Module 1 · Moisture, Air and Materials

1.5 Sorption, Equilibrium and What 'Dry' Means

8 min read

"Is it dry yet?" is the question every contractor, landlord and insurer eventually asks, and it has no answer until you define dry. No building material reaches zero moisture in a building — each settles at an equilibrium moisture content determined by the surrounding relative humidity, and that relationship is the material's sorption isotherm. Reading it is what turns a drying programme from a guess about elapsed weeks into a decision with a defensible endpoint.

Learning objectives

By the end of this lesson you will be able to:

Describe the sorption isotherm and locate the hygroscopic, capillary and saturated regions on it.
State a defensible drying target for a given material and environment.
Explain why a stalled moisture reading may indicate equilibrium rather than ongoing ingress.
Design a drying verification that does not depend on elapsed time.

Key terms

Equilibrium moisture content (EMC): The moisture content a material settles at when held at a constant surrounding relative humidity and temperature. Material-specific.
Sorption isotherm: The curve relating a material's equilibrium moisture content to the surrounding relative humidity at a fixed temperature.
Hysteresis: The gap between the adsorption and desorption curves: a material drying out holds more moisture at a given RH than the same material wetting up.
Air-dry: At equilibrium with the surrounding air. The only practical definition of 'dry' for masonry inside a building.

Softwood at 60% RH and 20°C sits at roughly 11% moisture content. At 85% RH it sits at about 19% — above the roughly 20% threshold where wood-decay fungi become viable, which is why that threshold and that humidity keep appearing together in Module 5.

Masonry works the same way with different numbers. A fired clay brick at ordinary indoor humidity holds well under 1% by weight in the hygroscopic region; the same brick after capillary uptake can hold 15% or more. The gap between those two figures is where all the diagnostic information lives.

Which means the important question is never 'what is the moisture content'. It is 'where on the isotherm is this material sitting'. A brick at 4% is enormously above its air-dry equilibrium and something is wetting it. A softwood joist at 14% is slightly damp, entirely stable and of no consequence. The same number means opposite things in different materials.

The three regions of the curve

Hygroscopic region (roughly 0–95% RH) — moisture held as adsorbed layers and in the finest pores. Governed by air humidity. This is where a healthy building material lives.
Capillary region (above about 95% RH, or on contact with liquid water) — moisture held in the pore network by capillary suction. Requires liquid contact or near-saturation to reach; this is what a wetting defect produces.
Supersaturated region — pores filled beyond what capillarity achieves, requiring pressure or prolonged immersion. Flood conditions.

The step between the first and second regions is steep and it is diagnostically decisive. A material in the hygroscopic region is responding to air; a material in the capillary region has touched liquid water. Establishing which one you are in tells you whether to look for a leak at all.

Setting a drying target you can defend

'Dry' has to be expressed as a target moisture content in a stated material under stated conditions, and verified by measurement rather than by the calendar.

Establish the target from the material's air-dry EMC at the humidity the finished building will run at — not at the humidity during drying, which is usually much lower.
Take a control reading from an unaffected area of the same material in the same building. This is the strongest possible target, because it accounts for the actual construction and conditions rather than a table.
Measure the same points, at the same depths, on a schedule. A moisture trend is evidence; a single reading at the end is not.
Declare drying complete when readings have stabilised at or below the target across successive visits — stability matters as much as the value.

A stalled reading above target is the case that needs judgement. It means either that drying conditions have stopped being adequate, that there is a source still active, or that the material has reached equilibrium with salts it contains. Those three have different remedies, and 1.4 is how you tell the third from the other two.

Hysteresis, and why a wall stays damper than it 'should'

A material that is drying holds more moisture at a given humidity than the same material wetting up to that humidity. The curves do not coincide, and the gap can be several percentage points.

In practice this means a wall that has been thoroughly wet will settle at a higher equilibrium than the textbook figure for its material, and will do so legitimately.

It is one more reason the control-sample method beats the published-table method. A control from the same wall, dried to the same conditions, carries the hysteresis, the salts and the actual pore structure with it — none of which a table can.

Common mistakes to avoid

Quoting a moisture content without naming the material. The same figure is alarming in brick and unremarkable in softwood.
Setting a drying target from the humidity during the drying programme rather than the humidity the finished building will run at, so the material re-wets after handover.
Declaring a wall dry on elapsed time. Drying rate depends on conditions that vary enormously; a month per 25mm is a planning figure, not a verification.
Reading a stalled measurement as ongoing ingress without checking whether it is equilibrium with hygroscopic salts.
Comparing a reading against a published table rather than against a control from the same building, which carries the real pore structure, salt load and hysteresis with it.

Lesson summary

Materials do not dry to zero; they settle at an equilibrium moisture content set by the surrounding humidity, and where a material sits on its sorption isotherm — hygroscopic region or capillary region — tells you whether it is merely responding to air or has been in contact with liquid water. Defensible drying means a target derived from the building's finished conditions, verified against a control from the same material in the same building, and confirmed by stable repeat readings rather than by elapsed weeks.

Next: what an electrical moisture meter actually measures, and why its scale is not a moisture content.