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Ventilation under minihomes and crawlspaces – what’s necessary and why

Our inspectors have run into an interesting oversight of late, and it’s one that – if not caught – could cost you a lot of money. Or your house.

In our region, there are a lot of people who live in either older-style trailers, or more modern mini-homes, and it’s common to “skirt” these structures to close off the bottom section. The skirting generally serves two purposes: it improves the appearance, and can help curb heat loss due to high winds in winter.

However, it’s really important to keep these areas open to the outside air, all year long, and the failure to do so can actually cause serious property damage.

Under the National Building Code of Canada, crawlspaces – that is, an unheated area under a home – have to be ventilated. The requirement is a pretty simple formula – 0.1m2 per 50m2 of area. If metric isn’t your thing, then think of it as 1 square foot of ventilation per 500 square feet of area. So if you’ve got a 72x16 minihome on blocks, with skirting, you’re looking at 1152 square feet, or just a touch more than 2 square feet of ventilation.  This can be achieved by using vented soffits, wired-off mesh, or gable vent enclosures.  Just make sure that the venting won’t be covered by snow.

Just recently, one of our inspectors had a contractor challenge this code requirement. [If you want to look up the code, it’s of the NBC, and, just in case you doubt that, clause 9.2.1 of CSA Z240.10.1, which is the minihome installation and securing standard for the country.]

Some may wonder why Code would ask for ventilation in cold months. After all, won’t the ventilation cause cold air to enter the space under the home?

Well, yes, but that air is already going to be cold. (The exception is if the skirting is insulated. In this case, that area must then be mechanically ventilated.) Skirting will help mitigate wind chill will be mitigated, and in truth, any temperature-sensitive elements (like the sewer connection) should be insulated anyway to prevent cold-related freeze-ups.

The issue is condensation. Without air moving through the crawlspace, moisture can accumulate and cause considerable damage. In fact, the section of Z240.10.1 that deals with skirting around minihomes explains why:


When skirting is used, crawl space ventilation and access shall comply with Articles and of the NBC.

Note: Failure to provide adequate ventilation can allow moisture to build up under and enter the building. Such moisture can lead to decay within the crawl space and high humidity in the building, resulting in problems with condensation.

This applies to any floors above ground. That bedroom addition on screw piles? The space underneath has to be ventilated. The failure to adequately ventilate a crawlspace can lead to catastrophic failure.

A sad, but illustrative example comes from a recent file where our inspector was asked to look at just such an addition needing extensive repairs. The joists under the home had been exposed to so much condensation that they had rotted almost completely, with almost every joist showing moisture content in excess of 30 per cent, even though they were well off the ground. Some joists showed as much as 60 per cent moisture content, and were visibly falling apart: wood is subject to rot related damage if moisture exceeds 20 per cent on a continual basis. The reason for this catastrophic failure? Insufficient ventilation: the space had been fully enclosed, and for years, moisture (from vapours seeping underneath from the home above and from water in the soil below) had been gathering with no way of dissipating, because there was no air movement to carry the vapours to the exterior.

This cautionary tale points to a key element of Codes: they may seem onerous, or excessive, but there’s always a reason for them.

Now you know why ventilating crawlspaces and the space under skirted minihome, trailer, or any other floor that is over unheated space is necessary.