No pretty pictures today but this information from my architect buddy
was too valuable not to share.
Here’s a great idea Eric Moser introduced me to. Crawl spaces are normally the dark underbellies of houses where the dirt floor is always damp if not downright standing in water in wet times of the year. Mold and mildew often abounds there, and if cancer-causing radon gas gets into your house, it usually gets there by coming through the crawl space.
The crawl space is as cold as the outdoor air in winter, because you have to ventilate it in order to remove at least some of the moisture so that your floor joists don’t rot… and sometimes, they rot anyway. And so you have to insulate the floor system, almost always with fiberglass batts. Problem is, anytime a plumber or other service person is in your crawl space, it’s really easy to dislodge some of the fiberglass, giving that unconditioned air direct access to your floor.
Plumbers aren’t the only creatures in your crawl space. Most crawl spaces are teeming with all sorts of vermin, from feral cats and squirrels (sometimes having a fight) to rats, bugs, and other creeping things. And all of those unwanted critters probably do more to dislodge your soggy floor insulation than the plumbers do.
When Eric introduced me to the idea of a conditioned crawl space, it sounded at first like the latest way to spend more money, which is unusual, because Eric is normally so practical. But the more I listened, the better it sounded. Here’s what you do:
A. Insulate the foundation walls with rigid closed-cell insulation, rather than insulating the floor system. Unless your house is on a steep slope, there’s probably a lot less surface area of walls to insulate than the area of the floor. So you may actually save money on the insulation.
B. Skip the foundation vents; you won’t be needing them. Savings are minor here, but every dollar counts, right?
C. Use a really good vapor barrier (at least 20 mils thick) and seal it tightly to the top of the foundation wall. Cover the rigid insulation on the foundation wall, and extend it all the way across the floor. Make sure all the joints are taped securely. You’ll need to insulate the band joist above the top of the foundation wall with rigid insulation, but don’t cover this with the vapor barrier, as the band joist needs to be inspected from time to time in order to satisfy termite inspectors or property inspectors if you’re selling the house.
D. If you really want to do the best job, install a 2” thick “rat slab” over the vapor barrier on the floor of the crawl space. This slab doesn’t need to be troweled, nor does it even need to be particularly level… just make sure that it’s not thinner than 2” in spots.
Building a crawl space this way has benefits beyond the elimination of mold, mildew, vermin, rot, and diseases for your family. Your plumber will thank you profusely whenever he has to service something in the crawl space, but it doesn’t stop there. You won’t have to worry about pipes freezing under the house anymore, because you’ll actually be piping a bit of conditioned air into the crawl space. It might be 10 degrees cooler than your living room, but it’ll be much warmer than the winter air outside. This also means that ductwork running through the crawl space isn’t subjected to summer heat or freezing temperatures in winter, so your equipment will be more efficient. You can also put your airhandling unit in the crawl space, where it can be serviced in a clean and dry environment. Like the attic units I blogged about earlier, this can save a couple thousand dollars or more in finished space, because you won’t be needing that HVAC closet next to the great room anymore.
Bottom line: you’ll likely spend a bit more money upfront on a conditioned crawl space. Estimates run as low as $1,500 if you do all the work yourself (without the rat slab) up to several thousand if a contractor does everything for you. But you’ll clearly save that money back before long on service and operation costs alone, and that doesn’t even begin to count the health benefits. What’s your family’s health worth?
Thanks for the great idea Steve and Eric! I will try to use this .
And if you would like to say,”That was fun!” at the end of your project contact me at