Michael Geoghegan is designing a house for a mixed, humid climate and he plans on using an insulated crawl space.
Advanced Energy, a regional energy consulting firm, has published a report on crawl spaces which suggests a dehumidifier can be used as an alternative to providing code-required supply air to condition a crawl space. “I’m a little bit concerned about cold floors in the winter time with this method,” Geoghegan writes in a Q&A post at GreenBuildingAdvisor. “Does anyone have any experience here?”
Detailing crawl spaces correctly isn’t simple. A recent Q&A Spotlight focused on methods for insulating a crawl space, and Geoghegan’s question — “What’s the best way of ventilating a sealed crawl space?” — raises overlapping concerns.
In this Q&A Spotlight, the discussion quickly turns to a broader issue: Are crawl spaces a good idea in the first place? Although they are common in the southeastern part of the U.S., they are rarely used in the Northeast, where full basements are the norm and crawl spaces are an anomaly. Who’s right?
Crawl spaces need an exchange of air
A dehumidifier might be useful for lowering humidity levels in a crawl space temporarily, says GBA senior editor Martin Holladay, but there should be no need to for a dehumidifier to become a permanent crawl space fixture. Because dehumidifiers use a lot of energy, it would be better to remove the dehumidifier once the crawl space has dried out.
Long-range, the question becomes how best to comply with a building code requirement that a sealed crawl space be conditioned. There are two code-approved approaches, Holladay says. Both approaches require the installation of a floor grille to connect the crawl space air with the conditioned home above the crawl space, and neither approach involves a dehumidifier. Builders can choose either:
But in the end, Holladay doesn’t like crawl spaces, and he thinks the idea of installing a dehumidifier in one is a “desperate, last-ditch effort.”
Crawl spaces may have poor air quality without continuously running exhaust fans, he wrote in a 2011 blog on the topic, and radon levels may be 10 times what they’d be in a vented crawl space. “It’s important to emphasize that even a well-detailed crawl space represents a problematic foundation design,” Holladay wrote at the time.
Because Geoghegan is still in the planning stages of a house, he adds, there is still time to “choose a foundation system that makes more sense than a crawl space.
“As I said, don’t start out assuming that you need a crawl space dehumidifier,” he adds. “And concerning the recommendations of Advanced Energy, my only reaction is that it’s usually safer to follow the building code than it is to violate the building code. But if you can negotiate with your building inspector successfully, you may be able to follow Advanced Energy’s recommendations — assuming they make sense to you.”
But in some regions, the bias is against basements
James Morgan suggests that Geoghegan not be deterred by Holladay’s dislike of the crawl space, adding that crawl spaces are the norm in his part of the country.
“I think it’s a regional issue,” he writes. “Here in the Carolina Piedmont we have a similar prejudice against the kind of basement that seems to function well in the North. If your climate and terrain are anything like ours (we’re in Advanced Energy’s home territory) the unvented insulated crawl space with a small supply air vent from the main HVAC system has many advantages and is generally considered standard best practice in the industry.”
Nor does Morgan understand why Holladay thinks basements and slabs are less susceptible to certain types of problems than crawl spaces. He adds, “As to why why good, responsible, intelligent builders continue to prefer crawl spaces in appropriate regional circumstances, how about a clean dry mechanical and electrical distribution and equipment space that is inspectable, upgradable, is relatively inexpensive to construct and suitable for almost all topographical conditions of the region?”
Moreover, North Carolina building codes require daylight drainage for a full basement, which is “not achievable on 90% of the lots here.” While slab construction is possible, it also has its disadvantages.
In the end, Morgan writes, Holladay should take some of the advice he offered in a recent blog: study buildings that successfully handle local conditions.
Crawl space detailing that works in the region
Danny Kelly, a builder in Charlotte, North Carolina, has wrestled with these issues and has eventually settled on the exhaust-fan approach to ventilating increasingly tighter closed crawl spaces. Radon is one reason, and moisture control is another.
“We assume all crawl air is bad air so best to exhaust it from the crawl rather than push it up into the house by pressurizing the crawl with a supply duct. It is very important that all penetrations are sealed,” he says. “Seal at mudsill and band joist, and [make sure that the] crawl door is weather-stripped and closes tightly. [You] need to be 100% sure that the incoming air is coming from the house and not from the exterior.”
Kelly also is learning toward insulating at the floor level rather than the exterior foundation walls (what he would call a “closed crawl” rather than a “conditioned crawl”).
“Cold floor complaints combined with increased hardwood floor issues is pushing us in this direction,” he says. “If you look at the Pinehurst study Advanced Energy did, there is actually an energy penalty in the winter on a conditioned crawl. Closed crawls outperform vented crawls in both energy savings as well as moisture control. There is very little difference in energy savings between a closed and a conditioned crawl.”
Kelly uses a fan made from PVC rather than metal, which lowers the risk of condensation.
Another detail that passes muster, at least in North Carolina, is the use of a dehumidifier in a crawl space. No matter what national model building codes might say, adds Stewart Akerman, the state’s residential building code specifically cites a dehumidifier as permissible. None of the approved methods, he adds, require a connection between the crawl space and the conditioned space above.
Putting local practices into the right context
Ultimately, Holladay and Morgan find themselves agreeing on one point: local building practices should be evaluated on their merits.
“I agree, and I’ve never suggested otherwise,” Morgan replies. “There are three common approaches to traditional local building practices. You can dismiss and disdain them as the habits of old fuddy-duddies who don’t know any better. You can adopt them uncritically because what was good enough for grandaddy is good enough for me. Or you can treat them with critical respect to see what you can learn from them in the light of current building science knowledge, material and skills availability and changing needs.
“I’ve never advocated any but the third approach,” he continues. “In searching for the best building practices of our time and place we’re part of an ancient and continuing tradition, and if we’re enabled to see farther than those who came before us it’s it’s only because we stand on their shoulders.”
Our expert’s opinion
Here’s GBA technical director Peter Yost’s take:
Crawl spaces generally get a bum rap because they are so often done so badly. One of my brothers just happened to call me last week regarding his crawl space in a relatively new home he purchased in Idaho (see Image #1, above).
And part of the problem with crawl spaces is their history. Originally, the code referred to them as “basementless spaces,” as if they were for folks who could not afford a “real” basement.
If your soils and geology make a full basement either too hard or too expensive to achieve, and your building design favors subfloor space for forced-air ducts and mechanical equipment, it’s hard to argue for slab-on-grade over a crawl. And achieving a safe, dry, unvented crawl space is just as logical and attainable as a safe, dry, full basement.
One thing I really don’t understand is designing a new home on a vented crawl space insulated and air-sealed at the floor level versus an unvented crawl insulated and air sealed at the perimeter. If you are going to pay for the foundation space, either put the space to good use or move to a slab-on-grade foundation. I have found in my work that:
- The times that code-level crawl space vents actually provide any significant ventilation and active drying is a fraction of the year.
- Insulating and, particularly, air-sealing at the floor level of a crawl space is a very difficult proposition. It’s possible (see Image #2, below), but hard.
- Monitoring any crawl space (either through sensors or regular routine visual inspections) is always a good idea.
In the case of the crawl space that was later air-sealed, radiant-floor hydronic heating also was installed in each floor framing bay (not visible in the photo). We monitored the temperature and relative humidity of the new crawl space for one full year, also taking quarterly moisture content readings of the mud sill and posts. While the new crawl space experienced seasonal high relative humidity spikes, they never translated into moisture content in crawl space framing above 17 percent.