Sweat, Mold, and Bugs: The Case Against Vented Crawlspaces
Throughout my time in the construction industry, crawlspaces have been at the center of some interesting, and often unpleasant, experiences. I’ve encountered various animals, been pinned to my belly with floor joists scraping my back, and found myself completely soaked due to poor moisture management. It’s the latter that I want to talk about today. Vented crawlspaces are a non-solution to a serious problem, especially here in North Carolina.
Why Vented Crawlspaces Are Problematic
The idea behind a vented crawlspace is to remove stagnant, moisture-laden air. In theory, this makes sense! But in practice, funneling in hot, humid summer air to solve the problem causes more harm than good. Here's why: condensation and vapor infiltration.
Due to air conditioning, our flooring system stays cool, meaning the temperature of the framing and insulation is often below the dew point of the humid summer air. When this hot air hits the cool surfaces, it condenses. The wood absorbs the moisture, and the insulation traps it. This leads to several problems. First, once the relative humidity of these surfaces reaches 70%, mold can start to form. Second, constant moisture exposure degrades building materials prematurely. Lastly, condensation on the floor system and crawlspace provides the perfect environment for pests.
Now, let’s talk about water vapor. Vented crawlspaces create a constant flow of hot summer air beneath your home. Water vapor tags along with this air and seeps into the house through subfloor penetrations. This increases your cooling load and decreases comfort. Essentially, you’re paying more for less comfort. If your home feels unusually humid in the summer, this is likely a key culprit. The phenomenon of hygric buoyancy—where hot, moisture-laden air rises—is especially problematic in two-story homes with second-floor humidity issues. The first place I’d check is your vented crawlspace.
What Else Could Go Wrong?
Your HVAC system could be contributing to the issue. Many homes have HVAC equipment in the crawlspace. This not only allows moisture to passively enter your home but actively brings it in through the HVAC system. Here's why: ducts always leak. A leaky return duct in the crawlspace can suck in crawlspace air and push it into your living space. Beyond water vapor and humidity, this air can also contain bioaerosols like mold spores, cockroach feces, and dust mites. For more on this issue, see my other blog post. But there's another significant issue that’s often overlooked: radon. Leaky return ducts can pull radon-laden soil gases from the crawlspace and pump them into your home. Think about it—fans are typically used to mitigate radon by drawing gases into vent pipes and exhausting them outside. When ducts leak, they reverse this process.
Supply ducts are problematic as well. Remember the cool first-floor system? Well, supply ducts are also cold and often have minimal insulation (R-4 to R-6). It's common for air to condense on these ducts, especially where cold air escapes through leaks. The other issue with leaky supply ducts is that they depressurize the building envelope. This causes your home to pull air in through crawlspace, wall, and attic penetrations. So, not only are you increasing your cooling load, but you're also affecting indoor air quality.
What Should Be Done?
Convert your vented crawlspace into a conditioned crawlspace. This involves completely air-sealing the crawlspace, properly installing a vapor barrier, and insulating the perimeter walls (with a termite inspection gap). Then, conditioned air should be supplied to the crawlspace. If radon levels are unsafe, soil gas mitigation should also be included at this stage. It’s essential to hire a professional with expertise in conditioned crawlspaces to ensure the conversion is done correctly.
Why It Matters
Vented crawlspaces can negatively impact building materials, comfort, energy efficiency, and indoor air quality. If you're experiencing issues with your crawlspace or excessive humidity in your home, consider contacting us for help!
