PÉCLETBuilding Physics
PÉCLET / Disciplines / 04p — Pa · RH — %

Moisture

Moisture moves slowly, invisibly, and it is the failure mode that outlives everyone on the project.

QuantityVapour pressureTransportDiffusion · CapillarityInteractionDrag to change interior humidity. Watch where the condensation plane settles inside the build-up.
Fig. 04 — Vapour through a build-upRaise interior humidity and watch the plane move outward
With interior humidity at 59 per cent and a moderately vapour-resistant inner layer, the condensation plane sits 47 per cent of the way through the build-up from the exterior face — inside the structural depth, where accumulated moisture would be a durability risk.
ExteriorCool · dryInterior59 % RHVapour drive ← outwardCondensation plane inside the structural depth
A magnified cross-section through a porous material, shown as a dense field of pores. Vapour particles perform a slow random walk from the high-humidity side toward the low-humidity side, drifting rather than streaming. A dew-point plane runs through the section; particles crossing it slow, settle and accumulate as condensate. Moving the plane changes where accumulation occurs.

The position

Water vapour diffuses through materials along a pressure gradient. Somewhere inside the assembly the temperature falls low enough for that vapour to condense — and where that plane sits, relative to the layers, is a design decision whether or not anyone made it deliberately.

Vapour control, drying capacity and material selection are one problem. An assembly that cannot dry in at least one direction is a liability regardless of its U-value.

What we ask

  • 01Where does the dew point fall inside this assembly, in winter?
  • 02Can this build-up dry, and in which direction?
  • 03Is the vapour control layer continuous, or is it a drawing convention?
  • 04What does this detail look like after fifteen monsoons?

How we answer it

  • 01Interstitial condensation assessment
  • 02Vapour control strategy and layer sequencing
  • 03Material permeability and drying-capacity review
  • 04Junction and interface risk review