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Understanding Open Crawl Space Vents: A Shift in Perspective

Understanding Open Crawl Space Vents: A Shift in Perspective

November 29, 2023

At one point, building code stipulated the installation of open crawl space vents. The basic principle behind this was to stimulate air flow in and out, thereby preventing moisture build-up and consequent structural issues. However, as our understanding of home construction evolves, so too do our methods. It's come to light that these well-intentioned open crawl space vents don't behave as initially anticipated. Instead of regulating air flow, they often trap outside air beneath the floors, leading to the air inevitably creeping into the living areas of the home. Hence, experts now suggest sealing these crawl space vents.


The open crawl space vent was once a staple in home design and it had a singular purpose – to allow free air circulation. The theory was that this circulation would keep humidity and moisture levels balanced, diminishing the prospects of mould growth, mildew, and wood rot. Sounds great, right? However, new research paints a different, less rosy, picture.

What actually tends to happen is that the outside air, carried with it all of the particulates, allergens, and humidity inherent to it, becomes trapped beneath your floors and nowhere to go but up, infiltrating your home. Humidity becomes a particularly grave issue during the summer. As warm, humid air enters the crawl space and meets the cooler surfaces within, condensation occurs. This leads to serious concerns about moisture build-up, paving the way for mould, mildew, and structural damage.


To counter the issues brought about by open crawl space vents, the best option is to seal them off completely. Sealing these vents creates what many specialists refer to as a 'closed' or 'encapsulated' crawl space. This encapsulation process involves installing a heavy-duty polyethylene barrier to completely cover your crawl space – including the floors, foundation walls, and possibly even the ceiling.

By implementing this solution, you restrict the flow of outside air into your crawl space, and consequently, into your home. Sealing your crawl space means you not only control the humidity levels within the space, but you also limit the potential for pests to find a cozy home below yours. Moreover, keeping out unconditioned outside air can also improve your home's energy efficiency, reducing heating and cooling costs.


Current building codes have evolved to reflect the problems that come with open crawl space vents. Most new constructions incorporate sealed crawl spaces into the design, shaping the modern understanding of what constitutes a healthy home environment. If your home currently has open crawl space vents, do not fret – making the transition to a sealed crawl space is usually a straightforward process.


Old building code once dictated crawl space vents be installed to allow adequate airflow in and out. Today, our understanding of air flow dynamics and moisture control have advanced, leading us to focus on encapsulation and sealing off these vents. If your house still heeds the old rules of crawl space vents, consider shifting to the current, more beneficial approach. Sealing crawl space vents may indeed prove to be a home improvement task that pays dividends in durability, comfort, and energy efficiency.