Cows don’t like draughts but they do like plenty of fresh air to keep them cool, whether it’s summer or winter.
Getting the balance right – and more particularly ensuring there is adequate air exchange for the size of building and number of cows involved – is the tricky bit and one that some dairy building designers believe is often not given adequate attention.
“Yes, it’s well understood that dairy buildings should be well ventilated but the over-riding principles are not well observed in practice,” says David Mair of Agri Design, Ayrshire. “There have been lots of studies to show the importance of ensuring adequate ventilation in such buildings – but it takes a bit more effort to come up with the correct solution.”
Cheshire designer and project manager John Mogg of Dairy Design Consultancy takes a similar view.
|Picture (above left): Kingshay consultant Rob Mintern uses a smoke bomb to test ventilation in a cattle building.|
“Ventilation requirements are the least understood aspect of dairy building design and specification,” he says. “I’ve seen far too many cattle buildings with very poor ridge design.”
High-level air extraction is the key issue. Farm buildings rarely have insufficient air inlet area, he says, but getting stale air out of the building is a different matter.
“Cattle require enormous quantities of oxygen to support their metabolism,” Mr Mogg emphasises. “Once exhaled, that large volume of warm, moist air has to be exhausted from the building so that it can be replaced by fresh air.”
The default solution – fitting ventilating sections within a ridge formed from cranked crown panels – is rarely adequate for the task, according to Mr Mair.
“But that’s the quickest and easiest solution for the builder and the cheapest solution for the customer,” he points out. “An open or protected open ridge will cost perhaps another £10-15/m and involve more work. But it will do the required job – as long as it is specified and installed correctly.”
Mr Mair urges farmers planning a new dairy building to grasp the principles of ventilation requirements, satisfy themselves that design proposals meet those requirements and then ensure first-hand that the chosen air extraction solution is installed to the correct specification.
“Unless the foreman on site appreciates why a ridge layout, for example, has been specified the way it has, there’s always a risk that important details and dimensions will be overlooked,” he says. “Yet they can be critical for efficient air extraction.”
John Mogg advises: “Don’t assume builders always understand ventilation requirements for cattle buildings. It’s easiest for them to continue doing what they’ve always done and in terms of ventilation that’s not always going to be the best thing,” he says.
“There is a cost element involved, of course, because customers will often be attracted by the cheapest option if they don’t appreciate the implications,” he adds. “But there can also be a heavy ongoing cost of not doing the job right in terms of the cows’ health and performance.”
Although ridge outlets provide the main source of air extraction, raised roof sheeting and spaced roof panels can be used to good effect in large or multi-span buildings.
A so-called “breathing roof” is created by inserting wooden battens or nylon mesh between courses of profiled sheet. The small ventilation openings should be weather-proof as long as sheet overlap recommendations are followed.
“A ‘spaced roof’, with sheets laid 12-15mm apart, provides air inlets lower down and extraction outlets higher up the roof,” notes Mr Mair. “The roof will not be weatherproof but when properly installed the heat from the cattle creates an updraught that will help keep rain out – and when the shed’s empty in summer, it doesn’t really matter if some rain does get in.”
Choosing the most appropriate ridge configuration for a dairy building should largely depend on its purpose, size and the number of cattle it houses.
“If I’m putting a roof over a collecting yard, I’ll recommend an open ridge as the lowest cost solution – it doesn’t matter if the rain comes in and it helps wash down the yard,” says Mr Mogg. “Otherwise, a protected open ridge, as long as it’s constructed to the correct dimensions, is an effective and largely weatherproof solution.”
Dimensions are important to provide sufficient outlet area and to achieve the chimney stack effect that draws out moisture-laden air.
Both types – whether open or capped – have upstands in fibre cement or steel sheet to create a draw as the wind blows across the opening.
But dimensions are particularly important when a cap is added to keep the rain out.
“If the builder doesn’t appreciate that there’s a relationship between the width of the opening between the top purlins and the height of the cap above them, you can end up with the angled flat sheet being fitted so low that it reduces the airflow,” explains Mr Mair. “It’s one of those details that has to be right for the installation to work as intended.”
Incoming airflow capacity is rarely a problem in farm buildings but needs to be considered in a new build nonetheless.
“The aspect of the building relative to prevailing winds, its size and the pitch of the roof and the proximity of other buildings all have an influence on incoming airflow,” emphasises Mr Mogg. “A 9m (30ft) high building will have an effect on a neighbouring building up to 21m (70ft) away, so that has to be taken into account in the design.”
Yorkshire space boarding is the most common form of side cladding but perforated or louvred steel sheet is an alternative. But as cows can be housed for long periods and are often in buildings in summer – for buffer feeding, for example – roller curtains provide a flexible solution.
Operated manually or under automatic control according to temperature, wind strength and rain, they can be fully open for maximum airflow or partially or fully closed as weather conditions dictate.
“You don’t want a draughty building but nor do you want one that’s sealed up,” says Mr Mogg. “Dairy cattle won’t be bothered by the sort of winter temperatures we get here – they survive perfectly well in much colder temperatures in Canada and elsewhere.
“Keep the wind and rain out by all means,” he adds. “But don’t stifle airflow because that’s when you’ll be greeted by coughing youngstock when you go out in the morning.”
|One of the most important design features of a dairy building is the ridge treatment – it will determine whether the building is sufficiently well ventilated when full of cattle|