Soffits and winter ventilation

As I write this week’s blog, snow is just beginning to fall in the Manakto, MN area, the first snow of the year. Schools west of here have been closing and they are advising no travel in southwest Minnesota due to the combination of snow and high wind. Of course, in spite of warnings all week, many are not prepared for this and there will be many reports of accidents and comments about drivers, etc.

Every year we have the same situation in our production facilities. Coming out of summer heat, many producers and contract growers don’t do a good job of preparing their facility for the winter housing season. Yes, you don’t want to prepare too early or you may get caught with hot pigs on late fall day. This year this has been an even bigger problem as temperatures for the first 2.5 weeks of November have been running about 20F above normal. However, next week they are talking about temperatures being 10F below normal so winter should be on every producer and grower’s mind.

The most common winter (and even late fall) preparation failure I see is attic inlets. Most often these are soffits that are covered with netting of some time. This needs to be cleaned (generally with a broom) so we can get adequate air-flow into the attic. If attic air intakes are restricted, static pressure in the barn will rise, often times resulting in higher electric bills since higher pressure requires more electricity to remove the same amount of bad air and moisture.

It’s a snow storm today so having doors on the north/west soffits closed to prevent snow drifting into the attic with 40-50 mph winds is the right thing to do. However, if you’re short of attic inlet area (and many barns are), you may need to open these doors once the storm has passed. If you have to open and close soffit doors (most of you will end up just leaving the doors closed until late March and be willing to live with higher static pressures) make plans this winter on how you’re going to resolve this ventilation restriction next summer. The recommendation is 1 square foot of attic opening per 400 cfm of air you pull from the attic through ceiling inlets.

Is your attic a restriction to ventilation? Here are some quick guidelines. If your barn is older and only 40 ft wide, a 5.5” south soffit opening is sufficient for 37 cfm/pig at the 1 sqft per 400 cfm rate.

If you have a 50 ft wide barn, a 5.5 ft soffit now must provide air for more pigs per lineal foot of soffit. In reality, it can only provide for 29 cfm/pig for a single wide curtain barn.

This means if you have a double-wide tunnel and only 1 soffit is open in the winter, you may be able to reasonably provide for only 15 cfm/pig before your attic inlet area starts to become a ventilation restriction. 15 cfm/pig most often is less than the amount of air you remove with the 4-24” fans operating in stage 2 of your ventilation system.

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