How to Keep a Garage-Shop Toasty Warm
Strawmyers’ new garage will do double duty — providing not only parking for two vehicles but a small shop space as well. The 26-ft. by 36-ft. structure, attached to his house and located in Indiana, will be one big space, built with 2×6 walls insulated to R-19 and 14 in. of cellulose in the ceiling. It sounds like he has the bulk of the project figured out, but he’s still wrestling with the details of how to insulate the concrete slab and now to keep the space warm.
“My original intent was just the ‘standard’ for this area of 2 in. (R-10) of rigid foam under the slab with 2 in. around the perimeter for a thermal break,” Strawmyers writes in a post at Fine Homebuilding’s Breaktime forum. “Had considered 3-in. (R-15) under the slab so it would be closer to the R-value of the rest of the structure; but that size would have to be special-ordered and wound end up being an additional $600 over the 2 in. For this type of application, do you think R-15 vs R-10 would make a ‘felt’ difference that merits the upfront investment?”
It’s no minor quibble, because Strawmyers knows all too well what it’s like working in an ice-cold shop in the dead of winter. “My ambition comes from spending way too many nights in my parents’ unheated, uninsulated pole barn in the middle of winter working on some unexpected vehicle problem, etc. and being just miserable the entire time,” he writes. “The goal when I get this thing built is to move all of my tools over to my place and have a tolerable working space regardless of the weather.”
Strawmyers’ project is the topic for this Breaktime Spotlight.
Is insulation beneath the slab a waste of money?
No chance the extra insulation will be worth $600, writes Rdesigns, neither from the standpoint of payback nor comfort. “Any difference in comfort would be virtually undetectable,” he says.
Insulating walls and insulating a concrete slab are fundamentally different, Rdesigns says, because the heat transfer rates are so different.
“R-10 meets or exceeds the current requirements of the Energy Conservation Code,” Rdesigns adds. “[By the way], in the coldest climate zones, the energy code requires either perimeter vertical insulation to a depth of 48 in., or perimeter horizontal insulation 48 in. wide, but not both.
“Insulating under the entire slab would be an impractical expensive, if that’s what you mean.”
But under-slab insulation is exactly what Strawmyers means.
After pouring the footings and stem walls, Strawmyers intends to add strips of 2-in. thick rigid insulation along the inside of the foundation walls as a thermal break. After placing a vapor barrier and pea gravel in the area of the slab, he would add 2 in. of rigid foam before pouring a wire-reinforced slab 6 in. thick.
“Would you still not recommend insulation under the slab if the actual slab [were] heated (radiant)?” he asks.
Perimeter insulation, vertical or horizontal, acts as a barrier to heat transfer, RDesigns says, but additional insulation under the slab isn’t needed because soil is a good insulator, provided it’s thick enough.
“Nearly all of Indiana is in Climate Zone 5, which means that if you use vertical perimeter insulation only, you need to extend it down at least 2 feet below grade,” he writes. “Admittedly, more insulation and full under-slab insulation will provide additional energy savings, but nowhere near enough to pay you back in your lifetime, even if you heated the place continuously.
“For the intended use of your shop (intermittent), I can’t see the value of installing radiant slab heat, luxuriously comfortable though it certainly is.”
No, a radiant-heat slab is just what you want
It may be a waste of money to Rdesigns, but not to Calvin.
“I don’t want to tell you what to do,” he writes, “but will be happy to suggest blowing your money on a radiant slab. Seems every time I have shop work it’s 10 degrees outside. That means pulling out the propane heater and putting up with the fumes and the still cold concrete. About tomorrow, the room is warm enough to be comfortable.
“If I’d have done it right, like we did in the house, I’d have a happy wife with a warm car and a shop that was comfortable [before] I needed it.”
Stawmyers had considered radiant-floor heat, using a residential water heater as the heat source. But he’s concerned that the slab would take a long time to get warm, much longer than if he mounted a propane heater on the ceiling directly over the shop portion of the garage.
“If the slab is heated all of the time, scares me to think what my added electric costs would be to run the dedicated residential water heater during the winter months,” he says. Plus, he has plans to add a vehicle lift in the garage at some point in the future and worries he might puncture the PEX floor tubing when drilling the floor for anchor bolts.
Strawmyers is right on one count, says MarkH, and that’s the length of time it takes to get the concrete slab warmed up, so sporadic heating may be a problem.
“I would keep some heat going all the time if you use the shop a fair bit,” MarkH says. “I haven’t heard many complaints about heated slabs at all.”
On the question of whether insulation under the slab will help or hinder, MarkH says this: “My unscientific thinking is adding more insulation under the slab may make it colder. This is because the ground temperature can be warmer than the ambient air temperature. Adding a lot of insulation under the slab is useless, because heat rises, and the ground temp is generally closer to room temp than outside air in winter. Even if the slab was heated, R-10 or less would suffice for insulation.”
Other options for heat
PaulCP’s advice is not to heat the slab with hot water but opt for an electric radiant system instead, with the grid placed near the top of the slab and tile used as a finish layer.
“Keep the heating wires as close to the top as possible,” he writes. “Set the wiring up on a simple timer so you hit the switch and come back in an hour and the floor will be warm. That combined with an air heater will make your shop very pleasant to work in ON DEMAND.
“Cheap to install and run, too.”
Count Perry 525 as another vote against a radiant slab. Not only does it take “forever” to warm up a concrete floor, he says, but in order to run a radiant floor system economically it would take at least 4 in. of closed cell foam below and around the perimeter of the slab.
“A better way to heat your work shop is radiant quartz short wave heating,” Perry 525 says. “The joy of quartz heating is it’s like the sun! Switch it on and the heat is there instantly, switch it off and its cold. It only heats the space you are working in, move away and the area is cold. It’s cheap to run.”
There’s an idea that DanH likes. He points out that electric radiant heat is “essentially instant and it keeps you warm enough to work comfortably.
“The downside (beyond operating cost),” he adds, “is that the heat is uneven — one side of you is warm and the other cold.”
Or, try a frost-protected shallow footing
Another approach altogether is a frost-protected shallow footing, says Andy Engel.
“Essentially, extending the foam under the footers and out a few feet from the building saves the concrete and excavation of a frost footing,” he says, and that’s the approad he used in building his own garage.
By controlling groundwater and trapping latent heat in the ground prevents the ground below the footings from heaving in cold weather, Engel says. “In my case, I had to insulate the ground 4 ft. out from the building. Piece of cake. The building has been through three winters and there are no apparent issues. And as my original post mentioned, it’s an engineered design put out by the American Society of Civil Engineers. The insulation details depend on the climate zone.”