The dew point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid water. Inside a bivy sack, your breath and body perspiration raise humidity. If the inner surface of the bivy (which is often close to outside temperature) drops below this dew point, condensation forms — soaking your sleeping bag and reducing insulation.

Moisture Vapor Permeability (MVP) measures how well fabric lets sweat vapor escape. Higher MVP (30k+) allows quicker moisture transport, reducing internal humidity and lowering the dew point. Low MVP fabrics trap humidity, dramatically increasing condensation risk, especially in cold conditions. Our estimator incorporates breathability to adjust the final risk score.

We use a simplified model: the inner surface temperature is assumed to lie between outside air temperature and internal temperature (from your body). Because bivy sacks have minimal insulation, the surface is usually only a few degrees warmer than outside air. The estimator adjusts this based on typical bivy construction (e.g., single-wall vs. double-wall), but defaults to a conservative approach.

Yes. Improve ventilation by opening the hood or vents (if available), use a small fan or vapor barrier liner, or position your bivy so wind can remove moisture. Also, lower your internal humidity by exhaling outside the bivy or using an anti-condensation mask. These steps are recommended when our estimator shows moderate/high risk.

High outside humidity (fog, rain) combined with cold temperatures and high internal humidity (heavy exertion) creates the largest dew point difference. For example, outside 2°C / 98% RH with inside 25°C / 95% RH can produce severe condensation even with good breathability. Use the estimator to explore your own scenarios.