Dewpoint Policy

Dew drops inspire poetry.  Dewpoint can trigger lawsuits.  Yet, they are both a result of the same laws of physic, water condensing out of air as the temperature cools.   We just don’t like when all this activity takes place behind closed walls.

Much of the recent discussion about bringing the PassivHaus Standard to North America has been the appropriateness of the airtightness and the insulation.  I went back to the source, and listened to interviews with Dr. Feist concerning the .6 ACH50 air tightness standard (or approx. .04 NACH), in comparison with ASHRAE requirements of .35 NACH.  This was one principle of PasivHaus on which Dr. Feist was adamant, citing the need to avoid structural damage from the exfiltration of moisture into the wall envelope.   Ah… so what we are really looking for is a minimum standard for wall assemblies to eliminate potential for dewpoint to occur within the interstitial cavities.

Air tightness is a good thing – not just getting an overall low ACH number, but avoiding specific points of penetration into the envelope.  One good approach is to perform a blower door test prior to drywall, with thermal imaging to identify any leaks.  These can then be sealed prior to the installation of any interior insulation.  But houses do shift, settle and shrink over time, so new points of access might emerge.

The toxic mold litigation cases have driven the demand for better modeling tools to determine the moisture  transport of any building envelope.  Programs such as WUFI and MOIST are freeware programs developed by our national scientific laboratories and research groups, and are reliable indicators of behavior of a specific assembly.  But what if there is a point of penetration in this perfect assembly? A puncture?  Everything changes.

If the primary concern with moisture in the building envelope is mold and rot (vs performance), it would seem the ultimate answer is to build with materials which can’t harbor mold and don’t rot… This is why we still have centuries old stone and brick buildings, or why old concrete structures survive even when surrounded by jungle.  Or polystyrene -  those fast-food containers which just won’t seem to break down in a land-fill… Maybe that’s a good thing in the right application – such as insulation. 

For myself, I think I’ll adopt a new policy.  Skip all the calculations, the modeling, and the lawsuits.  Use materials which don’t rot or mold.   Simple.