How much warmth does a bivvy bag add?

A bivvy bag typically adds 2-4°C to your sleeping bag temperature rating. Basic poly bivvies add about 2°C, breathable Gore-Tex/eVent bivvies about 3°C, and hooped bivvies about 4°C. This is due to wind protection and the dead air space between bivvy and sleeping bag.

How do I prevent condensation in a bivvy?

Condensation is the biggest drawback of bivvy bags. Use a breathable fabric (eVent is better than Gore-Tex for vapour transfer). Keep the opening as wide as possible. A hooped bivvy creates airspace that helps. In winter, moisture condenses as frost on the inside which is easier to manage. Consider bivvy + tarp as a system.

Is a bivvy bag waterproof?

Yes, bivvy bags are designed to be waterproof. Even basic poly bivvies will keep rain off. Breathable bivvies (eVent, Gore-Tex) are waterproof while allowing some moisture vapour to escape. However, in heavy rain, a bivvy alone gives less protection than a tent or tarp — many bivy users carry a small tarp as backup.

When would I choose a bivvy over a tent?

Bivvy bags excel for: emergency shelter (always carry one in winter), lightweight fastpacking, quick overnight stops, mountaineering bivouacs, and situations where pitching a tent is impossible. They are significantly lighter (200-500g vs 1-2kg for a tent) and pitch instantly anywhere.

CampCalcs

Bivvy Bag Calculator

Calculate the right bivvy bag size and temperature boost for your sleeping system.

Your Height

Shoulder Width

Sleeping Bag Comfort Rating

Bivvy Material

How We Calculate This

Size Calculation

Length = Height + 25cm

Width = Shoulder width + 20cm

Temperature Boost

Basic poly: +2°C. Breathable (eVent/Gore-Tex): +3°C. Hooped bivvy: +4°C.

Effective temp = Sleeping bag rating - Bivvy boost

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify conditions and use proper safety equipment outdoors.