Microwavable heating pads with organic fillers are a wonderful way to soothe sore muscles or just warm up on a cold day. Their combination of toasty warmth and good smell are a natural remedy you can enjoy every day without side effects. The rice-filled warming pad project we did here at Sew4Home is one of the most popular gift items ever featured. Most likely, it’s because they’re not only functional, they’re also really easy to make. Everybody who makes them seems to have a favorite filler. So we thought we’d do a little testing to see if we could find out which one is best.

Cotton on the outside, but what should go inside?

You can use other substrates, our original pad project featured fleece and cotton ticking, but the traditional choice for anything microwaveable is 100% cotton. It can get very warm without melting and has a nice feel against your skin. Of course, like anything you put in the microwave, even cotton will eventually burn if you cook it too long. The benefit to using the organic fillers is they require very little time to heat thoroughly, fifteen to thirty seconds is usually plenty.

But what about the filler? What should you put inside your heating pad to get the best results?

First of all, you want your filler material to be microwaveable; that eliminates anything with a metallic component, which will spark. It should stay warm for an extended period of time. It should have a nice smell or no odor when heated. And finally, it should have a nice feel against your skin. This last reason is probably why neck warmers filled with driveway gravel never caught on.

Talking with our Sew4Home team and then looking at what’s recommended on the web, we found a huge number of different fillers people have tried. The more exotic range from silica beads to cherry pits. But the ones that appear to be the most popular (and in our experience the most practical) are: rice, dried corn, and flaxseed.

All three meet the requirements of retaining heat, having a pleasant smell, and feeling good against your skin. Additionally, we like the fact you can buy any of these rather inexpensively in the bulk food section at most supermarkets. If you’re making more than a few heating pads, this is something to consider.

Some people swear by the convenience and/or cost of buying feed or seed corn at a local feed store, but there were also many concerns about “buggies” showing up in corn. We chose to use food grade fillers for all our tests and have not experienced any issues with pests – even in pads that have been used over and over for years.

We wanted to know which of our three finalists – rice, dried corn, and flaxseed – would perform best in a “highly scientific test.”

Which heating pad filler is best? Our scientific analysis.

To test our fillers we made three 5½” x 5½” test pads from scrap cotton fabric and filled each with 1½ cups of the various fillers.

We chose to compare them by volume rather than weight because that’s the limitation on your sewn warming pad.

Heat retention

Using a 1.65 kilowatt microwave, we heated each of the pads for thirty seconds. Using a food thermometer, we measured how warm the pad was right when we took it out. And then how warm it was after sitting out for five minutes.

As a reference, we heated a cup of water to 140° and found it had cooled to 124° in five minutes.

Rice: 140° out of the microwave. Five minutes later had cooled to 136°.

Dried Corn: 158° out of the microwave. Five minutes later had cooled to 142°.

Flaxseed: 144° out of the microwave. Five minutes later had cooled to 142°. This one retained the most amount of heat.

One additional plug for rice, which came in second: rice is hygroscopic, meaning it absorbs water and then releases it when it is heated. You may have put a little rice into a salt shaker to absorb the water that can invade – especially in humid climates – so the salt will shake more freely. When you use rice in your heat packs it delivers moist heat, which can be quite beneficial. Putting a small cup or bowl of water into the microwave when using any of fillers inside the heating pads can help with moisture.

Smell

We’d heard some people complain that grains, like the rice and corn, can have a cooked smell when heated. We didn’t detect any strong smell. But cooking odors may be caused when the grains are microwaved too long and burn a little.

Flaxseed is a seed and so much less prone to cook. However, of the three, the flaxseed actually had the most noticeable smell.

We often use essential oils to add a pleasant scent to our warming pads. The organic fillers tend to absorb and retain these oils, but remember that a little bit goes a long way. You need just a few drops to create a lasting aroma. Other scent options include, dried herbs, flowers, and teas.

Feel

This is the most subjective test. It really depends on the texture you like on your skin.

Rice: This has a nice “full” feel, almost like a batting fiber.

Dried Corn: It has a granular, pebble feel that’s pleasant when resting on your arm or neck.

Flaxseed: This flowed the most easily and conformed to where you laid it.

Cost

As mentioned above, we narrowed it down to our three finalists because they were readily available from the majority of supermarkets with bulk bins. Prices, of course, will vary based on your area and shopping options. The costs shown below are averages.

Rice: $1.15 per pound

Dried Corn: $1.30 per pound

Flaxseed: $2.99 per pound

Conclusion

If you’re going solely for heat retention, use flaxseed. But all three of the fillers we tested performed better than water and so should stay warm longer than a traditional hot water bottle.

For smell and feel, we found things to like about all three. So much so, that we’ve even mixed rice and flax for some of our projects.

We now expect the major scientific journals to begin clamoring for the publishing rights to our detailed research, but you can use it for free.

Conduct your own experiment

Of course, there are many other filler options; we simply didn’t have the time to test them all. We’ve heard good anecdotal evidence for using lentils, dried soy beans, millet, birdseed, and hard wheat. Put on your white lab coat and do some testing of your own. We’d love to hear about your own successes or failures with organic fillers.

If you’re ready to put your fillers into practice, check out our projects for Rice Warming Pads, a Scented Spa Set, and a Therapy Neck Wrap. These projects also include tips on heating, cooling, and cleaning.