Crawl spaces are like the Loch Ness monster: talked about, rarely seen, and potentially a bit scary.
The crawl space 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 mind
The reality is hardly any prettier (but hopefully no Loch Ness or Shelobs). Persistent problems live in crawl spaces — 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 crawl space floor, any damage or problems won’t be seen.
If a full basement leaks or floods, sumps and mops will be deployed immediately. In a crawl space, however, unseen moisture can damage wood, concrete, and make its way into the house.
The connected crawl space
That shouldn’t be a problem, right? I mean your crawl space 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 crawl space access door open.
When a blower door test is performed, sometimes the pressure reading doesn’t change, whether the crawl space access is open or closed. Big deal, right? Actually, it means that the holes (and air flow) between the crawl space 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 crawl space?
Yes, 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 crawl space, 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 crawl space.
This can act as a “crap in the crawl space” ventilation system. If the ducts handle cold, conditioned air in a humid crawl space, 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 attics
Another common problem is the vented crawl space. Venting can be a crapshoot in attics, and a vented crawl space 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 crawl space 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.
Crawl spaces can be a pain in the butt. Unlike basements, where the homeowners may see and fix any problems, crawl space problems can fester for ages. And problems that occur in the crawl space affect the whole house. Take care of your crawl space and you’ll take care of the whole house.