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