How to Keep a Garage-Shop Toasty Warmcomments (12) April 8th, 2013 in Blogs
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 2x6 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."
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