September 7, 2012

PHI airtightness maximums compared to current USA airtightness code requirements

OSB joints taped airtightly with TESCON Vana and Profil at Ecocor’s Montville project

It has been argued by many that the air-sealing requirements set by the Passivhaus Institute are too strict, unrealisticly low and thus too costly. However, the kind people on the other side of the Atlantic have shown that it is not hard to surpass these requirements by far, save both energy and prevent moisture and mold issues in walls.

We are starting to see more and more builders blowing past 0.6ACH50 as well. For instance Chris Corson ( already tested 0.54ACH50 on his Montville Magic project, and this was with a reused basement that was still pretty leaky.

The building codes in the USA do recognizing the value of air-tightness as well and IECC 2012 mandates a blowerdoor test of 3.0ACH50 or lower for residential construction (which is actually the same as for conventional construction in Germany).

The difference between 3.0 and 0.6 is a factor 5. And the challenge is that when building gets tighter and the holes get smaller, the air starts flowing faster through the remaining holes at 50Pa. This means it gets incrementally harder to lower the air change number. This is of course especially true for small buildings, that have relatively large amounts of enclosure/surface areas compared to their overall volume.

Surface to area ratio (metric, devide by 3.28 to get I-P values (source: bre_passivhaus_designers.pdf)

Since the heat demand of a building is based on the amount of air that leaks out of a certain volume of heated space, the PHI used the ACH50 (n50) leakage as threshold of 0.6 as their limit and not the (q50) permeability of the enclosure area. (see image at the bottom of page 2 the BRE document (m2/m3 values))

Small or spread out buildings can have surface to area ratios of 1.0m2/m3 – 0.3ft2/ft3 or higher or higher (a 7’x7’x7′ cube has a volume of 343ft3 and an area of 294ft, thus a ratio of 0.85m2/m3 – 0.26ft2/ft3). These kinds of buildings will have a much harder time to be airtight then larger, multi-story buildings that are more compact (<0.7m2/m3 – <0.21ft2/ft3)

PHI airtightness limit compared to USA codes requirements (excel file of calculation)

Very large buildings are most of the time much more compact and have very favorable surface to volume ratios, consequently they can reach low n50 (ACH) numbers without much effort. Hence PHI’s recommended target value for buildings over 4,000m3 (140,000CF) it that they both conform with q50 <0.6ACH50 (which should be quite easy) and stay below n50  0.6m3/hr*m2 (0.033CFM/ft2).

Similarly IECC 2012 requires commercial buildings to limit the permeance in a blowerdoor test of the enclosure to a max of  0.4CFM/ft2 (section C402. Building test in IECC 2012 chapter 4) As the graph above shows, this is a factor 12 higher than the PH requirement.

Note: a 1.3 conversion factor has been applied to adjust the flowrate for the pressure difference between 50Pa test and 75Pa test (as per Infiltec)

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