Question: What is a Hygrothermal Assessment?
Answer: A Hygrothermal Assessment analyses the combined movement of heat and moisture through building fabric to determine whether a wall, roof or floor construction is at risk of condensation, damp or mould growth. It considers the thermal properties and vapour resistance of each layer, the internal and external climate, rain exposure and the effect of insulation, membranes and vapour control layers.
Hygrothermal assessments are essential for new construction details and particularly important when retrofitting insulation to existing buildings, where poorly designed solutions can trap moisture and cause decay.
Question: When is a Hygrothermal Assessment required?
Answer: Assessments are commonly required for:
- Internal wall insulation and solid wall retrofit
- Insulating historic and traditionally constructed buildings
- Flat roofs, warm and cold roof constructions
- Timber frame and cross-laminated timber constructions
- Basements and below-ground structures
- PAS 2035 retrofit projects
- Building Regulations Part C and Part L compliance
- BREEAM and Passivhaus projects
- Investigating existing condensation and mould problems
Retrofit funding schemes and PAS 2035 increasingly require moisture risk assessment before internal insulation is installed.
Question: What methods are used for hygrothermal assessment?
Answer: Two main methods are used. The Glaser method in BS EN ISO 13788 is a simplified steady-state calculation of interstitial condensation risk suitable for conventional constructions. Dynamic hygrothermal simulation to BS EN 15026, using software such as WUFI, models heat and moisture transfer hour by hour over several years and is required for moisture-sensitive constructions, traditional buildings and internal insulation.
BS 5250 provides overarching guidance on managing moisture in buildings and identifies when each method is appropriate.
Question: What is interstitial condensation?
Answer: Interstitial condensation occurs when warm, moist internal air penetrates the building fabric and cools below its dew point within the construction, causing moisture to condense inside the wall or roof. Unlike surface condensation, it is hidden and can cause timber decay, corrosion, insulation degradation and mould without visible signs until significant damage has occurred. Hygrothermal assessment identifies whether this will occur and how to prevent it.
Question: Why is hygrothermal assessment important for retrofit?
Answer: Adding insulation to an existing building changes the temperature and moisture profile through the wall. Internal wall insulation in particular makes the original masonry colder and wetter, and if the wrong materials are used moisture can accumulate behind the insulation. PAS 2035 requires a moisture risk assessment for such measures, and hygrothermal modelling provides the evidence that the proposed system is safe and durable.
Question: What outputs does a Hygrothermal Assessment provide?
Answer: The assessment reports the moisture content and relative humidity within each layer over the simulation period, the risk of condensation and mould growth using recognised criteria, drying potential and the long-term trend in moisture. Recommendations are provided on insulation type and thickness, vapour control, breathability, ventilation and detailing at junctions.
Question: Why choose Syntegra Group for a Hygrothermal Assessment?
Answer: Syntegra Group provides hygrothermal assessments to BS EN ISO 13788 and BS EN 15026 using WUFI simulation for new-build details, retrofit projects and existing building investigations across the UK. Our building physics consultants combine moisture analysis with thermal bridging, U-value and airtightness expertise to provide a complete fabric performance assessment.
We support architects, contractors, housing associations and retrofit coordinators with clear reports suitable for Building Regulations, PAS 2035 and funder requirements.
