Hygrothermal Peace of Mind

Do you know what's happening within your wall during cold or hot periods? Our final wall assembly comes close to the “perfect wall” as it is often referred to and published by Building Science Corporation and gives you an additional peace of mind.

Interstitial Condensation Risk Analysis

When checking the condensation risk of this assembly under steady-state conditions during winter months as required by German standards, we noticed that when we have an exterior temperature below 20°F and an interior relative humidity above 35%. This is significantly lower than the 50% required by German standards, which would trigger interstitial condensation at the OSB board. Now, it is still debatable if 35% interior humidity under cold weather conditions with more or less continuous heating is a reasonable long-term interior condition. However, if we assume that our enclosure follows airtight building standards, and we have a rather high occupancy rate (e.g. 4 BRs) compared to the total square area (actually volume), these relative humidity rates become more likely, which in turn would increase the condensation risk.

We then investigated this assembly closer under transient conditions with the software tool WUFI. While our educational version only allows to utilize the standard ASHRAE material database, and limits simulation run-time scenarios to a maximum of two years, this provides enough insight into the actual condensation risk in our local Baltimore climate.

WUFI ResultsIn the Figure to the left we can observe that our proposed wall would face some condensation issues, if we assume airtight construction but do not actively control for interior humidity levels.

WUFI ResultsWe then compared this scenario with a proposed active dehumidification system that could hold the interior humidity levels below 50% during the summer months, an approach that we envisioned to spec out for our design anyway. The Figure to the right shows the simulated total water content in our assembly over two years, and it seems as there is enough drying potential under these conditions.

WUFI ResultsWe then also investigated the introduction of a vapor retarding layer on the interior and its impact on water content. In the Figure to the left we can see that a vapor control layer can successfully achieve the same as a system controlling for interior humidity, which still is an energy intensive process. The challenge will be the correct installation of this layer and transferring the strategy to construction practice.

WUFI ResultsWe also compare the wall assembly including the vapor retarder while actively controlling for interior humidity. This results in better hygrothermal performance, reducing the risk of errors on either side, be it construction process or system failures (compare Figure to the right). As shown in the figure on top of this page our ultimate solutions for the TownHauZ exterior wall system as proposed by our team even meets the stringent German standards, which requires reduction of the condensation risk under 50% interior relative humidity conditions during winter.