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Are You Making These Mistakes with Your Garage?

Garage Air Conditioning Mistake

Sometimes people do the craziest things. Take that photo above, for example. That’s a new home being built in Austin, Texas. The arrows point to three air conditioning ducts. In the garage. Yes, they’re air conditioning the garage.

Mistake #1: Air conditioning the garage

Now, the first mistake here may be adding air conditioning to the garage in the first place. Most of the time, a garage is used simply as a garage. That is, the homeowners store cars, lawn mowers, high school yearbooks, a Smoking Donkey cigarette dispenser, a “time capsule” in a shrimp factory box that you’ve had since your first job in high school, and that old Mr. Microphone. (Hey, Good Lookin’, we’ll be back to pick ya up later!)

If it’s just going to be used as a garage, there’s no need to air condition it all. It’s a waste of energy that will provide the owners littlemr microphone if any benefit. If they’ve got some really valuable stuff that can’t take the heat or humidity out there (like that Mr. Microphone), then bring it into the house.

Why is it a waste of energy? Because cars, lawn mowers, and junk don’t need to be air conditioned. Also, standard roll-up garage doors aren’t insulated or air-sealed, so you’re paying to condition space that’s constantly reheated by Mother Nature.

Are there good reasons to air condition a garage? Sure. If I’m a woodworker and don’t care if my cars sleep in the driveway, I’d air condition my garage in Austin. Or if I want to turn it into a mancave, momcave, or teen hangout space, it’s definitely gonna chill. Or maybe you can’t build a house in a certain price range or Austin neighborhood without air conditioning the garage and have any hope of selling it.

Mistake #2: Air conditioning the garage the wrong way

OK, so let’s say you’ve got a good reason to air condition the garage. That’s fine, but you should absolutely never, ever do it the way they did here. Matt Risinger, an Austin home builder who owns Risinger Homes and does things the right way, explains why in the video below.

The big potential problem here is bad indoor air quality. Let me show you why. An air conditioner is supposed to be balanced in its air flow. A 3 ton AC pulls 1200 cubic feet per minute (cfm) of air (nominal) from the conditioned space and then puts 1200 cfm of air back into the conditioned space.

What Matt showed above is a system that puts maybe 400 cfm of air into the garage. It’s still pulling 1200 cfm from the house (assuming a 3 ton system) but only has 800 cfm left to put into the house.

Hmmmm. 1200 out, 800 in. That’s like having a 400 cfm range hood running whenever the AC kicks on. That’s gonna depressurize the house.

One of the basic rules of air flow is that for every cfm in, you’ve got to have another cfm out, and vice versa. So if the AC is pulling 1200 cfm from the house and putting 800 back in, it’s got to find another 400 cfm somewhere. It does that, as Matt said in the video, by pulling from the garage, under the walls, through the can lights, and also down a gas water heater flue if the house has one.

That last one is potentially deadly. If the air conditioner pulls air down the flue while the water heater is firing, that means the exhaust gases aren’t going up the flue. That’s called backdrafting. It can put carbon monoxide into the house and into your lungs while you’re sleeping. Not such a good idea, eh?

Even if you don’t backdraft a gas water heater, you’re pressurizing the garage and depressurizing the house. That means all the bad stuff in that garage air wants to find a way into the house.

If that’s not enough for you, how about the fact that the building code doesn’t allow it? (Thanks, Sean!) Just because it’s not allowed doesn’t mean it doesn’t happen, as the video shows. Building inspectors don’t catch everything.

The right way to air condition a garage

mini split heat pump ductless head air conditionerIf you really must air condition a garage, you need to install a separate system for the garage only. The best way to do this is with a mini-split heat pump or air conditioner. It’s perfect for this application because it’s small, doesn’t require ducts, is very energy efficient, and can be turned off when you don’t need to cool that space.

The photo at left shows a ductless head at rough-in of a house (not a garage but you get the idea). If you build the garage well and size the system properly, one head should do the trick.

Of course, another, less expensive, option would be to use window units. They’ll do the same thing for less cost. You’ll get more noise and lower efficiency, but if it’s not used much, that’ll probably be acceptable.

There you have it. As with most things, there’s a right way and a wrong to air condition a garage. Then there’s the question of whether you should be air conditioning it at all.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.


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I had an interesting discussion with a trio of upstate New Yorkers on Facebook recently when I posted Risinger’s video there. They were horrified that anyone would even consider air conditioning a garage. I pointed out that the US uses more energy for heating than for cooling and that sometimes people in cold climates heat their garages and put heat strips in driveways, sidewalks, and roofs. That didn’t go over so well with two of them. Had we not had that discussion, however, this article may have just been about mistake #2 above, so thank you, Blake, Matt, and Dale. I’d really hate for Energy Vanguard to be thought of as “Energy Waste Vanguard.”


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This Post Has 22 Comments

  1. So much for codes…&nbsp
    So much for codes… 
    R302.5.2 Duct penetration. Ducts in the garage and ducts penetrating the walls or ceilings separating the dwelling from the garage shall be constructed of a minimum No. 26 gage (0.48 mm) sheet steel or other approved material and shall have no openings into the garage. 
    & common sense

  2. For many of us, an old
    For many of us, an old fashioned window unit would be an appropriate answer. With a low enough duty cycle, it becomes less important to invest in high efficiency. A heresy perhaps, but rational.

  3. Sean:
    Sean: Thanks for the code reference! And yeah, sometimes common sense is in short supply it seems. 
    M. Johnson: Not a heresy at all. A window unit accomplishes the same result as a mini-split for less cost. You get more noise and lower efficiency, but as you point out, if it’s not used much, it may well be the best choice.

  4. Air conditioning a garage if
    Air conditioning a garage if you are a woodworker! Wow! I am! I installed my old air handler from the home into the garage and added a hydronic coil. So my garage is heated. I am CZ 4 not 2. 
    I did not choose to AC the garage at this time. I did install a 220 CFM inline fan in the roof. (Garage is not finished with any dry wall.  
    I exhaust the hot air from the peak of the garage and try to pull in air from the ground level over night. Just watching some thermometers nothing recorded, so this is anecdotal only. 
    I find the solar gains find the mass of the garage floor and cast iron in the machinery and over a typical day over 90° F will warm the area up to almost the high for the day by 6pm. The ambient is then dropping and the inside is climbing. So the fan starts to pull the warmer air out and provide cooler air to begin dropping the mass temperature. 
    I have been cracking the garage door to get the lower opening. My observation is that just pulls cooler ambient air across the thermal mass of the driveway (faces west) and is probably not working as expected. So I am evaluating my options to find an alternative to this. 
    In spite of that observation. I can still obtain acceptable results for a Saturday AM of wood turning. 
    The winter works great. The only time the furnace came on last winter was when I moved the stat up from the min setting of 40° F. We had 2 mornings of 5°F lows. < Design Temp. Did not kick on. 
    I think the key here is air sealing; insulation and thermal mass. 

  5. Great thread – the issues
    Great thread – the issues raised, especially about IAQ impacts associated with conditioning a garage using the house’s system. 
    Doing so is not just against code but also absolute lunacy when one considers all the nasties kept in a typical suburban garage – fuels, insecticides, fertilizers, painting supplies, pool chemicals. 
    One variation I see is where a subdivision’s model home is sold off – As a model home the garage often serves as a sales office and is therefore conditioned. Obviously those ducts need to be completely removed (and the system downsized…yeah, right!) when the home transitions to conventional usage. 
    I have a close friend who converted a garage into a teen cave. He wanted me to add ducts to it from the main system. I told him I would do it only if he removed the garage door and framed the opening, so he went with a windowshaker – it works fine even though it looks like Hades, IMO. 
    I agree that a minisplit is absolutely the way to go, and I advise new home builders to put a 3″ conduit and power for a future minisplit in the garage if there is any chance some or all of it may be converted / conditioned. 
    I wouldn’t be too quick to condemn windowshakers for low efficiency – it is important to distinguish EER from SEER. Shakers are available in EERs of 11+, and that corresponds to a SEER of 15 or so, not as good as a mini, but not half bad. 
    At some point I hope SEER and EER ratings incorporate standby power – seems hardly fair that they don’t.

  6. You missed one common mistake
    You missed one common mistake. Most attached garages are 100% sheetrocked. Folks move in, decide to take in their garage for a gameroom, man cave, or something. Then they simply add another duct (no return) to serve the space. I have seen it done many times in Texas. One thing not considered was that now the A/C system is too small to serve the entire home. Secondly, they did not think about the garage walls NOT being insulated and cold rooms in a hot humid climate really make for interesting science discusssions.

  7. It is necessary to heat an
    It is necessary to heat an attached garage in colder climates if mean temp remains below freezing for long periods. Not doing so invites frost movement which can cause damage to the garage and house. In our climate almost everyone who does not use a garage has a remote car start and idle their vehicle for long periods to make them drivable. An efficiently heated garage would likely use less fuel and create less pollution. That said, it is not necessary to heat a garageto the same level of comfort as a house. I usually heat mine to about 45 degrees fahrenheit. One disadvantage of a heated attached garage. I have found myself downtown at minus 30 with only my house slippers on.

  8. I use my garage as my shop. I
    I use my garage as my shop. I live in Texas, so I want conditioned air in my shop on an as needed basis. I consulted the A/C installer who installed the original system when the house was built. He dropped two vents off the large system into the garage and added a manifold that I switch on/off as needed to bring cool air into the garage. No return in the garage. Sounds like this might have been a bad idea but I have been using it this way for a number of years without any obvious draw backs. Someone let me know if I need to do something different!

  9. Theresa in
    Theresa in the short the answer is both that is fine & stupid. the fine part is based on the end usage I am pretty positive it meets codes.  
    As for the its a stupid idea, well that depends on… do you really want sawdust & other items like fumes from stain, etc… found in the shop being able to get into your house? 
    The best item is a separate system for the garage & one for your house 
    As for the sizing argument for conversions – not buying that one in many cases as the house AC in many locals is almost guaranteed to be oversized. Should they fix the garage door & blow insulation in the attic & walls – yes. Model it & size it right

  10. Sean – thanks for your
    Sean – thanks for your response. I don’t understand how the sawdust, fumes, etc. you refer to enter the house if there is no return in the garage. Is it just from the presure imbalance and so the system is actually pulling air through the garage? All the walls, ceiling and overhead door are all super insulated since I knew when I built the house I would be using part of the garage for my wood shop.  
    Not debating your information. I am ignorant of these things and just trying to understand. At the time I added the vents it was $800 compared to the $3500 price tag for the split system. Penny wise and pound foolish? 

  11. TJ- some coal miners breath
    TJ- some coal miners breath coal dust all their lives & DON’T die of lung disease. Still, I would not recommend this as healthy practice. Your garage/shop may not house any glue, paint, or household chemicals. But what about the next owner who does woodwork, or repaints cars? Alison and Matt are correct. It’s a bad idea, not code compliant, and there are better ways. The fact that the cost is higher does not make it wrong or foolish.

  12. Have to add another example
    Have to add another example of this practice. My friend built a home in NE Atlanta. The design included a master bdrm over the garage. The code inspector REQUIRED a vent in the garage so that the floor above would not be cold. 
    At least he did not make my friend put in a return, and it was only one small vent. 
    My house – same county & municipality- presumably diff inspector- did nor require the garage vent. But I did see them in other homes in the area. 
    Just because the building passes inspections, does not mean it meets code, is built right, or is safe. We all have to take responsibility for our own health and safety. A little “active caring” for others is a good idea as well. 
    take care and Sail on. 
    __/) * 

  13. I live in Southwest Louisiana
    I live in Southwest Louisiana and am a certified building code official. It is against code to tie any HVAC system from the house into the garage. I can see why someone would want to air condition a garage in a hot climate but you have to have a separate system, a mini split or window unit will meet this need and the code, The main reason for not having ducts in a garage tied to the house system is carbon monoxide poisoning, the other reason is for fire safety. Garages tend to be places where flammable liquids are stored and if you have a fire in the house it can spread through the ducts. Having a return air duct in the garage is even worse because carbon monoxide can be drawn into the house from the garage and in case of a garage fire will suck smoke and flames into the living area of the house. Attached garages also require self closing hinges on any door leading to the living area of the house and all separation walls must be sheetrocked with at least 1/2″ drywall, ceilings in garages that have living space above must have 5/8″ Type X drywall. This is a good post and may help save a life, thanks for posting! I will post this on my page. If anyone has any other questions they can contact me through my website at , I will be happy to help.

  14. Hi, this is a very
    Hi, this is a very interesting discussion. I landed here through twitter post at a time when i’have been asked to air condition a welding shade. This is in abu dhabi where temperature is 52c at peak. here comes the worst part…the roof is uninsulated plane sheet metal and new added doors will also plane shutters . No matter how i convince client that it is not possible and getting a sandwich panel is a must…he keeps playing the record that do it. He has a advantage of chilled water network passing by his place and he wants to tap it. My cooling load is massive (210kw for 420 sq mtr area)… and a 3″ pipe will do. I plan to get fresh through coil (fahu) and internal fcu to cool to maintain temp…but i am worried about condensation? With this shutters open to bring the works in and out, the humid air will give shower to the area? Is increasing pressure could be solution?

  15. It’s such a waste of energy
    It’s such a waste of energy to have AC in the garage but I’ve seen people do it.

  16. It’s also “such a waste
    It’s also “such a waste of energy” for solo commuters to drive big SUVs, but I’ve seen people do THAT, too. 

  17. code inspectors from city’s
    code inspectors from city’s scare me different interpretations allowed, some incompetence and 1 city i lived in major NE 2 million city corrupt 
    ie 14 plumbing inspectors 13 were indited for kickbacks and bribes 
    wen in doubt hire a insured inspector if you cannot ascertain hoe its to be done. 
    p/s i use a window shaker in garage in fl works reasonable and not too expensive 12000 btu 115v 2 car

  18. Early aprile we leave for
    Early aprile we leave for Alaska for 6 or 7 months. Our vacation home is in Ariz and sits all closed up except for house air is set for 90 degree F..We leave an auto and golf cart in garage where it gets 105 to 117 degrees for months at a time.. Our garage is an old one with old metal door.. We put in an A/C and saved 6 golf cart batteries and a car battery’s life. Just to replace the shortened life of the golf 6 batteries was a cost of $1400.00..Attack has blowen insulation. Water heater is flushed and drained and we put towels under the garage doors.. thank yoy.. and any comments please do…

  19. We just bought a house in
    We just bought a house in Lewes, DE. We have two dogs and two cats….and the cats ruined our last house by peeing all over. We have decided that they will live in the garage in this new house…which is NOT A/C’d. It’s pretty hot….and we want them to be comfortable. I found this discussion in my search for options for adding AC. We DO have a window in the garage…so it sounds like a window unit is in order. I know….finding other homes for the cats could be an option. But…not really. They are “older” and it wouldn’t be fair.

  20. Well beyond the common sense
    Well beyond the common sense of a wise use of energy to cool / heat a garage – the largest characteristic is being overlooked – the National Fire Code. 
    There is a reason 1 hr fire rated full ceiling & walls separate the garage, so even if you were to run metal ducts as posted earlier you MUST install FIRE DAMPERS.  
    Secondly – with consideration to CO, you can NEVER share the HVAC from the home to the garage.  
    In Fla IF you opt to heat cool the garage w/ the homes HVAC – most code interpretations require you to REMOVE the garage door (to resist the temptation or future use to bring the Harley inside) and block the wall up to ensure it will never be used as a garage.  
    IF you choose to heat or cool your garage – it MUST be stand alone either split HVAC, Mini Split, RAC.

  21. Hello, In N Atlanta suburb,
    Hello, In N Atlanta suburb, our attached garage and slab catch the evening sun and get very hot. Was thinking of two things 1) Insulate steel garage door with two pieces foam insulation board that is silvered one one side, 2) A passive siphon vent one at top and one at bottom to get hot air out at night and replace with cooler air. Any ideas (also trying to get neighbor to plant(at my expense) a couple of privacy bushes to cut the sun in front of garage a bit. I would appreciate any ideas, helpful hints or criticism. Thank You

  22. 2012 International
    2012 International Residential code N1101.4 (R101.4.5) Change in space conditioning.  
    Any nonconditioned space that is altered to become conditioned space shall be required to be brought into full compliance with this chapter. So if you are in a jurisdiction which this code has been adopted without revision this may be very difficult to accomplish.  
    The biggest issue I see is: N1102.4.1.2 (R402.4.1.2) Testing.  
    The building or dwelling unit shall be tested and verified as having an air leakage rate of not exceeding 5 air changes per hour in Zones 1 and 2, and 3 air changes per hour in Zones 3 through 8. Testing shall be conducted with a blower door at a pressure of 0.2 inches w.g. (50 Pascals). Where required by the building official, testing shall be conducted by an approved third party. A written report of the results of the test shall be signed by the party conducting the test and provided to the building official. Testing shall be performed at any time after creation of all penetrations of the building thermal envelope.  
    I have not seen a garage door that is extremely well sealed, so I would wonder if it is even possible. 

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