Staying dry is not just about keeping the rain out, it’s also about letting water vapour escape. In this context, breathability is the ability of the fabric to transfer moisture vapour (not air) and is generally stated in terms of:
The human body can expel over 4 litres of moisture a day – you just don’t notice it until you wear a significantly less breathable material over your skin. The best breathable yet waterproof barrier is perhaps human skin; it stops fluids getting in, yet opens up when fluid needs to get out. The higher the activity/aerobic level and outside temperature the more the body sweats, so the more breathable your jacket needs to be. Just remember that even the most technologically advanced garments will not be as efficient as human skin at transporting moisture away.
High activity outdoor sports like trail running, cross-country skiing, mountain biking and mountaineering require a breathability performance of at least 20,000 gr/24hrs/m² (≤6 Ret). So go for jackets that have a Gore-Tex Pro Shell(2 and 3 layer) or Berghaus's own, more affordable AQ 2.5L. If however, weight and packability take precedence, coated technologies such as or Gore’s own Paclite are your best bet. To add another dimension to this Paramo have spent years developing a system that works with your body heat and sweat creation to keep you drier. The developer of this system, Nick, just could not get a jacket he was truly happy with when out in difficult conditions.
Activities that produce less heat, such as walking, climbing, and lift skiing, need breathability in the range of 10,000 to 20,000 gr/24hrs/m² (6 to 13 Ret) – though, generally speaking, the more breathable, the better! Buy the most breathable you can afford; jackets with Berghaus AQ or The North Face HyVent coating are good choices for those on a budget.
Really we do not recommend buying anything less breathable than 13 Ret unless your hobby or work involves standing still.
Also, bear in mind that all technologies mentioned so far will have a range of performances due to variations in the face fabrics they are applied to. Furthermore, jackets with a liner either bonded or hanging loose on the inside will in fact be less breathable, but give the impression they are better because the fabric soaks the moisture away from your skin.
Tip: To help minimise moisture buildup under your waterproofs, use your vents and front zippers to allow moisture vapour to quickly exit your clothing system. Think about the layers you have underneath, and perhaps take one off (remember, sweating is your body’s way of getting rid of excess heat). Alternatively, think about slowing the pace down a bit, or anything to reduce perspiration rate. Another tip is to keep your waterproof clothing clean and proofed which stops the jacket wetting out.