If you’re lucky enough to have a garage, you already know it can be used for more than keeping your car out of the snow. As Kent Jeffery explains in a Q&A post at GreenBuildingAdvisor, garages also are useful for car and equipment repairs, and for storing garden vegetables, cans of paint and anything else a spouse may not want in the house.
But in order for a garage to serve those purposes, the temperature has to be above freezing — and for much of the country that means a source of heat.
Jeffery is in the midst of a house construction project, but already is thinking ahead to what his heating options might be for the garage. He’d like to keep a temperature of 45°F with an occasional boost to 65°F while he’s working on a project.
The options he’s considering include a direct-vent natural gas heater, electric resistance heat, a ductless minisplit heat pump, an unvented natural gas heater, or a portable heater used only when he needs it.
Any suggestions? That’s the topic for this Q&A Spotlight.
The no-heat option
Writing from the Chicago area, GBA reader WD tells the tale of a neighbor who installed a fancy gas-fired furnace in his three-bay garage but then found he rarely uses it. Maybe that’s because an attached garage picks up enough residual heat from the house, as WD found in his own case.
His 1989 house is anything but superinsulated, so WD added some insulation to the access doors, put clear plastic over the windows, and added 2 inches of polyiso foam insulation to the moveable garage doors (not an easy task).
“The result was a garage that was noticeably warmer in cold weather and cooler in warm weather,” Jeffery writes. “It’s about 5°F warmer than ambient during cool weather. I’ve used the garage for winter construction since then and wore a jacket but didn’t need additional heating.”
The bottom line, he adds, is that “I had little or no justification for a heater.” If Jeffery’s garage is going to be superinsulated like the rest of the house, he may find that the last option (heat only with a portable heater when it’s really necessary) is worth a try.
That’s essentially the approach also adopted by Nick T. in Minnesota. He recently added insulation in the garage attic, and at least in mid-November was enjoying temperatures 30 degrees above outside ambient air.
When he needs heat, Nick uses an electric infrared heater plus a box fan in the door between the garage and house. His high-efficiency furnace does most of the work.
A radiant floor with a direct-vent water heater
Chuck Jensen decided to heat his 24 foot by 26 foot detached garage in Climate Zone 6b with a radiant floor system consisting of PEX tubing buried in the concrete slab.
“For the heat source I used the smallest on-demand Rinnai propane direct-vent [water heater] available,” Jensen says. “It is set up with a Taco pump feeding the input and a manual bypass valve for circulating without the Rinnai in the loop. Control is just a timer that runs the pump at intervals for a short period. There is no thermostat. The Rinnai fires up when the pump runs due to the flow.”
The heating system “buffers nicely,” he adds, and keeps vehicles and tools good and warm, and he likes the arrangement so much he’s repeating it in a new detached garage that he’s building at a new house. “Very simple and requires very little room,” he says.
But when the concrete floor has already been poured, this good advice comes too late, Jeffery replies. In fact, Jeffery has a natural gas boiler and PEX radiant heat in a detached garage/shop — too bad he can’t use this approach in the garage.
Gas or electric infrared heaters
For temporary heating for a weekend project, Charlie Sullivan suggests an electric infrared (IR) heater, a simple unit with a quartz element and reflector but no fan. “If you want to go all out you can mount those on the ceiling so there’s IR heat everywhere,” he writes, “but you can also get a portable one and just put it where you are working.”
IR heaters might be a good solution, Jeffery says, but the goal here is to protect canned goods, not so much provide personal comfort. “So,” he says, “really looking for a low-level ambient heat of air and objects… at low cost.”
Dana Dorsett suggests the heating load would probably be about 10,000 Btu/hour, but a 30,000 Btu/h heater might not be too much if the garage were normally at 45° or 50°F and Jeffery wanted to bring the temperature up fairly quickly.
“Non-condensing air-delivery gas-burners are going to have a problem at 45°F indoor temps, but a condensing version could work,” he says. “But ceiling or wall-bracket mounted gas radiant heaters don’t have that problem, and would be far more comfortable while the place is still coming up to its temperature, since it is heating the objects in the garage (including the humans) directly, not heating the air first.”
These heaters are common in large warehouses, he adds, “since comfort can be achieved at lower temperatures, limiting the comfort shock and shortening the comfort recovery when large doors get opened and closed.”
Be cautious of benzene threat
Donald Endsley points to a concern about attached garages that applies regardless of a heating system: elevated levels of benzene inside the house. “Benzene is a chemical pollutant that basically evaporates from gas and oil stored in the garage, and also enters the garage through unburned hydrocarbons in vehicle exhaust,” he says. “Basically to counteract this the garage needs to be at a lower air pressure than the house is. That means you really need to add a continuous exhaust fan to the garage.”
Longterm problems associated with exposure to benzene include anemia, leukemia, and possibly non-Hodgkin’s lymphoma and multiple myeloma, he says. It’s a known carcinogen with no known safe exposure level.
Although he hasn’t confirmed his hunch with any tests, Endsley estimates that a fan moving between 15 and 30 cubic feet per minute would be about right.
Our expert’s opinion
Here’s GBA technical director Peter Yost:
I think we all can agree on these points:
1. Insulating a garage door can be cheap and easy (either DIY or with one of many commercially available solutions, such as ), but air-sealing a door is really hard. I checked in with local builder members of the and they all agreed that while gasketing between individual door panels and the concrete floor can be done, air sealing at the head of the door and along the side tracks is damn near impossible.
2. True radiant heating (not in-floor “radiant”) is the best heating solution for quick response and situations where there is a lot of air leakage.
3. Radiant floor heating systems — hydronic or electric — are high mass, not quick to respond at all, and not a good match for situations with lots of air leakage.
Jeffery is actually asking for two types of heating: background, steady heat for perishables and intermittent task heating. That is a bit of a tall order. Fast-acting radiant systems are well-suited for the latter because thermal comfort is based largely on the operative temperature (about 50% contribution from air temperature and 50% from mean radiant temperature) and in a garage you can quickly boost the mean radiant and achieve comfort with a reduced air temperature.
But that low air temperature and high mean radiant temperature can’t be sustained over long periods of time because delivered radiant heat is inevitably re-expressed as conduction and convection. The air temperature will gradually rise, unless you have a lot of air changes. So while there is greater thermal comfort and efficiency of fast-acting radiant systems for task heating, those advantages don’t translate well for background steady space heating.
The bottom line? Insulate and air seal the garage as best you can to minimize what you need to deliver for background steady heat and use fast-acting radiant heat for your tasks. (For more information, see Space Heaters).
I would recommend electric ceiling radiant heating panels for these reasons: they provide good, unobstructed “viewing” angles up and out of the way on the ceiling; they’re easy to wire and more panels can be added as needed; and they modulate for both background steady heat and fast-acting radiant task heating.
The only drawback may be that their surface operating temperatures (around 165°F) may not lift the mean radiant temperature enough for all tasks in the garage, but you could augment the ceiling panels with a task panel installed on the underside of a shelf above your workbench; the task panels are standard wall plug units. (The ceiling panels are hardwired either for 110 volt or 220 volt service.)