An Analysis of Insulated Mugs

Note: This is not a math teacher post.

I love good design. 

While I don't obsess about everything, every once in a while I will obsess about the design of certain objects. In the past few years I have become (only mildly) preoccupied by the design of insulated mugs. In my mind, a perfect insulated mug will

• be a good size to hold and drink from when full
• be leak-proof
• have a top latch that is operable with one hand
• be easy to clean
• keep liquids at their original temperature for a long long time

I've analyzed four insulated mugs in order to compare them. These are pretty much the only insulated mugs I've used, chosen based on good Amazon reviews, availability in local stores, and in one case, being on an Amazon flash sale while also looking suspiciously like my favorite mug. See if you can guess which is which.

What bottles were analyzed

For all parts of this analysis, I will list results in the same order and with the same color coding as below (roughly corresponding to color of bottle, but having to improvise on silver).

• Thermos Stainless Steel King (24 oz)
• $29.99
• 10.3" tall, 3.4" diameter
• 16 oz
• made in China
• Klean Kanteen Wide Insulated Stainless Steel Bottle with Loop Cap (20 oz)
• $20–$40
• 10.2" tall, 2.7" diameter
• Made in China
• Cozyna Travel Mug (16 oz)
• $17.70–$19.99
• 10" tall, 2.5" diameter
• Made in China
• Zojirushi Stainless Steel Travel Mug (16 oz)  (cheaper options here)
• $27–$40
• 9" tall, 2.6" diameter
• Made in Thailand

Insulation Data

There are two main questions that I have in considering an insulated bottle: How much does the bottle cool the liquid down upon initially putting the liquid in, and how well does the bottle help retain the heat of the liquid. Here are the results.

Initial Cool-Down

For this I put all four bottles out on my balcony, open, for about an hour. The temperature out there was about 58ºF. Then I  brought the bottles in and filled each one with boiling water. I took two measurements: one about 3 minutes after I put the water in, and then another about 5 minutes after the first measurement.  For these, I left all of the bottles' tops off for the entire ~10 minutes of the process. Here are the results.

All four mugs caused the temperature to drop 5-10º in the initial pour, and all four of them had a pretty serious temperature decline in the following minutes. The rate of decline is noticeably steeper for the Klean Kanteen. The precise rates of decline were 0.723, 1.355, 1.120, and 0.831 degrees per minute.

Note: This data reflects both the extent to which the bottle is absorbing the heat of the liquid, and the exposure of the liquid to the ambient air. I'd like for it to more accurately reflect just the heat transfer from liquid to bottle (plus, I didn't get to take into account the heat transfer into the cap). So, if I were to do this again, I would put the cap on each bottle, and probably turn it sideways once to expose the liquid to the cap, and just take one reading after 10 minutes.

Extended Cool-Down

For this I took the bottles that were pre-heated from the Initial Cool-Down test, and I quickly dumped and re-filled them with newly boiling water, and sealed them with their caps. I then took periodic temperature readings, opening them only as much as I needed to in order to get a temperature reading, for approximately the next 6 hours. Here are the results.

 From this round it looks like Thermos is the big winner, but the Zojirushi is not that different. Klean Kanteen is quite clearly inferior. I might as well admit now that my all-time favorite mug is the Zojirushi.

I have one friend (the friend who lent me her KleanKanteen) who says that she likes that her drink cools down quicker, because a bottle that insulates too well will keep your drink too hot for too long. While I can understand the sentiment here, I totally don't agree. I'd prefer to have a steady temperature for my drinks. Let it be noted, here, that because of my very efficient Zojirushi, I've adjusted my practice to put beverages into the bottle at the temperature at which I want to drink them, or maybe 10º warmer.

Note1: I realize now that if I were to do this totally fairly, I should have gotten all of the mugs up to the same temperature before pouring in the boiling water, rather than starting them in their different states based on the different results of the initial cool-down test. However, for various reasons I have good reason to believe that the results will be the same. I've done this test several times, with different collections of bottles, and every time Thermos has come out on top, with Zojirushi very close behind, and Klean Kanteen performing significantly worse.

Note2: I took another set of readings when I woke up this morning. This was 14 hours from initialization, and the temperatures were 144.5º, 109.7º, 117.9º, 137.1º. The extended chart is below. I feel like it brings up several mathematical questions, which I'll leave for you to ask.

Note3: Most often, we drink from our bottles throughout the day. So the cool-down will generally happen faster, as the lid is being opened, and the volume of liquid is being reduced (and replaced with cooler air). So, it's unrealistic to think, even in the case of the Thermos, that you could pour your coffee in in the morning, sip from it throughout the day, and still have it 144º 14 hours later.

Design/Cap/Ease-of-Use/Cleaning

The four mugs have four different cap designs. Here's what they look like in their "open-and-ready-to-drink" state. They are all easy to clean inside using a bottle brush, difficult to clean using a sponge, and (I believe) all dishwasher safe.

Thermos (24 oz)

The Thermos has a simple top with basically one part and two gaskets. It's very easy to remove the gaskets, and the whole thing can be cleaned well with a bottle brush. There is a vent hole, so pouring from the spout is smooth.

The cap keeps a tight seal; no leaking.

The lock on the cap is secure; it's a ring that comes up over the button and mechanically prevents opening. It would be one-hand-operable, except...

This thing is a tank. It's large, and it's heavy. This is probably part of the reason that it performs so well in the insulation, in addition to the fact that it has the largest capacity. Also, note that I mention how smoothly it pours, but not how easy it is to drink from, because for me it's just too big to drink from. I don't ever use this bottle, because it's too big and heavy. I carry one water bottle and one insulated mug, in a backpack, every day at work, and size/weight are important to me. However, I like having this around, just in case I need to take 24 ounces of hot (or cold) liquid somewhere, provided that I don't have to carry it. Because it performs so well.

Klean Kanteen (20 oz)

The loop top is a screw top with a single silicon gasket. In order to drink from the Klean Kanteen, you have to open the bottle entirely, and you have to put your lips to a rather sharp (and possibly hot) metal edge. It's not comfortable. Plus, every time you take a sip, you expose the rest of the liquid to the ambient air, thus speeding the cool-down. And you have to figure out where to put that cap as you drink.

Because of the simple design, the cap is very easy to clean. Just pull out the one gasket and use a bottle brush with some soap and water.

Despite the easy-to-clean cap, I've had issues with inside of the bottle looking rusty from normal use. I never had any visible corrosion in my drinks, but I can't imagine that it never happened.

I'll note here that during the extended time cool-down data collection, I noticed the Klean Kanteen top being warm. If I put my hand around the neck of the cap, I could feel heat coming out. If I put my fingers inside the loop, the top of the cap was very warm. The Cozyna also had a somewhat warm cap but not nearly as much as the Klean Kanteen.

Also: there is an option to buy a Cafe Cap that you can more easily drink from. Unless they've changed the design in the past two years, that cap is not leak-proof (it's advertised as "splash-proof," so take note that it's not intended to be leak-proof). It's made entirely of plastic, with no insulating materials, so will insulate approximately as well as having an open bottle.

My previous Klean Kanteen bottles always had an issue where the gasket would sometimes come out of the top when I opened a bottle that had hot liquid in it. Sometimes the gasket would then fall on the floor (ew), and sometimes it would fall into the hot liquid of the bottle. Also, sometimes the gasket would crinkle, and I couldn't get it to seal properly. It seemed to be more likely to misbehave when the liquid had been put in hot but then had had time to cool, thus creating a suction when I opened it. It looks like the new gasket design may be better than before, so maybe it'll stay in. I haven't tested the new gaskets, and I won't, because frankly I have no need for the Klean Kanteen.

Cozyna (16 oz)

Like the Thermos, the Cozyna cap has one part with two gaskets. It's super easy to assemble, disassemble, and clean. The pour hole is quite large, so it's okay that it doesn't have an extra vent hole. It's comfortable to drink from, and it's easy to operate one-handed. In fact, unfortunately, it's also easy to operate no-handed. I've often taken my back pack off to find this mug with its top popped open. There is a lock, but the lock is fairly easy to shift just from rubbing against other things in my backpack. Or from brushing a wall. In addition, if the lock is not set the quick release will be tripped just from picking up the bottle by the neck, because the cap release sticks out and will be triggered just with the pressure of picking it up.

The design is pleasant, and the price is decent, but the misfiring cap is frustrating enough that I'd call this a bust.

Zojirushi (16 oz)

The Zojirushi cap is the most complicated one of them. It has a removable center piece that has 2 silicon gaskets on it, plus a plug gasket in the lid. The vent hole is adjacent to the pouring hole, so pouring is slightly wobbly, but not so much that I'd call it "not good." I have a faint recollection that I wasn't entirely comfortable drinking from it when I first got it, but now I've used it so long that I don't remember why that was. I'd call it comfortable to drink from. The lid release looks at first blush like the Cozyna, but unlike the Cozyna, I don't ever accidentally mis-fire the release. The lock is a switch on the side, and it has a fairly firm "click," so it's rare for it to lock or unlock by accident. The release is housed inside a rigid plastic frame, so you have to mean to press it. Both of these controls are easy to operate one-handed, while holding the bottle mid-air.

Because of the deep chamber of the vent hole in the cap, this is the most difficult one to clean. However, it's not that bad if you have a pipe cleaner around. I just dip mine in the soapy water that I use for the inside, and run it through the hole. Note: I only take the teeny tiny circular gasket off to clean it once every few months. It's in a place that just doesn't get dirty.

I've had two of these bottles. One got lost, and I loved it so much that I replaced it. Both had an issue of leaking, but never if I closed it in a specific order. When you close this bottle, you have two choices. You can assemble the cap parts, then close and latch the lid, and then screw the entire thing on to the bottle; OR you can assemble the cap parts, then screw the open lid onto the bottle, then latch the lid. I cannot figure out why this is, but if I first screw on the cap and then latch the lid, I never get leaking. And if I first latch the lid, the screw the closed cap onto the bottle, I get a bottle that leaks about 50% of the time. Actually, that percentage is totally made up and unreliable, considering I never close my bottle that way, and I haven't collected data on it.

This bottle is also rugged. I've carried it with me nearly daily at work, and I've dropped it several times onto tile or concrete, and all it has to show is a few dings in the paint and two very minor distortions in the circular base. And, I'd like to add, no noticeable reduction in insulation performance.

 Summary

Here they are in order of my preference.

1. The Zojirushi performs nearly as well as the Thermos, but at a much more manageable size. It has two flaws (less easy to clean, and leaky if you don't observe the proper order of operations) that both have workarounds. Plus, it has that adorable elephant on its logo. 
2. The Thermos is the best performer overall, but it's too big for daily carry and use. I'll keep it around for carting large quantities of beverage to places where we'll have cups (and I won't have to carry). 
3. I got a pair of Cozyna bottles on a lightning sale at Amazon. They looked so much like the Zojirushi bottles that I assumed the design was the same. The insulation performance is okay, but the fact that the lid release can so easily be accidentally triggered (even when intentionally locked) means that I don't ever want to toss this in a bag with other things at the risk of having it open. It'd be fine for having in a car drink well, or some other environment where you know it won't get bumped.
4. The Klean Kanteen is pretty to look at, but its performance is sub-par. I'm a bit shocked that this is the only insulated bottle that is widely available in all health food stores and sports stores around my area. Power of marketing, maybe? I'll try interviewing some of these stores, and see if they can answer why they chose to carry that brand.