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I have bought a Merino wool base layer for all the obvious reasons, they're raved about and have been sworn by by many a rider, and I was getting not only chilly in the winter weather but also wearing a cotton t-shirt and getting caught in the rain caused it to very quickly go cold, damp and heavy.

So I bought small and medium and settled for the small but even that is quite loose on me and a baggy fit. It certainly keeps me warm, no complaints there, but then I very quickly get too warm. They're supposed to be magic - keep you warm in winter and cool in the heat - but if it can't stop me from overheating when its cold outside, I struggle to believe it being any better in the summer.

My theory is that it needs to be a skin fit in order to achieve its wicking capabilities, and because it's loose, it isn't able to wick the heat direct from my skin, but rather heats up the air between my skin and the base layer causing a reserve of hot air which keeps getting hotter and thus causing my own mini personal sauna effect.

Am I right or totally wrong? I don't mind being wrong, but just want to understand the reasons I seem to be too hot and what I can do to resolve it.

I am also wearing a waterproof jacket over it which isn't very breathable. This could just as much be the problem, but I don't want to leave the jacket in case it rains as I don't know how Merino wool base layers - especially when baggy - cope when wet? (Another question maybe?)

Thanks.

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what you say sounds about right about merino's wicking properties. Merino is indeed renowned as a great material (I have some part-merino tops that are great) but I think anything work under a rain jacket will suffer. Do you have any other choice of jacket? In the winter I'll carry a rain jacket with me but generally won't wear it unless its actually raining. Mostly I will wear something windproof instead. I do sometimes overheat, but can regulate this easily with the zipper. –  PeteH Dec 9 '13 at 22:29
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Rain jackets commonly keep rain out and sweat in. –  Samuel Russell Dec 9 '13 at 22:33
    
Are you using a jacket designed with for cycling? (has some vents typically) –  Batman Dec 10 '13 at 3:49
    
As an aside, the work wicking refers to a fabric's ability to transport sweat, not heat. Cooling still depends on airflow (which you've blocked with the jacket) –  Useless Dec 10 '13 at 18:01
    
Give wool a hot wash and pop it in the dryer: it will be skin tight if it wasn't before. :) –  Kaz Dec 11 '13 at 6:15

3 Answers 3

up vote 15 down vote accepted

I am also wearing a waterproof jacket over it which isn't very breathable

Well there's your problem.

The most wickable, breathable material in the world isn't going to achieve those properties if you put a plastic bag over it. If you don't want to be caught out in the rain, keep the raincoat in a backpack/messengerbag/pannier/whatever until it's needed.

Regarding how tight the base layer should be, I would say it should be pretty tight, especially when you first put it on because it will shrink up in the wash and loosen up considerably when you sweat in it. However, if it's a little baggy it's not the end of the world, it just wont wick moisture away as well.

Also keep in mind that everyone has different heat/cold tolerances. Two different people may require entirely different layers at the same temperature based on exertion, weight, preference, etc. If you're a hot blooded type, it might need to get a little colder before you put on a wool jersey. I know I'd catch on fire for anything above around 55°F

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rei.com/learn/expert-advice/underwear.html is a decent overview of a few of the things mentioned. For what its worth, I'm personally a t-shirt and shorts type of guy until below 30F... –  Batman Dec 10 '13 at 3:51
    
Yes, replace the waterproof jacket with a windbreaker (or a vest) unless you need to keep the rain out. I'm a fan of the twin six windbreaker that has vent panels along the back of the arms and extend down both sides of the torso. Other companies have similar offerings. –  Benzo Dec 10 '13 at 3:54
    
Changing my over layer seems like a good option. Would a windbreaker layer not be just as hot as a rain jacket? Or are they more breathable. I can see my answer will lie in the layer I choose over my merino base. –  Ghandi Manning Dec 11 '13 at 16:12
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@GhandiManning They are more breathable, however their purpose is to inhibit the wind from penetrating your clothing so you don't get as cold, so the obvious tradeoff is that it will inhibit sweat's ability to evaporate. –  joelmdev Dec 11 '13 at 22:34
    
It always seems that there's a trade off, being new to cycling at this level I feel like the only way it to spend loads of money and trial and error different clothing. Something windproof to keep me warm, but as breathable as possible, but then an extra layer for when it rains, but then I'll get hot again. The vents on my existing jacket don't seem to help, and I use my front zipper to regulate temperature, but that's not great when raining. –  Ghandi Manning Dec 12 '13 at 13:39

Merino is not magic, if you get it soaked it's uncomfortable and in hot weather it's probably more insulation that you need. It's just better than most other alternatives when the weather gets bad, ( cotton for instance, which can be deadly when soaked in hypothermic conditions. Merino also doesn't get as clammy as some synthetics ).

Waterproof and moderate to heavy levels of exercise mean you are going to get soaked no matter what base layer you have on. The key to staying comfortable is to not get wet in the first place and the only way to do that is effective layering with breathable garments. Despite all the marketing there is no such thing as a waterproof breathable fabric that will be breathable enough for moderate exercise.[1]

When it's raining hard, you don't have much choice. So you want the base layers to retain some insulation when they get wet. Merino is good for that. In this case I actually prefer to use it as a mid-layer with a very breathable synthetic knit underneath. Your body heat can dry the inner layer and the merino will absorb some of the moisture before it condenses on the waterproof layer.

The problem with merino as a single layer is that it is not at all windproof. When biking in cold weather you really need at least some windblocking. A light nylon vest or windshirt works pretty well for that. Bike specific vests with windblock fronts and breathable mesh backs are really nice layering pieces. Softshell jackets with some wind blocking and good breathablity will also stand up to light to moderate rain for much longer than you might expect.

[1]- Well that's been my experience in the last 45 years, for some people it works I guess. Of all the one's I've tried eVent comes the closest to actually working as advertised.

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Merino definitely won't work as well to wick away sweat if it's not touching much of your skin, but the rain jacket trapping all of that moisture in also doesn't help. It's hard to say which is the biggest contributing factor. Perhaps try the merino on without the rain jacket once in fair weather, and see if it performs better than just a cotton shirt.

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With the weather being colder and the conditions generally being dark when I commute I feel like my rain jacket provides benefit for warmth and visibility (its high vis) but I will give it a shot on one of my lighter journeys to see how the merino top performs on its own. –  Ghandi Manning Dec 11 '13 at 16:15

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