A humidity indicator comprises a transparent tube (1) containing a fibrous strip (3) impregnated with a hygroscopic salt and folded in its dry state into a zig-zag shape, the tube (1) being closed by a cap (6) until use. When used for measuring the relative humidity in a concrete floor (8) with a view to judging its suitability for glueing a floor covering (not shown) to it, a hole (9) is bored in the floor (8), after which the indicator, whose cap (6) has been removed immediately before, is inserted in the top of the hole (9), sealing lips (7a) ensuring an airtight and vapour-tight joint. When equilibrium between the space inside the tube (1) and the hole (9) has been established, and if the relative humidity in these spaces is equal to or greater than the liquefaction value for the hygroscopic substance in the strip (3), the water absorbed by the hygroscopic substance will soften the adhesive holding the paper fibres together, and under its own weight the strip will straighten out at least partly, indicating to an observer that the humidity is above some predetermined value.
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