They are two different ways of publishing the same property — how readily a fabric passes water vapour outward — and they run in opposite directions. On a bivy bag this is the figure that decides your night, and the waterproof rating everybody reads first is rarely the problem.
The number that is not the problem
A bivy bag’s hydrostatic head, published in millimetres, describes resistance to water pushing in from outside. Essentially everything on sale in this family keeps rain out, so comparing two bags on that figure sorts them into a group that all pass.
What makes people damp is the moisture they produce themselves, in an enclosure with very little air in it. That is why breathability is the specification worth reading, and why the makers who take the category seriously publish it.
Two scales that run in opposite directions
A moisture vapour transmission rate is quoted as a mass of water vapour passing a square metre of fabric in twenty-four hours — written g/m²/24h. Higher means more vapour passes.
The catch is that there is more than one method of arriving at it, and the methods do not produce the same number for the same fabric. A maker publishing an MVTR should name the test beside it, and where it does, that name is as much a part of the figure as the digits are.
RET is the other convention, and it is an evaporative resistance value. It measures how much the fabric resists the passage of moisture, so the scale runs the other way: a lower RET means a more breathable fabric.
Two bags, one quoting an MVTR and one quoting a RET, are not directly comparable. Neither is a pair quoting MVTR figures from different tests. The honest position is to compare within a scale and a named method, and to say plainly when a comparison cannot be made — which is the position this site takes.
What neither figure promises
A breathability figure is a laboratory property of a fabric. It describes how the material behaves under the conditions of a test, and none of those conditions is a still, cold night on a hillside with a person inside the bag.
On a still night the fabric is cold, the air inside is saturated, and vapour settles as liquid on the inner face regardless of how the material was rated. That is condensation, and in this family it is the normal experience rather than a defect. A wetted-out outer face makes it worse, because a surface holding a film of water passes less vapour whatever the membrane can do.
What decides a bivy bag beyond the fabric
Construction, in three parts.
Taped seams and a waterproof zip are what turn a rated fabric into a waterproof bag; a needle hole is a hole, and this is a separate claim from the hydrostatic head.
Denier, published for the floor and usually heavier than for the top, predicts how the bag survives being pitched on gravel.
Shape. A flat bag packs smallest and lies against your face. A hooped version carries a short pole over the head end, adds weight and packed bulk, and buys the space that makes a long night bearable — and, incidentally, keeps the wet fabric off your sleeping bag.
Then read the entry — a full-length zip is easy to get into and adds a seam, a drawcord hood is lighter and slower — and the packed size against the pack you already own.
Reading a listing properly
Write down the hydrostatic head, the breathability figure with the test named, the seam treatment, the deniers, and the packed size and weight. If the breathability figure has no test beside it, that is a gap worth noting rather than a number worth trusting, because the scale is usually printed once, in the smallest type on the page.
What people ask before buying
Which scale is better to compare on?
Whichever one both products quote, with the same named test beside it. A high MVTR figure and a low RET figure both describe a fabric that passes moisture readily, but the numbers run in opposite directions and come from different methods, so they cannot be converted between by eye.
Why is my bivy bag damp inside when it has not rained?
Because the moisture is yours. A sleeping person puts a great deal of water vapour into a very small enclosed volume, and a cold, still night gives it a cold fabric to settle on. That is condensation, and it is the normal failure mode of this category rather than a leak.
Last reviewed 6 September 2026