Quick Answer
Mylar bivys are generally not breathable enough for humid conditions, making them unsuitable for prolonged exposure in high-humidity environments.
Understanding Mylar Bivy Breathability
Mylar bivys are essentially thin sheets of aluminized Mylar, a type of plastic film that provides excellent waterproofing but limited breathability. Their breathability is measured in terms of moisture vapor transmission rate (MVTR), which is typically around 1000-2000 g/m²/24h. For comparison, a breathable fabric like nylon may have an MVTR of 10,000-20,000 g/m²/24h.
Practical Implications for Humid Conditions
When used in humid conditions, Mylar bivys can lead to condensation buildup inside the shelter, causing discomfort and potentially even health issues. This is because the Mylar material prevents moisture from escaping, causing the air inside the shelter to become saturated. In extreme cases, this can lead to the formation of dew or even frost on the inside of the Mylar. To mitigate this issue, users can increase airflow by venting the shelter or using a more breathable material, but these measures may compromise the waterproofing of the shelter.
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