The Thermarest Pump Sack serves two purposes. You can use it to blow up your sleeping pad, but it can also be used for storing gear. It has a similar size to a 20 liter bag so it can hold lots of gear, but is it waterproof?
Is the Thermarest Pump Sack Waterproof? The Thermarest Pump Sack that came with your sleeping pad won’t be waterproof. They use a nylon design that’s water-resistant, but it’s not completely waterproof. Thermarest does sell a waterproof pump sack that you can buy separately, but it’s not cheap.
I’ll explain how to increase the waterproofing on your water-resistant bag below, but Thermarest does sell a waterproof pump sack. Thermarest’s Blockerlite Pump Sack is a 20 liter polyester bag with a waterproof outer coating that’s 100% waterproof. I use mine just like any other dry bag when I go kayaking.
Most Thermarest Pump Sacks Aren’t Waterproof
Are Thermarest Pump Sacks waterproof? I got conflicting information when I first started looking into this question. Some sources claimed Thermarest Pump Sacks were waterproof and others claimed they’re water resistant. After reaching out to Thermarest for a clarification I realized that people were talking about completely different products.
The pump sack that came with your sleeping pad isn’t 100% waterproof. It’s made out of a water-resistant Nylon material that repels water, but it will eventually leak. They do sell a Blockerlite Pump Sack that’s completely waterproof, but you have to buy that separately. I’ll talk about it more in the following section.
Do you really need a waterproof bag or is a water-resistant bag good enough? It depends on how you’re using the bag. If you’re storing the bag in your pack with a pack liner and raincover you shouldn’t have any problem. All it takes is a cheap compacter bag to keep out 99% of water.
It takes a lot of rain to get through a water-resistant bag. I’ve left water-resistant bags out in moderate rain and they rarely leak. A little bit of rain soaks into the fabric, but it would take a lot of rain to soak the inside of the bag. The only time you would have a problem is if you submerged the bag in water or get caught in a heavy downpour.
Use Waterproofing Spray and Seam Sealer On Your Thermarest Pump Sack
You can spray your Thermarest pump sack with a durable water repellent spray to help increase the waterproofing properties. Spray it with Kiwi Camp Dry or Nikwax and it will be more a hair shy of being waterproof. The seams are the only part of the bag that won’t be waterproof and you can always use seam sealer to deal with them.
Just take a brush or Q-Tip and run the seam grip along all the seams. It should be dry to the touch within 30 minutes, but it takes 24 hours to completely dry. You may want to check out my post explaining seam sealer alternatives if you have other gear that needs seam repairs.
This won’t make your bag completely waterproof, but it will definitely help. It should be able to resist moderate rain and water leaking into your pack. Just don’t completely submerge the bag in water.
Thermarest Blockerlite Pump Sack Is Waterproof
Thermarest does sell a pump sack that’s completely waterproof. The Thermarest BlockerLite Pump sack uses a polyester fabric that’s coated with a waterproof DWR coating making it completely waterproof. If your pump sack says Blockerlite than it’s 100% waterproof.
You can use it like any other 20 Liter dry bag. I use mine whenever I go kayaking or canoeing. It gets strapped down to the back of my kayak and I’ve never had any problems. I’ve flipped over my kayak multiple times and the only time it leaked was when I forgot to close the valve.
Waterproof vs Water-Resistant Pump Sacks
It’s important to understand the difference between waterproof and water-resistant. A water-resistant pump sack is very different than a waterproof sack. They both repel water, but a water resistant bag will eventually leak.
Water-resistant sacks repel water to a point, but they’ll eventually leak. After a long period of exposure water will start to seep through the bag and soak the outer water-resistant layer. You might be able to submerge the bag in water for a few seconds, but it won’t take long to soak the gear inside your bag.
Waterproof bags use a water repellant coating so that they can completely resist water. You can drop a waterproof pump sack in the bottom of your pool and the inside of the bag should stay dry. The only time a waterproof bag will leak is if the seams are damaged or the lid isn’t properly rolled closed.