"Hygrothermal consequences of applying exterior air barriers in wood-frame walls" - Jelle Langmans, KUL & KAHOSL

The execution of exterior air barrier systems in timber frame construction has gained increasing interest in the aim to meet the severe airtightness requirements of Passive houses. In practice, fewer joints and perforations of exterior air barriers are an advantage compared to the traditional interior air barrier systems. However, for cold and moderate climates the hygrothermal impact of exterior air barriers remains unstudied. Consequently, currently the execution of exterior air barriers is not recommended for these climate types.

The present paper performs numerical HAM*-simulations on light weight walls with exterior air barrier systems to explore the risks herein involved. Yearly simulations under realistic climate data were conducted with an adjusted version of DELPHIN 5, capable to model forced and natural convection in interaction with detailed heat and moisture transport. The applied model was extensively documented and evaluated in previous work.

The current article presents the results of a parameter analysis that studies the hygrothermal performance of various timber frame wall configurations with exterior air barrier systems. The impact of different insulation materials, air barrier materials and interior finishing materials are discussed.

It is demonstrated how the application of mineral wool insulated timber frame walls in combination with exterior air barriers results in increased moisture loads, which are driven by natural convection. Moreover, small air gaps between the mineral wool blankets and the adjacent exterior air barrier significantly increase natural convection and adds up to harmful moisture levels. Yet, the use of blown-in insulation can avoid these issues. Finally, the study concludes on the necessity of applying exterior air barrier materials which have a sufficient thermal resistance and vapour diffusivity in combination with low air permeability levels.

* Heat, Air and Moisture

Jelle Langmans is mandate holder and PhD student of 'Feasibilty of light weight constructions with an exterior air barrier '. He spoke for a large audience including architects, students, manufacturers, national and international scientists, ...

Some of his publications :

  • Langmans, J., Klein, R., Roels, S. (2013). Quantitative study on the hygric response of timber frame walls with exterior air barriers. . International passive house conference. Frankfurt, 19-20 April 2013.
  • Langmans, J., Klein, R., Roels, S. (2013). Numerical and experimental investigation of the hygrothermal response of timber frame walls with an exterior air barrier. Journal of Building Physics, art.nr. doi:10.1177/1744259112473934.
  • Langmans, J., Klein, R., Roels, S. (2012). Hygrothermal risks of using exterior air barrier systems for highly insulated light weight walls: A laboratory investigation. Building and Environment, 56 (10), 192-202.
  • Langmans, J., Klein, R., Roels, S. (2012). Numerical and experimental investigation of the hygrothermal response of timber frame walls with an exterior air barrier. . International Building Physics Symposium. Kyoto, Japan.