Why Sealed Crawl Spaces Perform Better in Climate Zone 4A! In Climate Zone 4A, vented crawl spaces create more problems than they solve. We strongly recommend sealed crawl spaces because they give you control especially over humidity. Here’s the science behind it: When ductwork is outside the conditioned zone (like in a vented crawl space), all duct systems leak. And because homes have far more supply duct than return, most systems end up running negative pressure. That means your house starts p