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What is UnderCool?

UnderCool is a lightweight garment designed to keep you cool and comfortable in any situation. It can be worn under virtually anything. Unlike other cooling garments, you can wear UnderCool directly on your skin. With UnderCool, you can stay cool and look it too.

How does UnderCool work?

UnderCool uses multiple layers to maximize the cooling effect and duration. The garment is made of comfortable wicking fabric to keep you cool and dry, and features mesh panels for maximum breathability. The lightweight and flexible phase change material are encapsulated in reusable cooling packs. UnderCool features velcro for easy on and off application and adjustable straps for the best fit.

 Photo of cooling packs made from phase change material.

What is phase change material (PCM)?

Phase change material (PCM) is a category used to describe a wide variety of materials capable of storing and releasing large amounts of energy during the melting / freezing process. PCM's can include parrafin waxes, hydrogels, and bio based plant feedstock. ThermApparel uses a bio-based PCM product.

How do PCMs work?

PCMs are substances that absorb and release thermal energy during the process of melting and freezing. In the case of our lightweight and flexible cooling packs, the PCM slowly absorbs your body heat over time to keep you cool and comfortable, even in humid environments.

Are PCMs safe?

Not all PCMs are created equal. ThermApparel uses a PCM which is USDA-certified and bio-based. That means no dangerous oils or hard to clean paraffin waxes. ThermApparel cooling packs out-preform paraffin, salt hydrates, gel and ice in a number of areas. Long term stability, safe handling, sourced from recyclable materials, won’t get you wet or burn your skin, our lightweight and flexible PCM cooling packs are reusable and can be recharged again and again and again.

How long does it take to recharge the cooling packs?

The cooling packs will freeze in any environment under 70F / 21C; about 20 minutes in ice water, 40 minutes in the freezer, 60 minutes in the fridge, or 10-12 hours if left on a cold surface in a cold room under 70F / 21C. We have tested recharging in the snow and it took about 5 minutes. Please note all recharging times are approximate.


How do you wear UnderCool?

UnderCool has adjustable straps and velcro across the front to accommodate the waist of any user. UnderCool can be worn both under and over clothing, according to your preference. We recommend wearing it under your clothes for maximum cooling.

UnderCool is a unisex garment that comes in 7 sizes (XXS, XS, S, M, L, XL, XXL) to fit a wide variety of body shapes and sizes because “one size fits all” really isn’t accurate. UnderCool needs to fit your body like a glove to give you the best cooling support.

Sizing is dependent on your natural waist measurement, not your bust or chest size. 

1. Stand up straight and tall
2. Bend at both sides
3. Measure at bend or below ribcage but above belly button.

Please refer to our sizing guide for correct sizing.

We recommend measuring your natural waist first before ordering. If you not able to, then we suggest going with the size of a skin tight undershirt for men or camisole for women.

How long does UnderCool last?

UnderCool provides you with a comfortable cooling sensation for 1-3 hours. SlimCool Cooling Packs average 1-2 hours and ExtendedCool Cooling Packs average 2-3 hours. Cooling times vary depending on ambient temperatures and level of activity. 

How much does UnderCool weigh?

An UnderCool Vest with 4 SlimCool Cooling Packs weighs 1.3 lbs.
An UnderCool Vest with 4 ExtendedCool Cooling Packs weight 1.9 lbs.

Will it "sweat" or feel "wet" while I wear UnderCool?

The PCM is sealed in heavy duty industrial plastic. While the PCM does melt as it absorbs heat, the liquid PCM is kept within the packs. If you experience any wetness, remove the packs as this could be a leak. The PCM will not harm you but may stain your clothes.

Because of the 70˚F / 21˚C melting temp of the PCM, the cooling packs shouldn't sweat or get you wet like ice or gel packs and won't burn you skin like ice packs.

Is UnderCool machine washable?

Yes! The UnderCool Garment is washer safe.
No! The Cooling Packs are NOT machine washable. (Trust us on this one. We've tried it and they won't make it. Also, we won't replace any packs that have been through the washer.)

Can I take UnderCool on a plane?

Possibly, by being prepared with the correct TSA guidelines ahead of time. Read our tips and guidelines.

What are the TSA Guidelines for UnderCool?

The TSA has specific procedures and guidelines when traveling. We recommend that you review their guidelines to make your trip as easy as possible.



 Photo of woman wearing UnderCool cooling vest while doing yoga.

Can I buy UnderCool?

Yes! You can buy your UnderCool on our shop.

Is UnderCool covered by Insurance?

It depends on your insurance provider and your plan. Most providers cover cooling vests on a case-by-case basis. Check out our insurance page to read more about the process.

How can I fix hard lumpy cooling packs?

IIf your packs freeze in funny shapes and you want them to freeze flat, don’t stress! It’s a quick and easy fix.

  1. Toss them into hot water and let them melt. 
  2. Once melted, lay them flat in ice water, the fridge, freezer, a cold surface or even the snow, (if you have some)
  3. Wait for them to refreeze - 20 minutes in ice water, 40 minutes in the fridge, 60 minutes in the freezer, overnight in a cold room laying on a hard surface or 5-10 minutes in the snow.
  4. After waiting for them to recharge, they will be flat and ready to go.

Why doesn't my UnderCool seem cold?

Let’s make sure you have the correct sizeSometimes if the vest is a size too large, you won't get optimal cooling. If you think your vest is too large, give us a call so we can set up an exchange (855-232-7233). 

  • Does the vest feel snug like it's giving you a hug?
  • Do the cooling packs sit firmly on your lower back? Or, do they make complete contact with no gaps between the vest and your back? The packs need to make contact with your skin and not an undergarment for optimal cooling.

The packs won't feel as "cold" as other companies' packs and that is one reason why our cooling packs are better. Other companies ice and gel packs and some of the colder PCM packs, feel colder because they freeze at 32˚F / 0˚C, but they are dangerous for your skin. You will have to wear a t-shirt between the other companies' cooling packs and your skin so you don't experience ice burn. No t-shirt needed with ThermApparel Cooling Packs. 

With the other companies packs being so cold, they are not actually cooling as effectively. When they come in contact with your skin, your blood vessels constrict, which decreases blood flow. The decreased blood flow helps to decrease swelling and inflammation by making it numb, but it doesn't help with cooling your system because it isn't cooling your blood.

With our 70 F / 21 C cooling packs, you will feel cool and comfortable at a temperature that feels natural to your skin and body. Your blood vessels won't constrict and your blood will cool, which will cool off your entire body instead of just the area where the ice or gel packs make contact. Our packs act like a heat sink to help pull the heat out of your body.

Why don't my packs feel cold?

Even though they don't feel cold, if they are still in a semi solid state, they are still pulling the heat from your body and cooling you down.

The main reason why they may not feel "cold" is they are designed to melt and freeze at 70˚F / 21˚C. After you take the vest off, feel your back. Is it cool? Does the cooling effect last awhile even though you are not wearing the vest? Try the same activity you were doing with and without the vest to notice a big difference.

Can ThermApparel Phase Change Material (PCM) Cooling Packs be left in the freezer?

Yes! The best place for long storage of ThermApparel Phase Change Material (PCM) Cooling Packs is freezer or fridge. We keep our cooling packs in the freezer and they have not been damaged, even after months. Read our blog post for more info. 

Is UnderCool a cooling vest?

Yes! UnderCool is a lightweight cooling vest designed to keep you cool and comfortable. Read our blog post for more info. 

How Does a Cooling Vest Work?

A cooling vest works by lowering or stabilizing body temperature. This helps make exposure to uncomfortable temperatures more bearable. Cooling vests are used by people suffering from heat sensitivity due to sports, industrial applications and individuals suffering from multiple sclerosis, Lupus, POTS and other neurological disorders.

There are 4 different types of cooling vests.

1. Evaporative cooling vests
2. Ice chilled cooling vests
3. Cool flow cooling vests
4 PCM (phase change material) cooling vests. (this is the kind ThermApparel sells)

ThermApparel’s UnderCool cooling vest works by using PCM (Phase Change Material) cooling packs. PCMs are substances that absorb and release thermal energy during the process of melting and freezing. What does that mean? Simply, they act as a heat sink to absorb your body heat slowly over time to keep you cool and comfortable, even in humid environments. Want more info? Read the whole blog.

What is Phase Change Material?

PCMs are substances that absorb and release thermal energy during the process of melting and freezing. What does that mean? Simply, they act as a heat sink to absorb your body heat slowly over time to keep you cool and comfortable, even in humid environments.

ThermApparel uses a non-toxic bio-based PCM that out-preforms paraffin, salt hydrates, gel and ice in a number of areas. Long term stability, safe handling, sourced from recyclable materials, won’t get you wet or burn your skin, our lightweight and flexible PCM cooling packs are reusable and can be recharged again and again and again. Want more info? Read the whole blog.

How do I insert the cooling packs?

The cooling packs are all the same size and fit into all of our sizes. There are a few tips that can help getting the packs into the vest:

  1. Make sure the packs are solid / frozen.
  2. Lay the vest on a flat surface in front of you with the cooling packs above the pockets easily in reach.
  3. Lean over the vest and place your forearms on either side of one of the pockets.
  4. Using slight pressure with your forearms, pull the fabric in opposite directions. This will allow the pocket to be wider than the pack.
  5. While maintaining pressure and pulling the fabric in opposite directions, use your left fingers to pull open the pocket. Use your right fingers to slide the pack in to the vest.
  6. Let go.
  7. Repeat process with the other three pockets.

You can also our video on how to insert the cooling packs on our YouTube page. 

I can’t afford an UnderCool, can ThermApparel help?

There are many foundations and organizations out there that offer financial assistance for various things. Please note ThermApparel can not guarantee the accuracy or your ability to qualify for any of the programs. ThermApparel does not monitor, substantiate or endorse the information offered on these sites. It is recommended that you review any information you get from searching on the Internet with your health care professional who is your primary resource to meet your individual medical needs. Check out our Financial Assistance Page for more details and contact info

What does shipping cost?

For all orders over $40 in the United State we offer a Free Shipping Option. For other countries or express services it depends on where it’s going and how fast you want it to get there. In the USA, orders ship free via Priority Mail with the USPS. There are faster options with FedEx, UPS and DHL. For shipping international, DHL is our preferred shipping method, but sometimes USPS is possible. Want a shipping quote? Send us an email or chat with us via live chat.

How much does an UnderCool cost?

UnderCool starts at $199.99. You can see all the pricing for all the bundles in our shop.

Does ThermApparel offer any discounts or free vests?

ThermApparel usually has about 2-3 sales a year. However, there are many discount codes for UnderCool out there. Dave Bexfield at has one on his site, but you'll have to sign up for his newsletter. We also run ads that include discount codes in the National MS Society’s Momentum Magazine and MSAA’s magazine The Motivator.

I have Lupus. Will UnderCool work for me?

If you have heat sensitivity issues as a result of Lupus, the UnderCool could work for you. LupusChick has been putting UnderCool to the test for us and she loves it. Read what she has said about it

I have POTS. Will UnderCool work for me?

If you have heat sensitivity issues as a result of POTS, UnderCool could work for you. Chronically Salty has been putting UnderCool to the test for us and she loves it. Watch her unboxing video and her concert survival tips video.

What is ThermApparel’s return policy?

You have 60 days from purchase to return or exchange your purchase if the vest and cooling packs are still in new condition. ThermApparel has the right to inspect each return.

Can I wear UnderCool under my Santa suit?

Yes! Santa Ed Taylor, one of our nation's top real-bearded Santa's, reviewed UnderCool for us and wears it in LA all the time. Check out our Santa page for all resources for cooling off Santa.

Do you ship outside the USA?

Yes! If you have an address and DHL, UPS, or USPS International delivers to you, we can ship directly to you. Send us an email for a shipping quote.

Where can I try on UnderCool?

Our home office is located in Rochester, NY. You are always welcome to stop by and try one on. We have a reseller in Australia too! If you're around, feel free to stop into RehaCare. You can try one one there too. 

How do I return for a refund?

Visit our returns and exchanges page for complete details.

How do I return for exchange?

Visit our returns and exchanges page for complete details.

How do I contact MS Focus for their Cooling Program?

Visit the MS Focus Cooling Program on their website or by calling 888-673-6287.

I am going camping with no electricity. How do I recharge the packs?

Snow, a cold icy river or 20 minutes in ice water should do the trick.

Do you sell the UnderCool Vest only?

Yes, you can purchase in our shop.

Do you sell the cooling packs only?

Yes, you can purchase in our shop.

Can you wear a bra with UnderCool?

Yes, you can wear a bra or sports bra with UnderCool.

My waist measurement is in between two sizes. Which size should I order?

ThermApparel recommends going with the smaller of the two sizes.

Questions about our vests or shipping? Reach out to the ThermApparel team toll-free at 855-232-7233, write to us through our website’s contact form or chat with us via live chat. We always respond because, otherwise, that wouldn't be cool.