μ and Sd: the two values that tell you whether a render that lets the wall breathe is fit for purpose
μ factor: how to read vapour diffusion resistance on a technical data sheet
The μ factor is a dimensionless value. It compares a material’s resistance to vapour diffusion with that of air. On a technical data sheet, look for “water vapour diffusion resistance factor (μ)”, ideally based on a standardised test. The lower the μ, the more permeable the render. But for a render that lets the wall breathe, μ alone is not enough.
Sd (m): the equivalent air layer thickness, the most useful value for comparing two products
Sd is calculated as Sd = µ × e, with e in m. It is the actual resistance of the layer as installed. Two renders with the same μ cannot be compared if the finished thickness differs. Keep permeability increasing toward the outside. In practical terms, avoid an exterior Sd greater than the interior Sd on an old wall.
Useful order-of-magnitude benchmarks on site in 2026: what counts as “open”, “intermediate” or “blocking”
Quick benchmarks for a finished 15 mm layer. To limit pathologies, the dew point calculation also helps check the risk of condensation within the build-up.
| Level | Sd (m) | Equivalent μ at 15 mm (-) | Interpretation |
|---|---|---|---|
| Open | ≤ 0.2 | ≤ 15 | Compatible with air lime, earth and plaster. |
| Intermediate | 0.2 to 1 | 15 to 70 | Often the case with an NHL render or a multi-coat system. |
| Blocking | ≥ 1 | ≥ 70 | Risky on damp masonry, especially with film-forming paint. |
Golden rule in historic masonry: increasing permeability from inside to outside (and why)
The physical principle: let vapour out without trapping water in the wall
In historic masonry, moisture comes from capillarity and water vapour transfer. If you apply a tighter layer on the outside, vapour migrates, cools, then condenses within the thickness. The wall dries less, salts concentrate, the facing layers debond, and frost does the rest.
Typical cases: stone walls, rubble stone, rammed earth, old brickwork with weak joints
These substrates are heterogeneous, with joints and binders that are often soft. A cement render or a film-forming coating greatly increases vapour diffusion resistance on the outside. You then push moisture inward or further up the wall. By contrast, a render that lets the wall breathe preserves a drying path, provided the insulation and finishes remain consistent.
Simple verification method: layer stack-up, identify the tightest Sd
Build the layer stack from inside to outside. Note the μ factor and thickness e from each technical data sheet. Calculate Sd (m) = μ × e (m). Identify the layer with the highest Sd. It must not end up on the outside. The practical rule is a global Sd that decreases toward the outside.
Comparing render families from a “breathable” angle: lime, earth, plaster… and what blocks moisture
Air lime and natural hydraulic lime: differences in vapour behaviour, liquid water and strength
On the technical data sheet, compare μ and Sd. At equal thickness, air lime is generally more open to vapour and more flexible. An NHL render slightly restricts vapour flow, handles liquid water better (driving rain) and gains strength more quickly. It is often the right façade compromise, as long as the outside remains more permeable than the inside.
Earth and plaster indoors: when they are compatible with old walls (and their limits)
Earth and plaster provide good hygroscopic buffering and make a render that lets the wall breathe if the substrate is not exposed to runoff or active rising damp. Limits: sensitivity to liquid water, salts and impact. Plan treatment of the wall base first, and use a non-film-forming finish.
Cement, overly tight bastard renders, film-forming paints and thick plastic coatings: why they trap moisture
The issue is not “cement” in itself, but the combination of higher rigidity and overly restricted vapour diffusion. You increase Sd, you block evaporation. Typical result on old buildings: rising damp moves higher, salts crystallise, the render blisters, then frost and spalling break down the stone or rubble masonry. The same logic applies to exterior polymer renders and film-forming paints.
When you block an old wall: the defects to recognise before re-rendering
Rising damp moving higher: the wall looks for a new evaporation zone
A render that is too tight (cement, thick plastic coating, film-forming paint) reduces evaporation on the façade. Ground moisture keeps rising, then the wall shifts its evaporation zone higher up, or inward. During an inspection, map the damp areas, look for a sharp break line, and measure the gradient from bottom to top.
Salts, blisters, debonding: mechanism and warning signs at the base of walls
Capillary water carries salts. When drying occurs beneath the render, salt crystallisation builds pressure. The result is blisters, powdering, hollow-sounding areas and debonding at the skirting zone. Before re-rendering, identify efflorescence, scrape to assess cohesion, and note the actual thickness in place.
Freeze/thaw in stone and interior damp patches: impacts on the façade and in living spaces
On the façade, moisture trapped in soft stone accelerates surface freeze/thaw, causing flaking and breakdown. Indoors, vapour can be blocked, which leads to damp patches and recurring moisture at thermal bridge locations. Aim for a render that lets the wall breathe and maintain hygrothermal continuity with the associated insulation.
Render + insulation: build a coherent system to avoid trapping moisture
On old walls, render and insulation should not be selected separately. You assemble layers that manage water vapour, capillary transfer and drying. The decisive criterion remains the Sd pairing and the continuity of the build-up.
With internal insulation: manage the cold side and choose vapour-compatible materials
Internal wall insulation cools the masonry and creates a cold side at the wall. Choose vapour-open, capillary-active insulation materials, with a lining that buffers moisture (earth, plaster, lime). Avoid default vapour barriers. Calculate or justify the diffusion behaviour and treat junctions (wall bases, cross walls) to limit condensation and salts.
With external insulation: how a vapour-open façade helps secure an old wall, and when it makes sense
A vapour-open external insulation system keeps the wall warmer and helps it dry outward. This is where a breathable render, or a render that lets the wall breathe, really matters. Be careful with very damp or salt-contaminated substrates. Beforehand, secure rising damp, runoff water and joint integrity.
Quote-checklist for choosing the system: substrate, moisture, Sd of each layer, finishes, and a possible route to an external insulation option
- Substrate. Type, cohesion, salt presence, areas repaired with cement.
- Moisture. Readings and causes (wall base, drainage defects).
- Sd. State the Sd of each layer in the quote, from the facing layer to the outer render.
- Finishes. Mineral paints, no thick plastic film.
- Option. If internal insulation is risky, provide a vapour-open external insulation alternative.
