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I have a Kokatat fleece union suit, which we kayakers also refer to as a "bunny suit", which I wear under my dry suit when kayaking in cold conditions. I also wear it when snowboarding or XC skiing because the beauty of it is that the top does not untuck from the bottom, leaving your skin exposed. It is really one of the awesomest pieces of gear I own.

While the bunny suit keeps me warm, the regular cotton underwear worn under it gets wet from sweat (I swear not for any other reason, LOL) even when the temps are in their 20s. I tried wearing my Prana yoga shorts but they too absorb moisture, albeit to a lesser degree. Ideally, I would like some material like what is used for Helly Hansen base layer.

I am looking for recommendations for what kind of underwear to wear inside by union suit in the winter that will be moisture repellent?

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When you are facing a serious sweating problem, maybe your overall setup is too warm. What layers/jackets do you wear above the one-suite-fleece? I am thinking of a very thin layer which is highly breathable and will just be a shelter against the elements (wind, rain/snow) like e.g. the Gore Active Shell.

Still, sweating to some degree is pretty normal. What I can recommend in general is merino wool. It feels comfortable and warm also when it is wet. It also dries fast. Please check this link. Also very very nice is that merino doesn't attract bacteria like common materials like cotton or synthetics. Especially for underwear this is a unique and great feature. Going on multi-day trips you (and your friends) are still fine when you have 2 pieces of underwear with you.

Relating the comment of @amphibient:

You are right, cotton absorbs the sweat which isn't that favourable. Synthetics and also wool will transport the moisture to the outside. Your outer layers have to work the same way, otherwise your system won't work. Wearing a high-end merino shirt won't help when you have a cotton pullover over it.

Regarding your one-piece fleece midlayer and your one-piece hardshell outer layer, this might be great in terms of being waterproof and warm (no wind will break in) but I think you don't have the versatile options normal setups have. I am able to open zippers under the arms or open the main zipper from the top or even from the bottom when I feel slightly too warm. This lets me react to changes in weather/activity nicely.

  • I am not sweating profusely but the regular cotton underwear still absorbs a lot of sweat and becomes unpleasant to wear. Believe it or not, I wear nothing under the drysuit except for the fleece bunny suit. The dry suit does not offer much thermal protection, only keeps you dry. The fleece is so warm that you need nothing else except underwear for sanitary reasons mostly. But without it, you'd be cold. – amphibient Jan 21 '15 at 22:34
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Not Cotton

Related: Does cotton really kill?

Any active base layer will suffice as long as it is not cotton. Cotton is great for keeping you cool, but terrible for wicking moisture and keeping you warm.

The classic "Union Suit" that your suit is modelled after was developed as, and is still worn by some as underwear, so you could use your suit as your base layer–I'd recommend washing it as often as your undies if you do–and then add layers over top of it as needed, otherwise I'd recommend choosing some other active-wear base layer like polyester long-johns, or underarmour, or some other base layer made out of moisture wicking materials.

  • In addition to this, there are loads of non-cotton, quick drying underwear available on the market. There are quick drying boxers, hiking underwear, even ones with lace. Cotton, as you pointed out, absorbs moisture and holds it, and looses it's insulating properties. – JFA Jul 24 '17 at 19:53
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Merino wool is amazing. There are many brands of merino wool underwear, such as Icebreaker and Ibex. It is simultaneously the coolest and warmest fabric I've encountered. It never smells (I wash mine every 10 wearings just because I feel obligated), it drys in minutes, and it wicks moisture like sheep evolved their fibers for millions of years (because that's what happened). The downsides are its expense and lack of durability. To mitigate the latter, never dry wool at high temperatures (eg hang dry, air dry, or use a heat pump dryer).

  • but will it be wet the whole time from sweat because it absorbs moisture ? – amphibient Apr 13 '15 at 18:08
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    It wicks moisture well, like synthetics and unlike cotton. So it will dry and disburse moisture as well as anything. When it is wet, it still thermally insulates well. – Streblo Apr 14 '15 at 22:31
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Cotton retains moisture. It is not a good choice for underwear, and you have been bothered by it's major flaw. Merino wool, mentioned here, is a better choice. It is superior to cotton in every way but price. It is more comfortable over a wider variety of temperatures. I own several pairs of merino boxers and briefs. However, it is warmer and when the weather is not cold it may be too warm. It also retains noticeably more moisture than high-performance synthetics, linen, or hemp, but cutting-edge fabric blends and technologies are improving merino in this respect, like Nuyarn (offered by KUIU) and Icebreaker's Cool-Lite. The superior new merino blend technologies are new and not yet available in underwear, though. I recommend merino from KUIU, Icebreaker, Smartwool, and Minus33. However, I too have a sweaty groin and I recommend you avoid merino underwear. It's ideal for long johns, shirts, and union suits, but not underwear. It also won't last as long as the alternatives, but that also depends a great deal on your body's sweat and how you wash the garment. I mostly wear UnderArmour boxerjocks. Well made and very long lasting. Get the smooth "original" fabric rather than being tempted by the mesh if you're mostly sitting down, sitting down on a patternless smooth fabric is more comfortable. However, I prefer natural fabrics next to my skin and no longer buy synthetics like polyester and nylon for this purpose. Synthetics worn on the skin immediately begin to smell upon wearing and are very "unfresh" by the end of the day. The best choice is linen or hemp, which both perform very similarly. They're what I'm starting to move to, but I don't have much experience yet. They retain less heat than polyester and release moisture on par, while feeling and smelling much fresher. Rawganique seems to be a good company offering many styles.

Cotton may be less expensive, but it holds sweat more than any other common fabric. Go ahead and pay the $15-30 for a higher-end pair of underwear that will last for years and be significantly more comfortable. You wear underwear every day. You'll be more comfortable every day. You'll notice it and won't regret it, it's a good investment. Underwear will cling to your skin far less often.

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