Quick Answer
Altitude significantly affects the performance of Mylar bivys, as they can lose their insulating properties and become less effective in extreme cold at high elevations, typically above 10,000 feet. The Mylar's ability to retain warmth is compromised, and condensation may occur more frequently.
How Altitude Affects Insulation
Mylar bivys work by trapping warm air next to the body, but at high elevations, the air is typically colder and drier. This can cause the Mylar to lose its insulating properties, allowing cold air to penetrate and reducing the bivy’s effectiveness. For every 1,000-foot increase in elevation, the temperature drops by about 3.5°F, which can have a significant impact on the bivy’s performance. To mitigate this, consider using a higher-R-value sleeping bag or a warmer hat and gloves.
Condensation and Moisture Issues
At high elevations, the air is typically drier, but when the body’s moisture meets the cold Mylar, condensation can occur. This can lead to a buildup of moisture inside the bivy, causing the Mylar to lose its insulating properties and potentially leading to hypothermia. To minimize condensation, use a breathable, moisture-wicking sleeping bag or liner, and avoid eating or drinking too close to bedtime.
Techniques for High-Altitude Use
To make a Mylar bivy more effective at high elevations, consider using a few techniques. First, use a sleeping bag with a higher R-value to help keep you warm. Second, use a bivy sack with a waterproof and breathable membrane, such as Gore-Tex or eVent, to prevent moisture from entering the bivy. Finally, avoid using the bivy in extremely windy or exposed conditions, as this can compromise its effectiveness.
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