well in some calf houses.

Light is also very important in animal housing, mainly for the animals but also for the person looking after them. Any improvements to natural light that can be made in conjunction with making improvements in ventilation should be availed of.

Please always prepare a Safety Plan or a written risk assessment before undertaking specific projects.
Think about the dangers involved especially the foreseeable, more risky, and more likely ones. Write down the risks and hazards and make sure everyone involved is made aware of them. Use the right equipment and wear appropriate PPE. As some of this type of work is not the normal day to day work on a farm it may be safer to employ someone competent in this area instead.

Siting new buildings

Ideally cattle sheds would not all be lined up together as this reduces the ability for air to flow in the sheds. They should be sited with the side at right angles to the prevailing wind. However if they are open sided buildings they should be orientated with the open side sheltered from the prevailing wind.