David Meyer’s Seattle-area home is built over a crawl space, and after stripping out the old insulation and vapor barrier he is ready to re-insulate and seal the area. After looking into his options, Meyer is leaning toward “encapsulation,” meaning the crawl space would be sealed (unvented), with the insulation on the walls, not between the floor joists.
He’ll have 18 inches of clearance to work with. The floor framing consists of what he calls the “old style post and pier construction method” with 4×6 joists on 48-inch centers, followed by 2×6 tongue-and-groove subflooring, and hardwood finish flooring on top of that.
“After cleaning up the crawl space I started doing research into how to insulate the area, and found the encapsulation method,” Meyer writes in a Q&A post at GreenBuildingAdvisor. “I like the method due to not having to run insulation in the floor joists, giving me more space to work down there, and also in keeping the area cleaner for rodent inspection purposes and for when I do plumbing or other type of work.”
There are, however, a few details yet to be worked out.
The house is currently heated with electric baseboard heaters, but Meyer is considering a switch to a gas-fired forced-air system. If he locates the furnace in the crawl space, it would heat the space and the ductwork would not have to be insulated. Is that a better option than putting the furnace into the attic and running a duct to the crawl space? Or heating the crawl space by placing an exhaust fan in one of the crawl space vents and opening a floor register between it and the house? Would the exhaust fan be enough to mitigate radon hazards?
Meyer’s questions are the starting point for this Q&A Spotlight.
You don’t need to heat the space
Don’t worry about the heat, GBA senior editor Martin Holladay advises, because you don’t really need it.
“A sealed, unvented crawl space in your climate will not require any heat to stay warm,” Holladay writes. “Once you have sealed the air leaks and insulated the walls, it will never freeze.”
Many crawl spaces do incorporate an exhaust fan and a register in the floor above, he adds, but those features have nothing to do with heating the space. They are included to control moisture.
Holladay adds that Meyer could install a register from his forced-air heating system to the crawl space. In that case, he should keep the floor grille, but not bother with an exhaust fan.
Here, too, the object is not to heat the crawl space but to provide humidity control.
As for radon control, he adds, start with a radon test to determine whether it’s a problem that must be dealt with. If so, Meyer will need to do more than rely on a crawl space exhaust fan alone.
A gas appliance will probably be too big
Dana Dorsett sees trouble ahead if Meyer goes forward with his plan of installing a gas-fired furnace in the house, particularly if he puts the furnace in the attic as the HVAC installer wants to do.
“Almost any gas-burner is going to be ridiculously oversized for a 1150-square-foot rambler in Fall City, especially after tightening it up a bit,” Dorsett says. “That would be mistake #1. Putting it in the attic rather than in [an] insulated, conditioned crawl space would be mistake #2.”
With an outside design temperature of between 20° and 22°F, and assuming Meyer performs some air-sealing work, the heating load is likely to be less than 15,000 Btu per hour — and that puts him within range of a 1.5-ton ducted minisplit.
There are two models that might work, Dorsett says: the Mitsubishi MVZ-A18AA and the Fujitsu AOU/ARU-18RLFCD. “With either of them you have the benefit of modulated ultra-quiet output (more so with the 18RLFCD than with the A18AA), and at your average wintertime temps you would beat their HSPF efficiency numbers if the ducts are all extremely short, and inside a conditioned insulated crawl space,” Dorsett says.
Under the right conditions, a 1.25-ton or 1.5-ton ductless minisplit also might do the job, and cost less to install, he adds. There might even be local rebates available to help pay for the conversion from electric baseboard to a heat pump.
“Even the smallest gas furnaces are likely to be 2x oversized for your actual loads,” Dorsett says. “If you’re committed to going with natural gas, a condensing gas water heater and a suitably sized hydro-air handler is probably a better solution than an oversized gas furnace + water heater.”
What if the house got bigger?
Meyer replies that he plans to expand the house into part or all of the garage in the next year, enlarging the house to a total of about 1,600 square feet.
“We are in the process of converting to natural gas for cooking, water heating (tankless gas water heater), space heating (forced-air furnace), and possibly clothes drying,” Meyer says. “The baseboard heat and the old electric tank water heater are costing us a fortune. The house is not too open, and has 4 bedrooms and 2 baths (‘1.75 bath’ in realtor speak).”
Even so, Dorsett says, the design heat load will be less than 25,000 Btu/hour, making only the very smallest gas furnace a reasonable option.
“Replacing the electric baseboard with hydronic baseboard, running off a condensing tank hot water heater would allow you to micro-zone with impunity, and never have to suffer the limitations of a tankless water heater,” Dorsett says. “It’s a lot easier to find places to route pipes than it is to route ducts (which is how ducts all too often end up outside of conditioned space). The 50-gallon all-stainless HTP Phoenix Light Duty is a good candidate for this type of combi-system, but there are others.”
A ducted HVAC system would only make sense if Meyer were planning on central air conditioning, Dorsett says, “in which case heat pumps (ducted minisplit or traditional) make a lot of sense.”
What comes first: insulation or vapor barrier?
Meyer’s plan to encapsulate the crawl space will include both insulation for the walls and a vapor barrier to seal the floor. Which comes first?
“Most people install polyethylene on the floor and walls before insulating the walls, but frankly, it doesn’t matter very much,” Holladay tells him. “If you insulate the walls first, with either rigid foam or closed-cell spray foam (and no polyethylene on the walls), everything will still be OK. You can do the floor last if you want.”
If Meyer chooses polyiso foam insulation for the walls, however, Dorsett suggests it would be better to lap the ground vapor barrier up the foundation at least a foot before installing the insulation to prevent the cut edge of the insulation from touching soil directly.
“Holding the polyiso in place with 1×4 furring through-screwed to the foundation puts a few minor holes in the vapor barrier,” he says, “but on the walls that doesn’t matter.”
Insulate the exposed footing, too
The photo Meyer has provided of the crawl space (see the image at the top of the page) shows that some of the concrete footing has been exposed. Holladay suggests that Meyer insulate the exposed areas with closed-cell spray foam, available in a two-part kit.
Meyer seems concerned about cost of adding a required thermal barrier over the two-part foam, but Dorsett says painting it with an intumescent paint would be enough in most jurisdictions.
Alternately, Meyer wonders whether he could use rigid foam to cover the exposed parts of the footing.
“There’s nothing wrong with your plan to cover the horizontal and vertical parts of the exposed concrete footing with rigid foam,” Holladay tells him. “And, as Dana Dorsett pointed out, there’s less heat loss through concrete components that are well below grade than through concrete components that are near grade. So you’re fine. “
Our expert’s opinion
GBA technical director Peter Yost added these thoughts:
On the West coast, it sure seems that there’s a preference for venting crawl spaces and placing the air and thermal barriers or control layers on the underside of the first-floor assembly. I have found it really hard to get continuity of the air and thermal control layers there, so I have a strong preference for installing them on the crawl space perimeter. It does mean you are conditioning or semi-conditioning an extra volume, but that penalty is overcome by what you gain in energy efficiency and moisture control by placing your crawl space firmly inside your building.
You should check with local HVAC folks about the 18-inch clearance in your crawl space, but getting a furnace into this space, much less getting it well installed, seems highly unlikely to me. I’m betting that the space is just too tight.
When in comes to testing for radon, of course you test at the lowest inhabited level of the house. But when do you test? If you test before your work is complete, there is some uncertainty concerning whether you will improve or worsen radon levels on the first floor. And don’t rely on just what your neighbors might say or know; research has shown there is very little correlation of radon levels in buildings that are right next to each other.
Another question is: What type of radon testing do you do? The most accurate radon test is the alpha tracker test, but to get useful numbers you need to test for up to three months when the building is closed up, as it would be during the winter. I have found that the Siren Pro Series 3 electronic meters correlate pretty well with alpha track testing, and they give results in just three days (although testing over longer periods is much more valuable). I like the fact these meters can be set for short-term testing (three-day averages that after three days drop the first data and add in new — a running average) and set for long-term cumulative data collection.
In the photo provided, there is evidence of water wicking up the concrete piling on the right, but also what appears to be a sheet capillary break between the concrete and the wood post. Make sure that you indeed have a capillary break underneath each post.
Finally, on the question of whether you need a thermal barrier or ignition barrier to protect spray foam in crawl spaces: The model building codes allow for spray foam thermal barrier exceptions in attics and crawl spaces where entry is limited to repairs or maintenance. So in this type of crawl space you can use one of these ignition barriers prescribed by the codes:
- Mineral fiber insulation, 1 1/2 inches thick.
- Wood structural panels, 1/4 inch thick.
- Particle board, 3/8 inch thick.
- Hardboard, 1/4 inch thick.
- Gypsum board, 3/8 inch thick.
- Corrosion-resistant steel, 0.016 inches thick.
Alternative ignition barriers — such as intumescent paints or coatings — can be approved by code officials based on testing that follows ICC-ES Acceptance Criteria 377, Appendix X. Be careful: even if someone claims a material qualifies as an alternative ignition barrier, you should really insist on the appropriate test information.
Finally, you may be able to leave spray polyurethane foam (SPF) exposed, so long as the SPF has been tested and approved per AC 377 Appendix X or ASTM E 970. (For more information on this topic, see Thermal Barriers and Ignition Barriers for Spray Foam.)