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Heating a Detached Garage Being Used as Home-School Classroom

WeekendDIY | Posted in General Questions on

Hi All – I am remote schooling 6 children in a detached garage in Boston, MA. The garage is 18×19, vaulted hip roof ceiling. The walls are insulated with old fiberglass kraft-faced batts (2×4 studs), covered with 1/4 MDF. The Ceiling roof is insulated has 2x6s  rafters and fiberglass batts and is drywalled with 2 coats of mud. The windows are single pane, maybe 70 year old divided lite windows. There are five, they are about 24×48, rope and weight sash. There are two garage doors, uninsulated and once egress door.

After initially looking to heat the space with a 7,500W heater, the electrical upgrades necessary (over $3K) were too expensive. Talked with an Aerobarrier company, who thought it would also be too expensive to use them since (we hope) this is temporary.

The suggestion was to decrease the volume of the space we are heating by getting 1″ 4×8 sheets of foil faced rigid insulation and laying them across the chords creating a ceiling (I would also tape the seams of said boards and will be duct taping the small interior gaps on the garage doors as well as adding weather stripping the garage door). Each child would get their own heater so that the heat is directly on their space, allowing area close to the child to be heated. Thus, we would not be trying to heat the entire garage. We would also put plexiglass on the interior of the windows to stop drafts.

The reality is that the garage space is not going to get school house warm – 68F-72F. Most likely we will end up in the 55F range, but with the heaters with each child, hopefully they can stay warm. I believe in this plan. Are there any other suggestions or alternatives to this plan that folks would recommend that would be temporary and would not break the bank? I am climate Zone 5.

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  1. Expert Member
    BILL WICHERS | | #1

    A ceiling as you describe would reduce the total volume of the space to be heated, which would make it easier to keep warm. Heat tends to rise, so high ceilinged areas do tend to feel a little cooler down below. The reduced surface area of external walls will also help reduce heat loss a little more. Technically that's not a code assembly though, unless you used something like Dow Thermax (more expensive), and even that MIGHT be an issue on a ceiling -- I'm only familiar with it's use on walls.

    I would recommend the use of 1/16" PETG on the windows instead of plexiglas (acrylic). PETG is an acrylic/polycarbonate blend, and it's MUCH more durable than acrylic but not as expensive as a polycarbonate. You don't need a lot of thickness either, since the insulating work is all done by the air you trap between the window pane and the temporary plastic layer (similar to how a storm window works). Acrylic alone is very brittle and tends to shatter on impact, PETG is more like polycarbonate and will bend instead, which is better and also safer.

    A 7,500w heater would probably need a 40 amp, 240 volt circuit. You can get 5,500 watt "garage" heaters that will work on a 30A, 240 volt circuit which will be a lot cheaper to install. I'd suggest you look into that before you put in multiple small space heaters. A typical space heater is 1,500 watts, and is designed to completely max out a typical 15A, 120 volt residential circuit. What that means is if you want to run 6 such heaters (one for each child), you need 6 separate circuits. I'd try getting a 5,500 watt (note that they run in the range 5,000-5,760 watts) installed, then supplement it with a single 1,500 watt heater to get up to around the 7,000 watts you were originally targeting.

    Make sure you insulate that garage door. Air sealing is a good thing too, but those garage doors aren't usually insulated much if at all. You can get kits for this purpose that consist of multiple rigid foam panels cut to fit the door sections. You can also just tape the same rigid foam board up that you were planning on for the ceiling.

    I'll mention again that you probably have some fire code issues here insulating this way. Rigid foam generally needs a thermal or ignition barrier (typically drywall, but sheet steel and hardboard can also be used in some cases) on the exposed face in a living space like this. I would be especially concerned with multiple space heaters and children in such a space. A single, larger heater permanently installed would be safer than multiple small ones.

    And another warning: do NOT use typical extension cords with heaters running for long periods. You need the flat "air conditioner" cords in 14 gauge for this, or the heavier 14 and 12 gauge extension cords. Do NOT use the small two-conductor 16 gauge homeowner cords here, they will get hot and are very risky.

    *** Think SAFETY FIRST, not COST.***


    1. WeekendDIY | | #4

      Thanks for the reply. Unfortunately in order to run a single large heater, such as the 7,500W heater we were looking at, the electrical in the garage would need to be updated. It’s not my home, but my kids are there, so I don’t know all the specifics, but I believe the house is 100amps and the garage is probably 40-60amps I think. The cost to upgrade was ~$3,700 and that is if the underground conduit (it is an old steel pipe) is still able to have wires fished through. Excavation was anothe 3k, which I think is absurd, but that is beside the point. Overall, we are trying to have our kids stay warm and be educated. We are not in Boston proper, just outside the city a few miles. I hear you on the fire code and I appreciate the concern, we’re just trying to do something temporary and within reason within a garage without spending thousands of dollars that the home owners are not really able to spend.

      1. Expert Member
        NICK KEENAN | | #6

        Is there a circuit breaker panel in the garage? If not, it's unlikely you have more than 20 amps at 120v. That would limit you to one 1500 watt heater.

        1. WeekendDIY | | #7

          Yes, there is a sub-panel in the garage with four breakers in there.

  2. Expert Member
    Dana Dorsett | | #2

    [edit- it looks like Bill beat me to the punch as I was typing away. :-) ]

    >"The suggestion was to decrease the volume of the space we are heating by getting 1″ 4×8 sheets of foil faced rigid insulation and laying them across the chords creating a ceiling "

    Any foam board would need half-inch gypsum board or similar between the foam and occupied space for fire safety. You MIGHT be able to get away with a fire rated foam board (eg Dow Thermax) if it were simply storage space or a basement, but probably not for a 6 kid classroom.

    The amount of heat going out of 500 square feet of R13 - R19 batt-insulated ceiling is about the same (or even less than) the losses from the fifty square feet of air-leaky antique single panes. Reducing that to only 350 square feet with foam board ceiling isn't the best bang/buck. Concentrate on improving the windows plus air sealing the building as a whole.

    Depending on the door design may be able to get a good bang/buck out of insulating the garage doors with cut'n' cobbled fire-rated Thermax, and upgrading the weatherstripping for those doors.

    For a temporary situation, air sealing the antique windows with caulks & tapes then using purpose made shrink-wrap type window treatments for the second is cheaper than cutting interior storm windows out of plexiglass. The windows are a fairly standard size- you may be able to find some surplus-new or used triple-track clear glass exterior storms on craigslist for cheap (or free), which would cut the window losses in half on a more permanent basis, and protect the antique windows. As a DIY even new custom-sized clear glass (not low-E) storms would run about $400 for a set of five at box-store pricing, which is cheaper and more impactful than a 350 square feet of 1" polyiso foam-board ceiling even if going with cheaper, non fire-rated stuff.

    If you still want to do something about the ceiling, 500 square foot rolls of PERFORATED (about 5 perms, not a vapor barrier) radiant barrier are about $70 at box stores, and don't have a fire rating issue. Supported by 16-24" o.c. strapping mounted perpendicular to the truss chords the radiant barrier would be able to support R13-R30 batts, potentially available for cheap or free on craigslist.

    You don't really need a 7500watt / 25,000 BTU/hr heater unless you need to heat it up from cold to pretty-nice in only an hour or so. With the windows treated and some air sealing you're probably looking at a load of around 6000 BTU/hr at 30F outdoors (roughly the mean temperature in Boston in January) , 68F indoors. Six seated kids and one standing adult puts out a total of about 2000 BTU/hr, or 600 watts, which isn't nothing, and you'd probably be using 100-200 watts of fluorescent or LED lights for another 340-700 BTU/hr of heat input. On sunny days you'd even be getting some solar gains.

    If you had a DIY-able 115V 1 ton mini-split (with pre-charged linesets) can be DIYed for about a grand, and can work on a standard 15A circuit with margin to spare. Most are capable of delivering 12,000 BTU/hr @ +17F,even more at higher temps, and would probably pay for itself in electricity savings in one winter compared to resistance space heaters. I doubt Boston is going to get to in-class publication education before next spring, given current COVID trends in the Bay State:

    1. WeekendDIY | | #8

      Thanks for the reply and all of the info - the mini-split option sounds interesting. As I mentioned, we are trying to not spend a lot of money for something that is temporary AND will simply just go back to a 2 car detached garage whenever this pandemic is over. Any foam insulation is temporary and we are not trying to legally convert this garage into living space, but I do appreciate the concern for what is code and what is safe, especially the information regarding the electrical. As I mentioned in my original post, the electrical would need to be updated in order to bring 240v out to the garage and update the panel. The estimate to do that was VERY expensive $3,700 and that is if they are able to use the existing conduit running to the garage. As you mentioned, two of the big problems I believe we need to address are the windows and the garage doors and hopefully air sealing them and insulating the garage doors will help.

      1. Expert Member
        Dana Dorsett | | #11

        >"...the mini-split option sounds interesting."
        >" ...the electrical would need to be updated in order to bring 240v out to the garage and update the panel. The estimate to do that was VERY expensive $3,700 and that is if they are able to use the existing conduit running to the garage. "

        Which is why a cheap 110V DIY mini-split makes some sense. Most of those currently on the market are re-branded (Pioneer, Senville, Mr.Cool, Gree, etc.) Midea units with Toshiba compressors. (Midea is the largest manufacturer of AC equipment in the world, having swallowed up most of their Chinese competition. All Carrier branded mini-splits are Midea too.) Just be sure to get a 110-120VAC model- they all have nearly identical versions with similar part numbers set up for 208-230VAC.

        I mis-spoke on my prior post- most of the 115V one ton units need/want a 30A breaker, not 15A, and #10/3 wire. (Though in practice a 1-ton 115V mini-split usually won't trip a 15A breaker under normal circumstances.) The 3/4 tonners (which would also handle the load) need/want a 25A breaker and #12/3 wire. As long as the feed to the sub-panel can handle at least 30A it'll be fine.

        While it pays to watch a few installation videos (YouTube has many) and read the manual, there is nothing too technical about installing them. Typical internet pricing for a 1-ton is about $800 + shipping (often free shipping) which includes a 16' or 25' pre-charged line set, with modest upcharges for longer refrigerant lines. In Boston bracket-mounting the compressor unit on the wall, on a side protected by roof overhangs is usually best. Leave 2' or more of clearance below to keep it above snow drifts. (In Worcester make that 4'.) A mounting bracket & line-set hide/conduit and some wire & breaker make up the remainder of the cost- it's a few hundred, not five. It's an afternoon project for a reasonably handy person (with an occasional helper to lift things) who has done it at least once, maybe a full weekend day for a first times. If you screw up big time and lose a bunch of the refrigerant charge any reasonable AC contractor would be able pump it down and re-charge for under $500.

        There is usually excess line set that has to be coiled up, since it's not a DIY to cut them to length and re-charged line sets. While many do fine coiling up and leaning the excess line pretty much vertically against the wall, in some cases that can create a vapor-lock problem with liquids collecting in the bottom of the loop on the suction line. It's better to place the excess line coil horizontal than vertical, avoiding the vapor-lock risk.

  3. walta100 | | #3

    Safety must be you first priority the windows sound large enough to qualify as an emergency exit if they still operate freely and are low enough. If you cover them with rigid plastic they can no longer be used. I would cover the windows with thin film like the one in this link.

    It is very very unlikely the electric service to the garage is adequate to heat the garage with resistance heat plus lighting and several outlets for the computers. My guess is two 240 volt 15 amp circuit is required for heating and two 120 volt 20 amp circuit one for lighting and a separate circuit for the outlets will be required.

    Two heaters like the one in this link hard wired and attached to the wall would be much safer than portables that can be knocked over.

    Understand electric heaters are very expensive to operate even with low electric rates. From what I understand you are likely to have some of the highest rate in the US.

    If we guess it will take 1 1500 watt heater running 24 7 to keep the garage warm. 1.5 kw per hour x 24 hours x 30 days with an electric rate of .20 per kwh = $216 per month

    Safety says no exposed foam and no extension cords allowed.


  4. Expert Member
    Akos | | #5

    I had similar service power limits for my wife's studio and ended going with a mini split for heat. Best decision ever.

    Not only much cheaper to run than resistance heat, way quicker to heat up the place and AC in the summer time.

  5. user-2310254 | | #9

    Is there a natural gas service? I don't link combustion appliances, but a garage heater would deliver a lot of BTUs for not much money (if a gas line is already present).

    1. WeekendDIY | | #10

      Unfortunately there is no gas running to the house or the garage.

    2. Expert Member
      NICK KEENAN | | #12

      Or bottled propane? According to Dana's calculations above something like this would be overkill:

      It was the smallest I could find in a quick search. It is vented and installed which I think is the minimum you would want for safety (as opposed to ventless portable heaters).

      A gallon of propane is around 90,000 BTU's. At a maximum load of 4,000 BTU's per hour (per Dana's calculation) a 5-gallon tank would last 100 hours. That's probably the cheapest way to get through the winter.

      1. Expert Member
        BILL WICHERS | | #13

        I would avoid non-vented gas-fired heaters in a small, enclosed space like this. Personally, I'm not a fan of non-vented appliances in general.

        An electric resistance heater will likely have the lowest installed cost, assuming the garage has sufficient electrical capacity. Electric resistance heat is expensive to run though. A minisplit is going to be a lot cheaper to run, and will use less electricity per BTU of heat output because it's essentially pumping some thermal energy from outdoors to indoors and not making all of it's heat output from scratch.

        Note that if the garage feed is a 40A, 240V circuit, that's 9.6kw peak, but a little less than 8kw for a heater since you don't want the heater to exceed 80% of the supply circuit capacity. You'd probably be able to get a 5kw heater to run on that circuit and still have enough capacity left over for some lights and computers. A 60A, 240V circuit is good for 14.4kw peak, or a little over 11kw worth of heater.

        It's also worth mentioning that 6 separate typical 1.5kw space heaters is still 9kw. If you don't have sufficient electrical capacity for a 7.5kw heater, you won't have enough capacity for 7.5kw worth of smaller heaters, either.


        1. Expert Member
          NICK KEENAN | | #14

          The heater I linked to is vented.

          1. Expert Member
            BILL WICHERS | | #15

            I looked a little closer (I read the manual) for the heater at that link. It DOES mention a vent, but it’s not really a ducted system. It basically has a vent that gets mounted in the wall near the heater. There is no ducted exhaust to vent combustion byproducts to the outdoors.


  6. 1869farmhouse | | #16

    While definitely not ideal, combustion heaters coupled with redundant CO detectors can be done safely. There would always need to be competent adult supervisions of course.

    I get the feel that there isn’t someone with the experience around to install a mini split, otherwise the electricity wouldn’t be a limitation. I spent several years in an old farmhouse heated by butane/propane/kerosene, whatever I could get cheapest at the time.

    1. WeekendDIY | | #17

      Installing a mini split is something that could be done by myself. The problem, as I have written above, is that it’s not my home and I am not familiar with the electric there. There is a sub panel in the garage, but I think the amps out there are limited. For example, a circuit for an outlet on the outside of the garage kept tripping when a small pancake compressor and job site table saw were running. I just don’t think there is enough juice out there for an uptick in load without sinking money into a garage that will not be living space after COVID.

      1. 1869farmhouse | | #18

        Probably worth verifying the amps then, but if there aren’t enough, is the home too far away to pull a length of 10 gauge wire? If it’s just temporary, could even be pulled through conduit above ground.

      2. Expert Member
        NICK KEENAN | | #20

        If the sub panel in the garage is fed from a main panel there should be a breaker at the main panel that shuts it off, that will tell you the ultimate capacity of the sub panel. If that breaker has two legs then it's a 240V circuit.

      3. Expert Member
        BILL WICHERS | | #21

        DCContrarian has the right idea here: check the ampacity of the feeder supplying the subpanel in the garage. Since there is a subpanel, you ALMOST surely have 240 volts out there already. A quick conversion is as follows (adjusted for the 80% rule):
        20A breaker: 3.84kw
        30A breaker: 5.76kw
        40A breaker: 7.68kw
        50A breaker: 9.6kw
        60A breaker: 11.52kw

        If you have a double pole (usually this means two handles with a tiebar between them, but sometimes there is one handle with a breaker twice as wide), you have 240 volts going out there. If you have a single pole breaker (one handle, and not twice as wide as the others), then you only have 120v service which halves all those wattage numbers. Note that all the same numbers apply if you have a fused feeder (instead of a circuit breaker) too.

        Assuming you don't have any huge loads out there, a 40A or larger will let you run a typical 5kw heater, and even have a little leftover for lights and computers.

        If a side outlet tripped with a compressor, that probably means that particular branch circuit was overloaded and not the entire subpanel. Did a breaker trip in the subpanel, which would mean it was a branch circuit issue, or did a breaker in the house feeding the subpanel trip? The other thing to check is balance between legs on a 240v system. If, for example, you have three 1,500 watt space heaters, and a 30A, 240V supply to the subpanel, you could potentially trip the 30A breaker if all three heaters were plugged into the same "side" of that 240v circuit (which would total 37.5 amps). If you just moved one of the three heaters to the other "side", you'd have only 25 amps on the higher loaded side, and 12.5 amps on the other, and you'd be OK. That's another easy thing you might check. With sever imbalance issues, you can often tell by feeling the face of the breaker. On a double-pole breaker, if one side seems noticeable warmer than the other, you probably have an imbalance issue.


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