Castelli Flanders winter base layer was designed for the coldest winter days. The recommended temperature range is actually 18°-43°F / -8°-6°C. Unfortunately, I did not have a chance to test it in such extreme conditions as 18°F / -8°C since it does not get as cold where I live.
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This is why the lowest temperature I tested it in was around 32°F / 0°C, and this base layer was more than capable of keeping me warm and comfortable. So if your winters are harsh and you need the extra warmth, then Castelli Flanders is the base layer you need.
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On the flip side, it can be too warm when temperatures are above 43°F / 6°C, so you might need to adjust your clothing accordingly. Check out below how I am using this base layer and tackling this issue.
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Scoring Castelli Flanders a 10/10 for warmth.