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Evaluating moisture risks in vapour-open biobased wall assemblies: design and methodology of a long-term field study

Authors
  • Christianne Luijten (TNO – Building Materials and Structures, Delft, The Netherlands)
  • Karin Kompatscher (TNO Buildings and Energy Systems, Delft, The Netherlands)

This article is a preprint and is currently undergoing peer review by UCL Open: Environment.

Abstract

The transition toward rapid, low‑carbon housing in the Netherlands has accelerated the adoption of biobased light timber frame construction (LTF) systems. However, the suitability of vapour‑open biobased wall assemblies in the country’s humid, temperate maritime climate (Cfb) remains insufficiently validated. This paper presents the design and methodology of a comprehensive five‑year field study aimed at experimentally assessing the long‑term hygrothermal performance, moisture risks, and thermal stability of vapour‑open versus vapour‑closed LTF walls insulated with five biobased materials.

Two identical full‑scale dwellings were constructed in Groningen, The Netherlands, differing only in the presence (vapour‑closed) or absence (vapour‑open) of a PE vapour barrier. Each dwelling incorporates five biobased insulation types: grass, cellulose, hemp‑jute, wood fibre, and flax, installed across multiple façade orientations. A dense array of embedded sensors measures moisture content (M%), relative humidity (RH), and temperature (T) at hourly intervals within wall layers and indoor spaces, complemented by local and KNMI weather data.

Laboratory characterisation of sorption isotherms, vapour diffusion resistance, and baseline moisture content supports the interpretation of field measurements. The monitoring strategy is designed to test three hypotheses: (1) vapour‑open assemblies maintain sufficient drying capacity to avoid exceeding critical timber moisture thresholds associated with decay; (2) moisture‑induced thermal resistance (R‑value) reduction differs between vapour‑open and vapour‑closed walls; and (3) hygroscopic biobased materials contribute to measurable differences in indoor moisture buffering.

The study provides one of the first long‑term, high‑resolution datasets for biobased LTF assemblies in a temperate, maritime climate. Its outcomes will inform moisture‑safe design principles for sustainable, vapour‑open construction systems in the Netherlands and similar regions, contributing to evidence‑based policy and building practice.

Keywords: Moisture buffering, Vapour-open dwellings, Hygroscopic behaviour, Light Timber Frame Wall, Biobased materials

Preprint Under Review
License
Creative Commons Attribution 4.0

 Open peer review from Peter Rickaby

Review
Luijten C and Kompatscher K

Evaluating moisture risks in vapour-open biobased wall assemblies: design and methodology of a long-term field study

1. The paper describes the design and methodology for research to validate theoretical moisture-related risks in light timber frame construction in the CfB climate zone (i.e. the Netherlands, the UK and Ireland). This topic falls within the aims and scope of UCL Open Environment.

2. The paper presents a justified and detailed experimental design and methodology for the research, which is intended to address a research gap confirmed through a literature review. In that sense, the research will be breaking new ground and will make an original contribution to the field. However, this can only be confirmed once the proposed analysis of data from the study is completed, after five years; this is intended to be the subject of one or more subsequent papers.

3. Hygrothermal studies of construction are becoming common, so the field is difficult to keep up with. However, the narrowing of the study to light timber frame construction with vapour-permeable insulation, in a specified climate zone, narrows the field significantly. The bibliography included in the paper suggests that the authors are familiar with the current state of knowledge of the chosen topic.

4. The research design and methodology appear to be robust. Consideration of the following questions might strengthen the research design and methodology.

• How will a 'realistic' internal environment be simulated, especially with respect to the dynamic change in internal temperature and relative humidity during occupation? Might it be possible to study the impact of intermittent occupancy and heating (which is common in UK homes)? Many homes are heated only during short morning and evening periods, especially where households are in fuel poverty.

• Will there be deliberate ventilation (as is legally required in most dwellings) and will it be continuous or intermittent? If real ventilation is to be simulated, will moist air be extracted from the 'wet' rooms (i.e. kitchens and bathrooms) and a balancing supply of external air be supplied to other rooms? This may have a significant impact on humidity within the spaces and the structure.

• How does the positioning of the construction assemblies and the humidifiers relate to the position of relatively 'wet' spaces (i.e. kitchens and bathrooms) within the test dwellings?

• In Dwelling 2 (with vapour barrier), are the junctions and edges of the vapour barrier sealed, and if so, how? Was the vapour barrier also incorporated in the roof construction? If so, in just Dwelling 2 or in both dwellings?

• Why was the interstitial monitoring equipment not placed in the north-east wall assemblies? Might there be some morning effects potentially observable there (different from evening effects observed in the north west wall)?

• Are the various panels sufficiently isolated from each other that mould or rot appearing in one assembly cannot infect adjacent panels, thus distorting results?

5. The paper is relatively short (eleven pages including illustrations and bibliography), but it is the first of a series. Subsequent papers reporting five years of data collection and analysis are likely to be much longer. Given the timescale, an interim paper reporting preliminary results may be appropriate.

6. The paper is clearly written and concisely presented. The description of the experimental design and methodology is detailed and comprehensive.

Note:
This review refers to round 1 of peer review and may pertain to an earlier version of the document.

 Open peer review from Carsten Rode

Review
The paper informs about plans for measurements of moisture conditions in walls with biobased insulation in the exterior walls of two identical test buildings - one with, and one without the use of vapour barriers. Several insulation types and several wall orientations will be investigated. evral years pf test are beiung planned.

However, since there are no results from the test house at this moment, the measurements are just about to commence, it seems, the paper has only limited interest to its readers.

Furthermore, the paper contains several elementary editorial flaws that disrupt the reading.

The paper seems very much focused on Dutch conditions and with Dutch references. The presentation and interests should be broadened to be more international in focus.

Altogether, the paper must be rejected for these reasons.

Note:
This review refers to round 1 of peer review and may pertain to an earlier version of the document.