5.15 Hygrothermal modelling, surveying, monitoring and analysis

1. Hygrothermal modelling

Understanding the general principles and typical behaviour of older buildings is invaluable whether looking after an old dwelling or planning its retrofit.

Part of our understanding of what is happening has been derived from observation. However, modelling and calculation have proved invaluable in identifying which processes are most important, and evaluating the extent of the risk.

In many situations, where tolerances are close, it is worthwhile undertaking specific calculations for that situation, to help retrofit to proceed as safely as possible.

2. Glaser method

The Glaser method is an approach to assessing the risk of interstitial condensation at the design stage.

The method essentially carries out two separate calculations for locations though a construction, namely, the temperature and the dew point, for a given set of temperature and vapour pressure conditions either side. If at any location the expected temperature is below the dewpoint then moisture build-up at the point is predicted.

When assessing the dangers to a particular construction assembly arising from moisture, this method obviously has limitations and these should be recognised to avoid designers, manufactures and suppliers relying on it inappropriately. In retrofit more so than new build, other moisture sources and mechanisms are likely to dominate over interstitial condensation, but condensation predictions remain a very useful, early indicator of good assembly design.

The report (projects.bre.co.uk/moisture/Moisture.ppt, 2004 BRE Scotland) concluded that (in a new-build context) the Glaser method is suitable for lightweight structures (i.e. all timber construction) with little thermal or moisture storage capacity, but more complex models are needed for heavy (i.e. masonry) construction: more specific material properties and climate data are also needed.

Joseph Little sums up the limitations of Glaser succinctly:

“Glaser method vs. numerical calculation

The Glaser method (under I.S. EN ISO 13788) has been (and still is) widely used to assess risk of interstitial condensation. It is a simplified, steady-state method assuming that moisture transport is by vapour diffusion alone. It doesn’t take account of capillary transfer of liquid water, moisture storage capacity of materials, driving rain, construction moisture, solar radiation and other important phenomena. Therefore, following the scope of its own relevant standard (I.S. EN ISO 13788), the Glaser method is not suitable for assessing components where the effects of the above mentioned phenomena are not negligible.

Numerical hygrothermal simulation (under I.S. EN 15026) provides far more reliable information on the risk of moisture problems within building components and on the design of preventive or remedial treatment.” (Joseph Little, 2016)

Further reading:

‘Breaking the Mold’ part i by Joseph little, https://passivehouseplus.co.uk/articles/insulation/breaking-the-mould-part-i , accessed 2^nd August 2023.

3. Hygrothermal modelling software

Forty-five computerized hygrothermal modelling tools were identified by the Canada Mortgage and Housing Corporation, of which eight satisfied their 7 point criteria. Those eight were:

• DELPHIN4 is a two-dimensional model for transport of heat, air, moisture and salt in porous materials. It arose from the old DIM program developed at the Technical University of Dresden, Germany. A 30-day trial program can be downloaded at no cost.
• EMPTIED is a one-dimensional model for heat and moisture transport, with some considerations for air leakage included. It can be obtained at no cost by request from Canada Mortgage and Housing Corporation.
• GLASTA is a one-dimensional model for heat and moisture transport. It is based on the Glaser method, but includes a model for capillary distribution within the layers of the assembly, and may be suitable for assessing drying potential. It is available from a Belgian company called Physibel Building Physics Software.
• MATCH is a one-dimensional model for heat and moisture transport. Dr. Carsten Rode developed it as a DOS program at the Technical University of Denmark. The DOS version can be obtained from Bygge-og Miljoteknik, a Danish insulation supplier. A Windows© version is available from Rockwool A/S, but only in Danish. The distributor reports that an English-language version of the MS-Windows© program is now available but the program was not available in time for this review.
• MOIST is a one-dimensional model for heat and moisture transport. It was developed at the National Institute for Standards and Testing in the United States, and can be downloaded free of charge from their Internet site. It models moisture transfer by diffusion and capillary flow, and air transfer by including cavities that can be linked to indoor and outdoor air.
• 1D-HAM is a one-dimensional model for coupled heat, air and moisture transport in a multi-layered porous wall. The program uses a finite-difference solution. The moisture-transfer model accounts for diffusion and convection in vapor phase, but not liquid water transport. Heat transfer occurs by conduction, convection and latent-heat effects.
• UMIDUS is a model for coupled heat and moisture transfer within porous media. Diffusion and capillary regimes are modelled, so moisture transport occurs in the vapour and liquid phases. The model can be downloaded at no cost, but only Brazilian climatic data are included.
• WUFI is designed to calculate one-dimensional coupled heat and moisture transport. Heat transfer occurs by conduction, enthalpy flow (including phase change), short-wave solar radiation and long-wave radiative cooling (at night). Convective heat and mass transfer is not modeled. Vapour-phase transport is by vapour diffusion and solution diffusion, and liquid-phase water transport is by capillary and surface diffusion.

In the UK it would appear that WUFI is the best known and perhaps currently is the most widely used.

4. What can WUFI do?

WUFI expert, Joseph Little:

“WUFI is a menu-driven, user-friendly software developed by the Fraunhofer Institute for Building Physics (Germany) and validated using data derived from outdoor and laboratory tests. It allows numerical calculation of transient hygrothermal performance of building components, exposed to realistic climate conditions.
It can be used for assessing:

• the risk of interstitial condensation
• the risk of moisture accumulation from driving rain
• the risk of mould growth
• the risk of rot or corrosion in structure
• the risk of freeze-thaw damage
• the hygrothermal effect of retrofit measures
• the time required for construction moisture to dry out in masonry walls
• the moisture performance of roof and wall build-ups under unanticipated use or in different climate zones”

From: http://www.josephlittlearchitects.com/architecture/about-wufi accessed 11th December 2016 (no longer available)

Wufi is sensitive to the skill and experience of the user and the data on specific materials in buildings (which requires lab testing). However it captures trends well, if not the exact magnitudes of actual measured results – though there are not many studies available that look at the validation of hygrothermal modelling (remember you can regularly check the AECB knowledge base).

Other software may be of interest to retrofitters exploring options including:

• DELPHIN simulation program for the calculation of coupled heat, moisture, air, pollutant, and salt transport http://bauklimatik-dresden.de/delphin/index.php?aLa=en
• The result of PHIUS and Fraunhofer collaboration, WUFI Passive: http://www.phius.org/software-and-technical-resources/wufi-passive-and-other-modeling-tools/wufi-passive-3-0

Note: the AECB is not recommending any particular product nor has it undertaken any assessments of hygrothermal modelling software as part of the CLR project.

The CLR approach to modelling – combined with monitoring and project data analysis – is illustrated in the CLR case studies in Module 6. In Lesson 5.16 we explain the format and methods underlying the growing number of case studies.