If a full basement leaks or floods, sumps and mops will be deployed immediately. In a crawlspace, however, unseen moisture can damage wood, concrete, and make its way into the house.The connected crawlspaceThat shouldn’t be a problem, right? I mean your crawlspace is entirely separate from your house … right?Actually, no. When conducting a blower door test, we check the house in several configurations, one of which is with the basement or crawlspace access door open.When a blower door test is performed, sometimes the pressure reading doesn’t change, whether the crawlspace access is open or closed. Big deal, right? Actually, it means that the holes (and air flow) between the crawlspace and first floor are so large and so profound that it doesn’t matter if the door is open or not.All that grimy water, dust, and pest waste can float up into your house and get breathed in. Ugh! What could be worse? How about locating ducts in that crawlspace?Mechanical monstersYes, thank goodness there’s no giant mechanical system moving tons of air into the house. Oh, wait — there is: your ducted HVAC system. Often your HVAC system ducting is run through the crawlspace, which is just an awesome way to move all the dust, moisture, rodent poop, and chemicals into the rest of your house.A well taped and mastic-sealed duct shouldn’t be an issue. In theory. The reality is there are usually loads of poorly installed and unsealed ducts running through the nasty crawlspace.This can act as a “crap in the crawlspace” ventilation system. If the ducts handle cold, conditioned air in a humid crawlspace, then condensation is in your future. If the ducts have a foil sleeve, the condensation may accumulate within the sleeve. There is a scary potential to create a breeding ground of awfulness.Venting problems are not just for atticsAnother common problem is the vented crawlspace. Venting can be a crapshoot in attics, and a vented crawlspace can be a problem in humid climates. The idea behind ventilation is improving the interior air and conditions.What if the exterior air is worse than the interior air? Then a hole into the crawlspace will let in problems. This can be adorably compounded when some right-thinking owner puts a dehumidifier down there. It’s a noble thought to try dehumidify the whole planet.There is one last issue to discuss: any efforts to insulate the ceiling joists with fiberglass batts. (I detailed the thermodynamics of this approach in an article called “Should I Insulate My Basement Ceiling?”) Practically, fiberglass batts are not great. Between gravity, moisture, and cold, there can be problems. It’s a rare install where three-quarters of the batts aren’t hanging down or fallen off entirely.Crawlspaces can be a pain in the butt. Unlike basements, where the homeowners may see and fix any problems, crawlspace problems can fester for ages. And problems that occur in the crawlspace affect the whole house. Take care of your crawlspace and you’ll take care of the whole house.Erik North, the owner of Free Energy Maine, is an energy auditor and home performance specialist in Westbrook, Maine. He is also the author of the Energy Auditing Blog. Crawlspaces are like the Loch Ness monster: talked about, rarely seen, and potentially a bit scary.The crawlspace is that empty, barely accessible pit under the house filled with swampy water, cobwebs and spiders the size of dinner plates. (That link was the Google result for “spiders the size of dinner plates.” Enjoy). At least that’s what we imagine.Out of sight, out of my mindThe reality is hardly any prettier (but hopefully no Loch Ness or Shelobs). Persistent problems live in crawlspaces — ones that stick around because the homeowner never sees them. My all-time most ridiculous find was a running hair dryer, warming water pipes that had frozen. The homeowner’s daughter had done it without telling anyone and forgotten about it in the intervening two months. The electric bills were … impressive.There are other widespread problems that remain similarly out of mind. Moisture is a major one. Whether it’s running water, standing water, or moisture evaporating from a sandy crawlspace floor, any damage or problems won’t be seen. RELATED ARTICLESBuilding an Unvented CrawlspaceGBA Encyclopedia: CrawlspacesHow to Insulate a Basement Wall CAD Details from the GBA Library: Building Plans for Conditioned CrawlspacesFine Homebuilding: Sealing a CrawlspaceAn Open Discussion on Closed CrawlspacesBLOGS BY ERIC NORTHHow to Avoid MoldSealed-Combustion Appliances and Hot Tub PartiesDoes More Wall Insulation Reduce Cooling Costs?Should I Insulate My Garage Door?