Moisture Behaviour in Solid‑Wall Construction

A practical explanation of how moisture moves through older brick, stone and blockwork walls

Most older homes across Northamptonshire were built with solid brick, stone or early blockwork walls. These structures behave very differently from modern cavity construction, especially when it comes to moisture. Solid masonry absorbs water, releases it slowly and responds strongly to temperature changes. Understanding this behaviour is essential when deciding how to insulate or renovate an older property.

This page explains how moisture moves through solid walls, why some walls stay damp longer than expected, and how external wall insulation interacts with this natural behaviour.

1. Why solid walls handle moisture differently

Unlike cavity walls, which separate the outer leaf from the inner leaf, solid masonry is a single mass.
This means:

• rain and humidity can be absorbed directly into the wall

• moisture moves inward and outward depending on temperature and vapour pressure

• the wall cools quickly in winter, slowing down drying

• shaded or exposed elevations behave very differently

In older Northamptonshire homes, we often see walls that are structurally sound but thermally unstable — warm on the inside, cold on the outside, and holding moisture for longer than people expect.

2. The main sources of moisture in solid walls

Moisture can enter or remain in a wall through several pathways:

a) Rain and wind‑driven rain

Exposed elevations — especially west‑facing walls — absorb more water during storms.
Northamptonshire has areas with moderate wind‑driven rain exposure, which affects drying times.

b) Ambient humidity

Cold masonry attracts moisture from indoor air, especially in winter when internal humidity is higher.

c) Ground contact

Older walls without damp‑proof courses can draw moisture upward through capillary action.

d) Existing coatings

Dense paints, cement renders or old pebble‑dash can slow drying and trap moisture in the wall.

e) Thermal behaviour

Cold walls dry slowly.
Warm walls dry faster.
This is the core principle behind why external insulation helps stabilise moisture behaviour.

3. How moisture actually moves through solid masonry

Moisture movement in solid walls is driven by three main mechanisms:

a) Absorption

Brick and stone absorb water through pores and capillaries.
The rate depends on material density, exposure and surface condition.

b) Vapour diffusion

Water vapour moves through the wall from warm, humid areas toward cooler, drier areas.
This is why cold external walls often attract internal condensation.

c) Evaporation and drying

Moisture leaves the wall when the surface is warm enough and exposed to airflow.
Shaded elevations dry more slowly than sun‑exposed ones.

This cycle repeats constantly throughout the year.

4. Why some solid walls stay damp longer

In many older homes we survey, the wall is not “wet” in the sense of water ingress — it is simply slow to dry.

Common reasons include:

• cold external surfaces

• shaded elevations

• dense cement render preventing outward drying

• trapped moisture behind old coatings

• high internal humidity

• freeze–thaw cycles slowing evaporation

This is why homeowners often describe a “cold damp feeling” even when there is no visible water ingress.

5. How external wall insulation affects moisture behaviour

External wall insulation does not seal the wall.
It stabilises it.

By keeping the masonry warmer, EWI:

• reduces the time moisture stays in the wall

• lowers the risk of interstitial condensation

• reduces freeze–thaw stress

• supports more predictable drying

• shifts the dew point outward, away from the internal surface

This is one of the reasons EWI works naturally with older solid‑wall homes.
If you want a deeper explanation of how the full system build‑up works, we cover this in our guide on how external wall insulation systems work.

6. Why internal insulation behaves differently

Internal insulation (IWI) keeps the wall cold.
This means:

• the masonry remains at external temperature

• moisture stays in the wall longer

• the dew point can move into the structure

• cold bridges around joists and openings become more critical

This doesn’t mean IWI is “bad” — it simply behaves differently.
We explain the differences in more detail in our EWI vs IWI vs CWI comparison.

7. How to recognise moisture‑related behaviour in older walls

Homeowners often notice:

• cold internal surfaces

• patchy condensation

• mould in corners or behind furniture

• walls that feel “cold to the touch”

• seasonal dampness in winter

• render that cracks or debonds due to freeze–thaw

These are usually signs of thermal instability, not necessarily water ingress.

8. How EWI helps stabilise the wall

By warming the masonry and protecting it from direct rain exposure, EWI:

• reduces moisture retention

• improves internal comfort

• reduces condensation risk

• protects the wall from weathering

• supports long‑term durability

If you’re unsure whether your wall type is suitable for external insulation, we outline typical construction types in our guide to which walls can use external wall insulation.

If you’re considering insulation for a solid‑wall home

Every building behaves differently.
The only reliable way to understand moisture behaviour is to look at the wall itself — exposure, coatings, construction type and thermal patterns all matter.

If you’d like to discuss your property, we’re always happy to take a look and talk through the options in a straightforward, practical way.