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Q&A Spotlight

How to Heat a Garage

Exploring some low-cost options for keeping an attached garage comfortable for both people and vegetables

A hydronic heating system for a garage. GBA reader Chuck Jensen produced this radiant floor system for his garage with an on-demand direct-vent gas water heater and PEX tubing in the floor.
Image Credit: Chuck Jensen

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…

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  1. toymaker | | #1

    my Wisconsin garage
    My unheated 34 x 12 foot garage shares a long wall and end wall with my house. The shared wall is insulated to R32 with thermal breaks. The garage ceiling and outside walls are insulated to R20 and the 10 foot door to R8. The foundation is insulated to R10 with exterior foam. I have carefully monitored the garage temps. Over 3 years the garage has never gotten below 38 degrees, even with below zero stretches outside. Its usually around 50 at more usual Wisconsin Winter temps of 10-30. On the days when I need to work in the garage I just leave the inside passage door open and the temps jump 10-15 degrees after about an hour. This door is on a landing 3 feet below the house floor so the cool air slides right down the basement stairs to be recirculated by the forced air furnace. I had anticipated needing to add a heater to the garage for working and considered a direct vent wall heater, but so far it is not necessary. Gotta love lots of insulation.

  2. Tim C | | #2

    my Wisconsin garage
    I've got a bit less insulation than John, and mine still reliably stays above freezing all winter (but it's usually around 40). The bigger issue for storing things is that it also stays cool in the summer - and without any dehumidification it can be pretty moist in there.

  3. ecdunn | | #3

    Insulate the floor and wear a jacket
    My 1977 uninsulated attached garage is pretty cold in winter. I don't waste the space out there by storing a car. It would get in the way of my table saw! I have spray foamed the attic in the whole house and insulated above the garage ceiling to isolate that space from the house, up there. I have put an inch of foam on the exterior. I intend to put down an inch of rigid foam on the floor and cover that with ¾" T&G plywood as I feel that is where a lot of the cold feeling comes from. That will create a great wood shop floor. I may go further and tear off the drywall and insulate those walls with batts. Add in a task electric radiant heater and a jacket and I will be very comfortable on the few days I go out there in the winter. Also, I have made a point of storing paint and other toxic supplies in a detached shed out back.

  4. rlewis9184 | | #4

    My Iowa Garage
    I have a 3 car garage that's roughly 800 sq ft and has 12 foot ceilings. The three walls are 2x6 with fiberglass batt insulation, vapor barrier, and drywalled. The ceiling has about 15" of cellulose insulation blown in giving it an R-value of 57. One wall is partially shared with the house but there is no heat gain from the house into the garage. I installed a Modine Hot Dawg 75,000 BTU ceiling mounted furnace that is power vented and with a separate combustion so it gets no air from inside the garage. Without the furnace the garage stays about 32-40 degrees when the outside temp is 0-15 degrees. Since the doors face the west and the outside is painted brown, when the sun is out it will heat up to 50 degrees. Wind is the killer and drops the temp quickly because it blows in around the door frame - can't help that.

    When I go to work in the garage - change the oil, wash the cars, etc - I usually fire up the Modine and set the thermostat to 65 degrees which is perfect. It will take about 30 minutes to stay steady at that temperature. Once it stays 65 the furnace will kick on about once every 15 -20 minutes and stays on for about 5 minutes. Many times I will leave this on overnight and once the floor and walls get warmed up the furnace only kicks on once an hour for 5 minutes. In my opinion, this is perfect for the occassional user since you only pay for it when you need it and the Modine furnace plus installation was about $800.

  5. demouser | | #5

    Chicago area
    A rule-of thumb I have used for an upper Midwest climate and a moderately insulated garage is about 10,000 BTU per car space. Also look at the Williams wall mount gas furnace. It fits between the wall studs and is easy to install.

    A modest garage hydronic heating system can use a commonly available water heater, mixing valve and pump. No need for an expensive setup like shown in the article picture unless perhaps you get into 4+ car size or extremely cold climates.

    IMHO one should avoid non-vented furnaces. They add to much CO2 and humidity to the garage making for headaches and rust problems.

  6. riverguy99 | | #6

    My $10 garage heater
    I likie to keep things simple. Our forced-air furnace is in my oversized two-car garage with a 10 ft. ceiling. The main plenum supplying heat to the upstairs of the house is exposed for a short distance right there in the garage, so I cut a 6 x 12 inch opening in it and used a Home Depot sheet-metal adaptor for installing a standard floor register into the 12" duct. Since the opening is about 8' from the floor and hard to reach easily, instead of using a standard floor register, I made an aluminum door that is hinged at the bottom and closes with a magnetic cabinet catch at the top. I attached a 2' long rod to the door so I could reach it easily to open/close it. When open, the furnace heats the garage along with the house at no noticable loss of airflow inside the house. The garage has insulated walls and I insulated the 16' overhead door with rigid foam panels. The standard flexible seals all around the door do an acceptable job of sealing it from air leaage. The flat aluminum door on the plenum has a strip foam seal on it. On a winter day in the mid 30's, the garage stays at 68 degrees very nicely with this system. The same system could be used in any garage where you can get access to a heat-delivery duct, peraps needing to cut into a wall or ceiling to access it. It's cheap, and it works great!

  7. GBA Editor
    Martin Holladay | | #7

    Response to Skip Thomsen
    The main disadvantage to your system -- or any furnace installed in a garage -- is that when your garage register is closed, the forced-air system pulls some garage air into the ductwork and sends that garage air into your house. Since most garages are used to store gasoline, paint, solvents, and lawn chemicals, this is a very bad idea.

  8. Gough | | #8

    Heating a garage
    Here in the Pacific NW, electric prices are low compared to the rest of the country. Even so, a lot of the electric furnaces that come standard in manufactured houses get swapped out quickly. As a result, they are often available for free or for a nominal fee from HVAC contractors. That's what we've used for several garages and shops. While not as nice as a radiant slab heating, the initial cost is hard to beat and the response time is great.

  9. shaldon | | #9

    Heating a garage
    One of the options was a min-split heat pump but I haven't seen that used by anyone - cost maybe? We're looking at that for our new shop where, even if we had natural gas, we'd be concerned about flame.

  10. axgold | | #10

    Chuck Jensen system
    I installed PEX in my garage floor with the intent of adding solar hot water heating but when I saw the system that Chuck Jensen installed for his garage, I thought that might be a simpler solution; however, I am somewhat intimidated by all the gauges and other aspects of this system without professional help....and as a consummate DIYer was wondering if there is a source that anyone can refer me to that helps me understand what all would be required for such a system.

  11. Digger837 | | #11

    Double duty furnace.
    Found a great deal on a 60k btu house furnace 100$ 92% installed hat in our garage/work shop and also installed a 4 in filter that we use to filter out any air born saw dust. This unit heats the garage very fast and filters the air even faster. I used to us a 10k but propane salamander snd it was costing me maybe 3$ a day but temps were from 60-80 be cause it didn't have a thermostat. Down side to this system is the concrete slab remains cold for hours and I would love to have installed a Pex before the garage was built. (we store all chemicals in a detached shed.)

  12. DavePinNC | | #12

    How I heated a Charlotte-area garage
    I've put in several Pex-loop heated floors in garages over the years and they have all been gas-fired water heaters (tanked and tankless) - until the last one. For that one, the client wanted to stay all electric so instead of a gas-fired heater I used a 60kw tankless electric unit. The plumbing was good bit simpler, and since it was a well system it didn't even need an expansion tank. Did a few other tricks with the slab to help retain heat, and it seems to be working great. The owner keeps the floor at 60 degrees and a couple of weeks ago said he had only actually heard the pump running once since the install (late fall). This past week has been very cold (for our area) with some night-time lows near single digits. I sent an email to the client today and he responded that he did hear it running a couple of times on the morning after the 10 degree night, but he went back in around mid-day and it was not running. He checked the floor temp with a digital thermometer both times and found it to be 56 in the morning and 58 mid-day (thermostat still set on 60 with a 4 degree kick-in). So I will have to conclude it is practical to heat a garage (26'x32') with an electric tankless heater. I have attached a picture of the control setup. The floor has 485' of 1/2" pex in a single loop.
    If anyone has any questions about the system, shoot me an email. You can get me thru Profitt Custom Homes dot com.

  13. zulu53 | | #13

    I Believe in KISS
    Here is what worked in Calgary through one winter. R12 door. One exterior R24 wall. 1 interior wall to unheated space (partly earth with R24) - 1 interior wall to heated space. 100W light bulb on continuously when temp started to drop below 0C overnight. Worst period (-40C day to -45C night for one week - garage dropped to +1C ( I was ready to put a space heater if the cold snap continued, but it did not luckily. Only use car for going to work and shopping in the clod snap. When day temp above -20C then a few trips for outdoor activities. Bottom line - in 90% of NA attached garages do not need heating other than from an incandescent light bulb - and you get light too!!. Keep it simple - DO NOT over-design, any increased savings will be overwhelmed by the cost of the solution (cost in $ and the consumption of materials of construction - e.g. more iron=more mines).

  14. hughw | | #14

    I guess I'm resurrecting an old thread again, but it doesn't address one major question I have regarding heating a garage, the ICC 2018 code regarding energy.

    We're in the process of designing a fairly large new home on Martha's Vineyard which will have a two car attached garage. Going back to the original thread, our client would like some heat in the garage "for storing garden vegetables, cans of paint and anything else a spouse may not want in the house" and also so that the cars will not be really really freezing cold when they get in on a very cold day....50 degrees would be nice, 40 degrees would be ok.

    My question is not how to accomplish this with heating and insulating systems, but how to meet, or not meet, code requirements. We really don't want to install a system and insulate to meet Zone 5 requirements. Is there a work around that would allow some minimal insulation, perhaps R20 wall and roof along with a couple of inches of EPS below the slab, but not run afoul of the code? I did see something about defining an Unconditioned Space as a space below a certain minimum amount of Btu/sqft for its climate zone, and below a design temp of 50 degrees (or maybe 40), but can't find that reference again..

    1. Expert Member
      Michael Maines | | #15

      Hugh, if your clients can afford to heat their garage they can afford to meet current code requirements for conditioned spaces. It's not a stretch.

      1. hughw | | #16

        That's true....there are some detailing issues where it may be very difficult to properly insulate all the walls, achieve continuity, etc....also, I have no idea how to deal with air leakage with garage at.

        1. Expert Member
          Michael Maines | | #17

          Hugh, section N1102.1 (R402.1) in the IRC does provide an exception to meeting conditioned space requirements if the peak design rate energy usage is less than 3.4 Btu/h•ft² or 1.0 watt/ft². You could do a manual J calculation to determine if you meet those criteria.

          There is some information here on making garage doors relatively airtight: It's not very promising for overhead doors, unfortunately.

          1. qofmiwok | | #18

            Mike, you ever tried Thermotraks? I wonder how it does with airtightness.

  15. qofmiwok | | #19

    Our main house and shop (6B) are 2x6+2" exterior insulation, but the remaining part of the garage is just 2x6. One wall is the house and one is the shop, and most of the remaining 2 walls are garage doors and man-doors, so the current plan is not to do exterior insulation on the rest. But I'm questioning that right now since it's our last chance to change. There is R20 EPS below the slab; required by code if we planned to heat it at all. We're thinking of running PEX in the concrete just in case we want to use it to keep the garage at 50F all the time. The PEX isn't that expensive, just wondering if it's worth the trouble based on these comments.

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