Unless a barn is well ventilated, horses are better off outdoors, even in cold or stormy weather.
A poorly ventilated barn is unnaturally damp and dusty, which can lead to respiratory problems, and will also get too hot in summer.
Young foals are especially vulnerable to effects of inadequate ventilation and contaminants suspended in the air: dust from feed or bedding, mold spores, ammonia from urine and wet bedding. These irritants may trigger inflammation in the lungs and airway lining, opening the way for invasion by viruses and bacterial infections.
Some contaminants may cause allergic responses in horses, compromising respiratory health and producing performance-limiting problems that are often lumped together as COPD (chronic obstructive pulmonary disease, commonly known as heaves) and now called recurrent airway obstruction (RAO).
Stabled horses are always exposed to a higher level of dust in their environment than horses outdoors. Even if a barn is ventilated, there is more dust trapped in an enclosure. Proper ventilation is extremely crucial to respiratory health.
Many of the older style barns were poorly ventilated, with high dust levels. Many of the barns built prior to the 1980's were built for cattle and often the people who bought those farms for horses adapted the barns by building stalls in them--usually box stalls, with complete partitions, and no way for air to circulate between the stalls.
Often, older barns and stables don't have very many windows, limiting the options for bringing in fresh air and taking the old air out. The only option is to open the door at one end of the barn or the other.
Many horses have allergic airway disease or heaves due to improper barn ventilation because of dust and poor-quality air held within the confines of the barn. Horse owners tend to keep horses stabled for 5 or 6 months of the year; the animals spend more time indoors than outdoors, and some owners who are showing or competing with horses keep them in stalls year-round.
In winter, people want to keep the barn warm, for their own comfort, and keep show horses from growing a long winter hair coat. It's difficult to have adequate ventilation under these conditions when trying to keep a barn warm.
In summer, show horses are kept indoors to prevent sunburn and bleached hair coats, or to "protect" them from possible injury running around at pasture.
Although stalls should be designed with the best interests of the horses in mind, ventilation is often inadequate. We give each stall a window, which is good, but the stall should not be closed up at the top or bottom. The door at the front of the stall should be open at the bottom.
Studies have shown that dust and ammonia are greatest in the lowest 3 feet of the stall. Even if the door is half open at the top, you still won't get good stall ventilation because the bottom doesn't have enough airflow. It's very important to have air coming from the floor upward.
Cold air settles to the bottom in an enclosure, and warm air rises. If there's no air current to move the lower air upward, the air nearest ground level is always poor quality. If a horse lies down, he has no choice but to breathe air laden with dust and ammonia.
To improve ventilation we need to create air movement to circulate the air from the bottom of the stall on out.If there's circulation of air from the bottom up, some of the dust will settle and some will be moving into the ventilation path. If there's a window open in that stall, cold air will come in the window, go to the ground and circulate out under the stall door.
Every barn should have an inlet and an outlet, so the air can move through. Opening a door on each end of the alley may not be enough. It depends on which direction the wind comes from.
If it's from the east it will move west, and if this is the way your barn is situated, that's fine. But if the wind comes from the south and this is where the windows are located, it will come in the windows and all the dust will go into the other side of the barn, into those horse's stalls, since their windows would be the air outlet.
You may have improved one side of your stable but doing that may have made the other side of the barn dustier. Natural ventilation thus has limits, depending on where your barn is located relative to the prevailing winds. In winter, you may not want to open the doors at each end of the aisle because it's so cold outside.
If you are building a new facility, have an engineer look at the air flow--and design an ideal inlet and outlet.
When renovating an old barn, examine the structure of the building. A good engineer could tell you what you'd need, to assist the natural airflow, before you spend a lot of money on changes, and can also recommend where the fans should be located and what that would do.
If the barn has less than ideal construction/ventilation you can place fans in the best locations to maximize airflow. Someone who knows about ventilation engineering could advise you on where you need more air coming in and where it should go out and give you a better idea about where to place fans and what kind to use.
They can also advise you on which windows should be open and maybe recommend where you need outside vents rather than using a fan.
You may be able to improve the natural ventilation without unnecessary expenses, making the best of what you have, with the least cost. Horsemen can learn more about ventilation from the swine and poultry industries.
They have perfected their ventilation systems to create a healthy environment for those animals--concerned not just about dust but also the pathogen load in the enclosed area. Their barn ventilation designs are very sophisticated and horse owners can benefit from their experience in improving ventilation within the buildings that house their animals.
Whether you are building a new stable or renovating an older building, it would pay to contact someone who could advise you on proper ventilation, or show you how you can improve on what you already have.