Recommendations for the optimal and lasting sealing of joints from a hygro-thermal perspective

Purpose of the work

Windows according to the Passive House Standard 2016 require a ten times more airtight airtightness class (EN 14351) [1] than around 1990. The installation layer of the windows is constantly moving closer into the area of the insulating layer with possibly moisture-sensitive materials. Therefore, joint sealing of construction and connection joints must also be evaluated by applying parameters related to building physics like thermal conductivity, diffusion behavior, heat and moisture storage capacity, and the capacity for movement absorption.

The presentation focuses on the following questions:

1. Is it possible to define an optimal diffusion behavior of a joint, i.e. which product on the outside and which on the inside?
2. Are there differences if the climate conditions vary? This is analyzed by looking at the example of Seattle and Miami (both USA).
3. What is the influence of the concept of a small entry of water according to ASHREA 160 on the questions posed in 1. and 2.? (Comment: We assume that even with professional sealing, 1% of rainfall infiltrates the structure.)

Method of approach

For the parameter study, a layer set-up with “synthetic layers” is entered into WUFI PRO to guarantee a product-independent evaluation. The goal is to obtain a general recommendation for the influencing factor “diffusion resistance” for inside and outside in relation to the climate conditions. The influence of the insulation layer is simulated with a µ-value of 1 (analog mineral fiber). The inner as well as the outer layer were varied with five different µ-values between 10 and 1,500,000.

Conclusions

Analysis of the questions:

It can be concluded that for the climate conditions examined, there should always be a “breathable side” with the lowest possible diffusion resistance. For the Seattle climate, the German requirement of “more airtightness on the inside than on the outside” makes sense, although in the case of Seattle, it is also of great impact to build breathable to the extent possible, i.e. with a µ-value of <1000, ideally <100. It can be seen that problems may occur even if the principle of “tighter inside than outside” is applied, if you consider a very airtight material with, for example, µ 10.000 with a completely airtight material µ 1.500.000, and weather-related influences like minimal entry of rain [ASHREA 160] as well as/or other moisture-related processes (drying out of the structure). In addition, it can be noted that for extremely high outside pressures from moisture accumulation (Miami, Florida), it may make sense and is clearly more positive than the mere principle “more airtightness on the inside than on the outside” to do exactly the opposite, always considering that in Florida, a breathable/permeable side already has a positive influence on the moisture management of the joint.

Note

For more information, please contact the reference author at: kethorn@hanno.com