Exactly! Don’t forget about the radon. My house has two crawlspaces under room additions plus a conditioned basement and both crawls are open to the basement and neither had any insulation. Both crawls were nice and dry, so the first thing we did when we bought the house was to install plastic on the crawlspace floors, seal the vents, and insulate the exterior walls and rim joists. Well, it turns out our plastic and insulation job was not air tight enough and we also needed to add a radon ventilation system because recent testing shows 35pcl in the basement and 25pcl upstairs (should be under 4pcl) – yikes! We are now clearing out the crawlspaces and figuring out our best solutions to the radon issue.

I live in a double wide (27 x 67 feet) on cinder blocks (40 inch crawl ceiling) outside Knoxville, TN. After realizing that its crawlspace was flooding, I found a company that installed a sump pump and drainage matting and 8 mil poly on the entire floor, but said they couldn’t encapsulate because it was a mobile home. It appears that the IRC agrees with them, but I find it frustrating. (All other companies refused to touch it or sent me to their diy store). They recommended the automatic foundation vents, but I was skeptical (both for turning to always open/closed and the tendency of warm air to be humid). Instead I replaced 2 with tiny windows while the others have a foam cover when not in use as vents. I leave the vents closed most of the time as the floor air seal (with R13 fiberglass supported by bellyboard) isn’t very good while running a 70 pint self draining dehumidifier to control humidity. With the vents open, drafts are strongly felt. With the vents continually closed, the musty smells become noticeable. As it is, temperature in winter is about 55 F in the crawl. I didn’t insulate the walls yet, but that was primarily due to termite troubles. Once I verify they’re under control, I wish to use termite treated foam board from just above any intermittent water to just below the top.
I already verified low radon and no combustion outside the seldom used fireplace.
I considered the atmox system, but it seems like a $3k weather station that I could approximately replicate with climate data (and I occasionally hear reports of failure to control smell).
It seems my options are to install fans to circulate air from the far corners to the dehumidifier, invest in an official crawlspace dehumidifier, and/or add an exhaust fan on the vent nearest the bottom of a hill (where I put in a simple French drain on the exterior). Is it reasonable to install an exhaust fan and then only run it on a timer to coincide with the warmer part of the day, when the drafts are least bothersome and the relative humidity is lowest? Under this plan, it may switch to a brief midnight run in summer to expel smells. I would also duct the exhaust fan to the crawlspace floor since most odors likely linger there and air down there will take longer to impact the living space.
While I’m aware of the hvac diverting option, I already often feel chilly in winter and hot in summer, so redirecting 10% into the crawl doesn’t seem feasible unless I first enhance the air seal and/or insulation to need it used less.

Yep, thanks. Definitely a requirement in my area. We couldn’t find a non-toxic and breathable non-tumescent paint so we used stucco.

Cindi, btw, your R23 EPS on the crawl walls should be covered with an approved fire-resistive barrier, but you may have already taken care of that.

Our house is almost passivehouse, so yes it was modeled with the crawl as an underground floor which is part of the conditioned space. Per modeling it appears we will hardly need any heat in the whole house except on the coldest weeks. Is the modeling good enough to really take into account the heat from above coming through the floor? I’m not sure. We also don’t know how well the Zehnder will modulate temperatures through the ERV. (Some say it will others say it won’t.)

None of the heat has been turned on in the house yet, and so far it and the crawl have been pretty warm. (It’s only starting to get really cold here now.) But as for how much they will ultimately run, that will be determined by the thermostat, not the modeling. If they end up running a lot I can put a split head in there, but I suspect they won’t. (We have Daikin VRV Life multi-splits with capacity for several more heads.) We will have Emporia monitors on the breakers so we’ll know how much power the resistance heaters are using.

BTW regarding thermal comfort, we always wanted radiant floors but once we realized how little heat we needed based on our envelope, everyone said our floors would never feel warm.

Cindi, modeling heat loss to ground can be tricky, because determining ground temperature under your crawlspace floor can be tricky. How did you size the electric resistance heaters and frequency of operation? I am assuming you didn’t try to match the total heat loss through the crawlspace floor/walls?

In CZ 6b, they’ll potentially run all year round.

Personally, I would stay clear from electric resistance heat because of safety (having electric resistance heaters out of sight = not great), and efficiency.

Did you include the temperature of the floor above in your model, since your goal was improving thermal comfort? Note: improving thermal comfort does NOT equal satisfying heating loads.

I will let you know soon, Paul! I’m doing a full conditioned crawl, and the house is heated by splits, so I put electric resistance heaters in my crawl to make sure it stays warm. The modeling showed they wouldn’t have to run often but I will be collecting data on how often they do run. I’m in CZ 6b (Ketchum, ID) and used R20 sub-slab and R23 EPS/concrete block perimeter.

I am in a very low humidity area and am venting mine with my Zehnder fresh air system. 50 cfm in and 50 cfm out. So it should be close to perfect balance. Because we are in a high radon area, people here are used to slight positive pressure. They had never done a whole house fresh air system like this before, nor a fully conditioned concrete crawl. But once they wrapped their heads around it a balanced crawl made sense. Of course there is an elaborate radon venting system as well, but we’re hoping that with the good air sealing and Zehnder air exchange, we might get by with a passive system and not need a fan.

We didn’t grade to one low spot but to 4 floor drains placed throughout, which go to a sump pump pit.