Morgan Martin’s dilemma is what to do about the crawl space beneath the house, identified in a recent energy audit as a source of air leaks and energy loss.
The joist bays above the crawl space have been insulated with fiberglass batts, but the subflooring has not been air-sealed. A local green building company recommends removing the batts and taping and caulking all the gaps in the floor (that is, the crawl space ceiling). Once that’s accomplished, the builder lists three options:
- Put the fiberglass batts back in place with no additional insulation;
- Apply rigid polystyrene insulation, then reinstall the fiberglass batts; or
- Forget the fiberglass and spray BASF Spraytite closed-cell foam on the underside of the floor sheathing.
There are two complications: “Please know before I go on that I have chemical sensitivity issues,” Morgan Martin writes in this Q&A post at GreenBuildingAdvisor , “so this is why this decision is such a big deal for me.”
There’s also a concern about rats. The crawl space had been infested with them, and while it’s since been cleaned and sealed, Morgan questions whether the polyethylene sheeting that covers the dirt floor will be enough to keep them out in the future, and that may affect whatever option he chooses.
But the main issue seems to be the potential health effects of polyurethane foam, discussed in a number of previous Q&A posts. “We have the option of doing a 2-in. layer of the foam and then putting the fiberglass back up over it or doing a 4.5-in. layer and not using the fiberglass at all,” Morgan says. “Even with all this reassurance it will be safe for me and my family, I have severe reservations about using the spray foam — even though the green builder feels it is safe and is the best option. While I trust him, I always need to do as much research as I can due to my chemical sensitivity.”
Foam shouldn’t pose problems
“During that time we completely air out the house and run the ERV non-stop to help make sure there are no pockets of air that might contain any chemicals,” Sean writes. “While this is probably overkill (especially based on the VOC readings after just a few hours), we like to make sure.”
Another suggestion comes from Torsten Hansen, who says that he has clients who are concerned about off-gassing from polyurethane foam he provides them with a sample to live with before they decide. “I am sure your contractor will be happy to do the same,” Torsten says. “One lady put a string through our sample and wore it around her neck before she was confident enough in the product to proceed.
“Caulking an taping the crawl space ceiling would be a nightmare,” Torsten adds. “If you find that you are okay with the spray foam, your contractor can apply a thin coat of closed cell material to the area, which should do the job.”
When properly sprayed, foam is inert, says Bob Coleman. With careful application and venting as the spray cures, there should be no residual chemicals left. But, he adds, the installer has to know what he’s doing.
Make the crawl space a conditioned space
GBA senior editor Martin Holladay agrees with Morgan’s builder that the most effective way of sealing the floor over the crawl space is with spray polyurethane foam.
But, he says, the best approach is to seal all vents and insulate the crawl space walls, and in the process turning the area into a conditioned space.
This would move the thermal and air barriers to the crawl space walls, eliminating the need for air-sealing the floor above. “That way, it doesn’t matter if the floor above your crawlspace isn’t air-sealed,” Holladay writes. Moreover, crawl space remediation experts routinely seal spaces where dirt floors are covered only with a layer of polyethylene sheeting, so that shouldn’t be an issue.
Hansen also favors a conditioned crawl space, and refers Morgan to research by Building Science Corp.. “Conditioned crawl spaces work very well, and closed-cell foam is the most foolproof material available to construct them,” he writes.
If Morgan Martin has concerns about chemical sensitivity, adds James Morgan, the best solution is to install rigid foam at the foundation walls. It’s a “foolproof answer,” he says, and it can always be removed if there’s a problem. The same can’t be said for sprayed foam.
“By the way,” he adds, “vented crawl or no, you absolutely have to deal permanently with the rat issue, period. And if you have any ductwork and/or mechanicals in your crawl space, sealing that baby up is really a no-brainer.”
Our expert’s opinion
Peter Yost, GBA’s technical director, offered these thoughts:
I am a big fan of unvented crawlspaces for three reasons:
1. Getting a continuous air and thermal barrier on the crawlspace perimeter is much easier and certain than on the underside of the first floor assembly.
2. Crawlspace vents rarely provide anywhere near enough air exchange to really consider the space vented, especially in humid climates.
3. Most people either do use or are sorely tempted to use crawl spaces for something: storage, HVAC components, timeout space for teenagers, etc.
There is plenty of information on GBA to get the unvented crawlspace details right (see the reading list above). While there may be some very minor energy penalty for increasing the total volume of this conditioned space of the home, I have yet to see actual utility bills or modeled results that bear out any significant increase in total space conditioning costs.
In terms of the best approach for a home where one or more occupants are chemically sensitive: the empirical approach of test samples in proximity, especially overnight, is crude but the only one I have any faith in. The problem with that approach with site-formulated and site-applied spray foam of any kind is that an aged sample or even a fresh one may not be the same as what actually happens when the spray foam is applied on site. Everyone, but particularly chemically sensitive individuals, should look for experienced spray foam installers, ones who have been certified and adhere to the ABAA spray foam training and certification process.
And in terms of dealing with rats: regardless of what approach you take — vented or unvented crawl space, for chemically sensitive clients or not — a warm, dry, and fully contained crawl space is critical. I bet it is more likely with an unvented approach, but I show my bias. As the saying goes, if you can find holes big enough to throw a rat through, you have a more than just venting to worry about.