In Southern California, during new construction with a finished garage, the cost of adding insulation at this point should be low.

But if the space will not have any active cooling or heating elements (outside of a tankless water heater and, perhaps, a cat), is there any value in insulating it?

Everyone loves going from a cool house in to a hot garage or vice a versa. Would insulating it make any difference? Would it not get as hot in the summer or as cold in the winter?

Seems to me that it would retain whatever temperature it have longer (whether hot or cold) with the insulation than without, and one could "reset" the temperature of the garage by opening the door for a few minutes to the outside (for example, open it in the cool morning in the summer hoping to better maintain that during the day). Just not sure if it would be enough to notice or bother with.

Historically the garage has mostly been closed vs opened several times a day for cars. Whether that will be the case at the new house is up for debate.

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    Is it an attached garage it a separate building? Dec 4, 2018 at 17:32
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    Don't forget that insulation helps with sound. When your teenager starts a band, that extra insulation may soften the sounds emanating from your garage. It could keep your relationships with your neighbors on good terms.
    – B540Glenn
    Dec 4, 2018 at 19:42
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    @B540Glenn, it's also good for the standard hammering, sawing, vacuuming, swearing, etc. that goes on in a DIYers garage, too. Dec 4, 2018 at 20:19
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    If you ever have a fire in your garage, insulation can slow the spread of it. A tenant demonstrated that convincingly on one of our rentals. Dec 4, 2018 at 21:41
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    ...All I can think about is the poor kitty on winter nights if you don't. =(
    – jpmc26
    Dec 4, 2018 at 23:16

4 Answers 4


Oh, yes

It's a cornerstone of passive solar design.

We spent $2.4M on a large building we intend to never heat. We insulated it to the nines. Here's what that does.

Extremes of temperature are moderated by building's thermal mass*, which (with the insulation) helps it resist changes in temperature. This affects

Condensation, which is significantly reduced or eliminated. This makes life much easier for the contents of the building.

If you don't have enough thermal mass*, you can add some by installing large barrels of water or antifreeze.

* ”thermal mass" is the terminology used in building design, but it's actually a misnomer. Mass/weight does not store heat. Atoms do. And atoms vary in mass dramatically from 1 for hydrogen to 207 for lead. But an atom's heat storage ability is about the same, 20-30 j/mol/degK, most 24-26 j/mol/degK. Different materials contain different mixes of atoms, so their heat capacity by mass is all over the map. Water is ideal.

  • How are the water "thermal mass" / heat sink's situated in the building? Are they in a basement? Attic floor? Maintenance areas of each level? Are they sealed drums.. I find this interesting - more details would be appreciated. Dec 4, 2018 at 22:54
  • "Mass/weight does not store heat. Atoms do." ...Atoms are mass... Maybe you mean chemical bonds do? Also, if you're gonna be technical, "heat" is a type of energy transfer, and is not a synonym of thermal energy (at least going by the formal thermodynamic terms).
    – jpmc26
    Dec 4, 2018 at 23:17
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    @jpmc26: What Harper wrote was clear to me, even if the use of "atoms" was imprecise - the amount of heat you can store is not a function of the mass, but also has a factor of the specific heat of whatever compounds that mass consists of. Dec 4, 2018 at 23:31
  • @jpmc26 you disagree because you disbelieve because my statement is bold. Atoms possess mass as one of their characteristics. However mass is highly variable, from hydrogen (1u) to carbon (12u) to iron (58u) clear to uranium at 238u. So 6 pounds of water and 207 pounds of lead have the same number of atoms. When you look at the thermal storage capacity of substances, it correllates very poorly to its mass, and very well to the number of atoms in it. Check it out. It's not chemical bonds, that would only come into play if we burned it. Dec 5, 2018 at 0:23
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    It is not just atoms which store heat. Hydgrogen has a specific heat of about 14 kJ/kg/K, helium has a specific heat of about 5. Helium atoms are four times heavier than helium (so a kilo has four times fewer atoms), but helium only stores three times less heat. It's a lot more complex than that. Similarly, a polythene and iron. Dec 5, 2018 at 16:26

A few things come into play. One is heat transfer from your home (or to it, in your case). If you keep your garage more comfortable, you improve the efficiency of the home's HVAC system. In my case, the uninsulated garage stays 20-30 degrees warmer than outside, mostly due to solar gain and heat loss from my home's conditioned space. This means my home loses less heat through the adjoining wall due to a lower temperature differential. Whether this will result in a net payoff is hard to say.

Another is comfort in the garage, as you mentioned. You can't "reset" the garage's interior temperature in a few minutes. Your slab acts as a massive heat sink, and all the garage's contents and wall structure also absorb heat. This means that, while air temps may change quickly with the door open, they'll return to roughly the same temp when you close the door. By keeping it cooler with insulation, which slows solar gain, it'll probably be cooler during the day as well.

Again, the finances are difficult to calculate because of the wildly varying factors involved. If you can do it cheaply, I would.

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    Insulation alone without heating or cooling will blunt the daily temperature fluctuations - it won't cool off quite as much at night or heat up quite as much during the day. That's usually a good thing. Dec 4, 2018 at 17:34

We paid to insulate our garage in this house despite having absolutely zero plans to ever heat/cool it. 20 years of living here and I think we made the right choice. The garage is south facing in a very hot climate, yet because of that insulation the temperature in there never gets nearly as extreme as it does outside. Contrast that with my parents house with a south facing garage with a west wall of concrete block--that would easily get as hot as the outside and in the evening it was often hotter out there than outside.

On the other hand, our previous house with a north facing garage we didn't insulate.


Remember that your attic is going to get a lot warmer than the surrounding air. In southern CA especially, the sun will bake your roof and, even with proper venting, you'll still have an ambient summer temperature in the 110+ range in your attic.

Without insulation in the attic, you're going to let that 110+ seep through the drywall and into the garage. In other words, you're potentially making your garage another near-attic. This will increase your heating and cooling costs because you're baking 2 sides of your climate controlled house instead of just one (and walls are typically too small to get much more than a R15, while most attics can easily take R60 or more). I would at least get an R30 over your garage.

Insulating your door is another option, but I find it overkill. Make sure you have good gaskets around your door first.

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