Reserved for the coldest conditions, insulation layers are designed specifically to provide additional warmth. Insulation layers should be warm, lightweight and as compressible as possible, with full zips to allow venting and breathability. There are two main options when it comes to selecting the type of insulation: down or synthetic fill. For now, nothing can compete with down’s ability to loft, which offers incredible amounts of warmth for very little weight and bulk. That said, synthetic fill garments certainly have their benefits - they absorb a lot less water than down and retain thermal efficiency when wet. If you need help deciding between down and synthetic jackets, check out our guide here.