How to Interpret Hygrothermal Modelling Results

Hygrothermal modelling results may include temperature, relative humidity, moisture content and moisture movement within a defined construction assembly.

Important considerations include the magnitude and duration of elevated moisture conditions, whether moisture accumulates over time, and whether the assembly demonstrates drying under the modelled conditions.

Results should be considered at relevant material layers and interfaces rather than relying only on whole-assembly averages.

The significance of a result depends on material sensitivity, the assessment method, the applicable evaluation criteria and the assumptions used to develop the model.

A modelling report should communicate both the findings and their limitations so that project teams can understand which design decisions the results support.

 

A transient hygrothermal model simulates changing heat and moisture conditions within a defined construction assembly.

Depending on the software, model configuration and available material data, outputs may describe conditions at selected positions within a wall or roof, across individual material layers or for the assembly as a whole.

Common results include temperature profiles, relative humidity histories, material moisture content and changes in total moisture storage.

Some assessments may also evaluate moisture-related performance criteria, such as mould growth potential or the suitability of particular materials under the simulated conditions.

These outputs provide different information. A temperature curve does not independently establish material moisture damage, while a whole-assembly moisture trend may not reveal a localised condition at a sensitive interface.

Interpretation begins by identifying what each output represents, where it has been calculated and why that location matters to the assessment.

 

Relative humidity curves show how the humidity conditions at a defined position within an assembly change over the simulation period.

These curves may help identify seasonal variations, recurring periods of elevated humidity or conditions that persist for extended durations.

A high relative humidity value should not be interpreted in isolation.

Its significance depends on the temperature at that location, the duration of exposure, the adjacent materials and the evaluation criteria being applied.

A brief increase in relative humidity may have a different implication from sustained exposure over weeks or months.

Similarly, the response of a moisture-tolerant material may differ from that of a material sensitive to prolonged humid conditions.

There is no single relative humidity percentage that can be used as a universal pass or fail threshold for every building assembly and material.

 

Moisture content describes the amount of water stored within a material or a defined part of the construction.

Depending on the output and material, moisture content may be expressed by mass, volume or another defined unit.

The unit and reporting basis must be understood before comparing results.

Different materials have different moisture storage characteristics. The same numerical moisture content does not necessarily indicate the same performance condition in timber, masonry, insulation or other construction products.

Moisture content results can help identify whether a material is gaining or releasing moisture over time.

They may also assist with assessing whether modelled conditions approach relevant material-specific criteria.

A moisture content value should therefore be interpreted in relation to the material, the reporting unit, the exposure history and the intended assessment criterion.

 

A single moisture content value provides information about a particular point in time.

A moisture content trend provides additional information about how the material or assembly responds throughout the simulation.

Some assemblies may experience seasonal moisture increases followed by periods of drying.

Others may show moisture levels that remain elevated or increase over successive cycles.

Persistent upward trends can indicate that moisture inputs exceed the modelled capacity for moisture release over the period considered.

However, early simulation results may also be influenced by assumed initial moisture conditions. A reduction or increase during an initial adjustment period should not automatically be interpreted as long-term performance.

The assessment should distinguish initial-condition effects, seasonal fluctuations and any sustained moisture accumulation relevant to the design.

 

Drying behaviour describes how moisture stored within a construction assembly may decrease under the modelled environmental conditions.

In a hygrothermal simulation, drying may be reflected by reductions in material moisture content or total assembly moisture storage.

The timing and extent of these reductions can help indicate whether moisture introduced during one period is released during another.

For example, an assembly may experience increased moisture storage during cooler months followed by drying under warmer or drier conditions.

The presence of drying does not automatically establish acceptable performance.

A material may still experience prolonged elevated humidity or moisture conditions that are relevant to its durability.

Drying trends should be interpreted alongside material-specific exposure conditions and the overall moisture balance.

 

Building envelopes are exposed to environmental conditions that change throughout the year.

Temperature, humidity, rainfall exposure and solar conditions may influence moisture behaviour, depending on the modelled processes and boundary conditions.

Seasonal patterns can help explain why moisture conditions rise during some periods and decrease during others.

In cooler conditions, some assemblies may experience increased moisture exposure near colder construction interfaces.

In warm or humid climates, moisture movement may follow different patterns, particularly where indoor cooling affects temperature and vapour-pressure relationships.

These patterns should not be generalised solely from the building's climate zone.

The relevant behaviour depends on the complete assembly, orientation, exposure, indoor conditions and model assumptions.

 

Moisture conditions are not necessarily uniform throughout a wall or roof assembly.

Different layers may experience different temperatures, relative humidity levels and moisture storage conditions.

An assembly-wide moisture value may therefore conceal localised conditions at a material interface.

For example, the conditions adjacent to a moisture-sensitive lining or sheathing material may be more relevant to a particular risk than the average moisture content of the complete wall.

Interpretation should identify which layer or interface each result represents and why that position was selected.

This is especially important where insulation, membranes or materials with different vapour properties create distinct temperature and moisture conditions within the assembly.

A result should not be described as representative of every part of the building envelope when it applies only to a defined modelled location.

 

Different modelling outputs answer different questions. The table below provides a practical guide to their interpretation.

Model output	What it may indicate	Interpretation consideration
Temperature	Thermal conditions at defined locations.	Must be considered with relevant moisture conditions.
Relative humidity	Humidity conditions at a modelled location.	Duration, temperature and material sensitivity matter.
Material moisture content	Moisture stored within a material.	Units and material-specific criteria must be understood.
Total assembly moisture	Overall moisture storage changes.	May not reveal localised vulnerable interfaces.
Drying trend	Moisture reduction over time.	Drying alone does not demonstrate acceptable material exposure.
Moisture-related performance indicator	Potential concern under a defined evaluation method.	Criteria, applicability and limitations must be established.

 

A modelling result becomes more useful when it is evaluated against criteria appropriate to the material, exposure and purpose of the assessment.

Depending on the project, criteria may relate to prolonged elevated humidity, material moisture content, mould growth potential or other moisture-related performance concerns.

Different criteria may use different variables, exposure periods and assumptions.

A threshold developed for one material or assessment purpose should not automatically be applied to another.

The selected criteria should be documented, together with the reason they are appropriate for the construction being investigated.

Where a recognised assessment standard or project-specific requirement applies, the evaluation should reflect its defined scope and conditions.

The presence of a numerical threshold does not remove the need for professional interpretation of the underlying model assumptions and exposure conditions.

 

Hygrothermal results can provide temperature and humidity histories that may be used in an appropriate mould growth evaluation method.

Such evaluations consider whether the modelled conditions could support mould growth under the assumptions of the selected method.

The relevance of a mould growth indicator depends on factors including the material or substrate, temperature, humidity duration and the limitations of the evaluation model.

A predicted mould growth indicator is not the same as a physical observation of mould in an existing building.

Similarly, a result below a selected evaluation threshold does not establish that every possible moisture-related risk has been eliminated.

For a broader discussion of moisture-sensitive materials and prolonged exposure, see Moisture Accumulation and Mould Growth Risk in Building Materials.

 

Hygrothermal modelling begins with defined initial conditions, including assumptions about moisture stored within the construction.

These assumptions can influence the early part of a simulation.

A construction modelled with relatively high initial moisture may show an early drying trend that reflects those starting conditions.

Conversely, a model beginning with relatively low moisture content may require time to demonstrate how the assembly responds to recurring environmental exposure.

The simulation duration should be appropriate for the assessment question.

Where long-term moisture accumulation is a concern, a period that is too short may not reveal whether moisture levels continue to rise over successive cycles.

Interpretation should distinguish the influence of initial conditions from recurring seasonal behaviour and longer-term trends.

 

A hygrothermal model represents a defined construction under selected environmental and material assumptions.

It does not automatically represent every physical process or construction condition that may occur in practice.

For example, conventional one-dimensional assembly modelling may not adequately capture localised steel framing, complex junction geometry or unintended air leakage.

Material data may also be uncertain or differ from the characteristics of products ultimately installed on site.

Indoor conditions, external exposure and initial moisture assumptions can materially influence results.

These limitations do not make modelling unhelpful. They define the circumstances in which its conclusions can reasonably be applied.

A well-communicated assessment should identify the modelled construction, assumptions, evaluation criteria, exclusions and any additional investigation needed.

 

A useful modelling report should make the relationship between the results and the proposed design clear.

It should identify the construction being assessed, the relevant environmental conditions and the locations at which moisture behaviour has been evaluated.

The report should also explain the criteria used to interpret the results and whether any modelled conditions warrant further attention.

Key questions include:

• Which wall or roof assembly was modelled?
• Which climate and indoor conditions were assumed?
• Which material layers or interfaces were evaluated?
• Do the results show recurring or sustained elevated humidity?
• Does moisture accumulate over successive periods?
• Are there identifiable drying periods?
• Which materials are sensitive to the modelled exposure?
• What evaluation criteria were applied?
• What assumptions or exclusions limit the findings?
• What design decisions can reasonably be supported by the results?

The objective is not for every project participant to independently reproduce the simulation.

It is to ensure that the findings are understandable, traceable and relevant to the proposed construction.

 

The purpose of interpreting modelling results is to understand whether the assessed construction performs appropriately under the defined conditions and criteria.

Where results indicate potentially adverse moisture conditions, they may help identify which interfaces or materials require closer consideration.

The findings may support further investigation of material selection, membrane arrangements, insulation position or other aspects of the building envelope.

However, a model result does not automatically prescribe a particular design change.

Any proposed response should be considered in relation to the complete construction, other performance requirements and the moisture mechanism being addressed.

Where an alternative arrangement is proposed, additional assessment may be appropriate to establish whether it addresses the identified concern.

The value of the report lies in translating modelled conditions into clear, proportionate and technically supported design decisions.

 

• Confirm the exact construction assembly represented by the model.
• Review the indoor and external boundary conditions.
• Identify the material layers and interfaces being evaluated.
• Check the units and reporting basis of moisture content results.
• Review temperature and relative humidity together.
• Consider the duration of elevated humidity conditions.
• Identify seasonal moisture accumulation and drying trends.
• Distinguish initial-condition effects from recurring behaviour.
• Consider the sensitivity of the materials involved.
• Confirm the evaluation criteria and their applicability.
• Review the simulation period and relevant model limitations.
• Identify whether separate air-leakage or junction analysis is needed.
• Confirm which conclusions are supported by the results.
• Document any design questions requiring further investigation.

 

What does a relative humidity graph show in hygrothermal modelling?

A relative humidity graph shows how humidity changes over time at a defined location within the modelled assembly. Interpretation depends on temperature, exposure duration, material sensitivity and the relevant evaluation criteria.

Does high relative humidity always mean condensation has occurred?

No. Elevated relative humidity does not necessarily mean liquid condensation has formed. Moisture-related material risks may also arise without visible liquid water, depending on the material and duration of exposure.

What does increasing moisture content indicate?

Increasing moisture content indicates that the modelled material or assembly is storing more moisture during the period shown. The significance depends on whether the increase is temporary, seasonal or sustained, as well as the material-specific criteria.

Can an assembly pass if moisture increases during winter?

Seasonal moisture increases do not automatically indicate unacceptable performance. The assessment should consider the magnitude and duration of exposure, relevant material criteria and whether the assembly demonstrates appropriate drying under the modelled conditions.

Can hygrothermal modelling predict mould growth?

Temperature and humidity histories from hygrothermal modelling may be evaluated using an appropriate mould growth assessment method. The result represents modelled growth potential under defined assumptions, not confirmation of mould in a physical building.

Does a favourable hygrothermal result guarantee that a building will remain moisture-free?

No. A favourable result applies to the defined model, assumptions and evaluation criteria. It does not guarantee performance under every possible construction, weather, operating or maintenance condition.

Moisture & Condensation Risk Analysis

Explore the wider principles of condensation risk, moisture behaviour and building envelope assessment.

Dew Point Calculations vs Hygrothermal Modelling

Understand the differences between simplified condensation calculations and transient moisture modelling.

Moisture Accumulation and Mould Growth Risk

Review how moisture exposure, duration and material properties influence potential deterioration.

Drying Potential in Wall and Roof Assemblies

Understand why moisture release and drying behaviour matter when evaluating construction assemblies.