Technical Lab Knowledge Base

Dew Point Explained

A practical building-physics note on dew point, condensation risk, wet insulation and why the position of insulation matters as much as its thickness.

Dehumidifier beside a condensation-covered window in a flat with a family in the background

Condensation control depends on warm surfaces, correct wall build-up and reliable air exchange before moisture becomes mould risk.

Knowledge base:
Moisture safety
Risk:
Condensation + mould
Design check:
Hygrothermal review

Dew Point

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Core idea

The dew point is not the defect. The defect is allowing dew-point conditions to occur on a cold surface or inside a hidden layer that cannot dry safely.

Fast definitions
Dew pointPhysical condition
CondensationRisk mechanism
Exterior firstSafer envelope logic

The dew point is the temperature at which water vapour in air begins to become liquid water. In a building wall, the question is not whether dew point exists somewhere in the calculation. The question is where that critical condition falls, how often it occurs and whether the material can tolerate and dry the moisture.

If the envelope is poorly insulated, interrupted by thermal bridges, leaky to indoor air or under-ventilated, some surfaces can become too cold. Corners, window reveals, slab edges, roof-wall junctions and hidden interfaces behind insulation are especially sensitive.

This is why insulation continuity matters. A well-designed exterior insulation strategy helps keep the structural wall and interior surfaces warmer and more stable, reducing the chance that condensation forms where it is hard to see, dry or repair.

Monitoring block

Why it matters to monitor

Moisture risk usually grows before it is visible. The usual causes are a wrong wall build-up and insulation strategy, and insufficient room ventilation / air exchange. Tracking temperature, indoor humidity and cold junctions gives time to correct these details before damp surfaces become mould conditions.

Surface temperatureFind the coldest corners, reveals and structural junctions before they become wet zones.
Ventilation / air exchangeHumidity spikes from cooking, showers and drying clothes raise dew-point risk when room air is not replaced reliably.
Drying capacityA safe assembly is not only warm; it can also release incidental moisture before damage accumulates.
Temperature profile through a wallsame physics / risk depends on layer position
room sidestructureexterior insulationoutsidedew-point risk markerA warm structural core is more forgiving than a hidden cold interface behind internal insulation.
Risk signals

Condensation

Cold surfaces can collect moisture when indoor air reaches dew-point conditions near the surface or inside a layer.

Mould risk

Persistent dampness creates conditions where mould can grow, especially in corners, reveals and poorly ventilated areas.

Lower insulation performance

Wet insulation can perform worse than dry insulation and make the wall less predictable than the design calculation.

Finish and layer damage

Repeated moisture cycles can damage plaster, paint, boards, fixings and hidden layers before the issue is visible.

Design note
Design note: dew-point and condensation risk must be checked for the complete wall assembly, local climate, indoor humidity, ventilation strategy, airtightness and thermal-bridge details. These diagrams explain the logic; they do not replace project-specific hygrothermal calculation.

Material Risk

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Material behaviour

Dew point becomes dangerous when the material around it can hold water, lose thermal resistance or pass moisture deeper into the structure.

External insulation is not only about moving the temperature drop outward. The material itself matters. Some insulation types resist liquid water and recover more predictably, while fibrous or porous materials can lose performance quickly when moisture is trapped inside them.

With exterior EPS, graphite EPS or Neopor-type insulation, the critical temperature zone is moved outward into a layer that is not designed to act as a capillary moisture reservoir. The assembly still needs correct joints, render, drainage, flashing and airtightness, but the insulation is less sensitive to temporary moisture than fibrous open insulation.

With mineral wool, wood-fibre boards such as Steico-type insulation or similar vapour-open fibrous materials, the risk logic changes. If installation, airtightness, render, flashing or water protection is wrong, moisture can form or be trapped inside the insulation layer. Once that insulation becomes wet, its thermal resistance can drop sharply.

EPS / graphite EPS / Neopor-type layer
The safer logic is that the temperature drop is moved outward into a moisture-resistant exterior insulation layer.
Mineral wool / wood fibre layer
If moisture is trapped inside fibrous insulation, the layer can lose a large part of its thermal resistance.
AAC / gas block behind wet insulation
Moisture increases heat loss and can reduce mechanical reliability when the structural material stays persistently wet.
Failure sequence
01

Moisture enters insulation

Air leakage, rainwater, poor render, weak flashing or weak vapour control allows moisture into the layer.

02

Fibrous insulation gets wet

Mineral wool or wood fibre can hold moisture in its structure when drying is not sufficient.

03

Thermal resistance drops

The layer conducts more heat and no longer performs like the dry design calculation.

04

Dew point moves inward

The colder zone can shift back toward the wall surface or into the AAC itself.

05

Wall becomes wet

AAC can lose thermal and mechanical performance when persistently wet, and hidden mould risk rises.

System rule
System rule: the wall should be designed as a hygrothermal system, not only as a U-value number. Material type, water absorption, drying ability, airtightness, render, flashing and junction detailing determine whether the dew-point zone remains harmless or becomes a failure mechanism.

Internal Insulation

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Retrofit rule

Internal insulation is technically possible in selected retrofit cases, but it should be treated as an exception, not the preferred strategy for external walls.

Preferred strategy
Professional recommendation

Use a continuous exterior insulation envelope wherever the project allows it.

For new construction and deep retrofit projects, external wall insulation keeps the load-bearing core warmer, protects the structure and makes thermal continuity easier around slabs, corners, window reveals and roof-wall junctions.

Internal wall insulation may look easy because it can be installed from the room side. In building physics, however, it is usually the weaker and more sensitive strategy. The structural wall behind it becomes colder and less connected to indoor warmth.

That colder hidden zone can move the dew-point and condensation risk toward the interface between the insulation and the wall. If warm indoor air leaks into that zone, moisture can form where it is difficult to see, dry or repair.

Hidden risk points

Colder structural wall

The wall behind the insulation becomes colder and less able to buffer indoor temperature swings.

Hidden condensation risk

Moisture can form behind the insulation or inside the wall assembly, where it is difficult to detect.

More thermal bridges

Floors, partitions, slab edges, corners and window reveals interrupt the insulation layer.

More sensitive detailing

Airtightness, vapour control, sockets, pipes, window reveals and junctions become less forgiving.

Exception cases

When internal insulation may be used

Internal insulation may be considered when exterior insulation is not possible, for example in protected historic buildings, strict facade restrictions or partial retrofit situations. In these cases it must be designed with project-specific hygrothermal assessment, airtightness detailing, vapour-control strategy and ventilation design.

Risk note
Risk note: internal insulation can work only when the wall build-up, indoor humidity, vapour control, airtightness and drying path are deliberately resolved. It should not be used as a default solution for external walls without verification.

Wall Logic

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Comparison setup

Same wall, different insulation position: exterior-first insulation keeps the core warmer; internal insulation can create a hidden cold interface.

Layer position
Exterior-first insulationwarmer structure / lower hidden risk
risk kept outwardMain insulation outside the core keeps the structural zone warmer and more stable.

Exterior-first insulation

Main insulation is placed outside the load-bearing concrete core. The wall stays closer to the indoor thermal zone, while the colder part of the temperature drop happens outside the structure.

  • Warmer structural core.
  • More stable interior surface.
  • Lower risk of hidden condensation.
  • Better default logic for new construction and deep retrofit.
Next checks

Related Tech Lab pages

Continue with the main wall-system and continuous-envelope notes to connect moisture safety with product layer logic.

Passive House Block WallsContinuous Thermal EnvelopeWarm Window & Door Installation

Technical references

Use these as orientation sources. Final design should be checked by the project engineer using the actual wall build-up and climate assumptions.

ISO 13788: internal surface humidity and interstitial condensation calculation methodsISO 6946: thermal resistance and thermal transmittance calculation methodsPassive House Institute: wall and construction system requirements
Final check
Final check: this explanatory graph does not replace a project-specific hygrothermal calculation. Final risk depends on the full wall build-up, local climate, indoor humidity, ventilation, airtightness and thermal-bridge detailing.