A whole-building test for air tightness must be performed. See Section 3.8 for further details. If testing at 75 Pa, report the flow coefficient and exponent from the blower door tests.
The certification requirement is as follows:
For buildings of five stories and above that are also of noncombustible* construction:
q50 <= 0.080 CFM50/ft2 or q75<= 0.110 CFM75/ft 2 of gross envelope area
For all other buildings:
q50 <= 0.060 CFM50/ft2 or q75<= 0.080 CFM75/ft2 of gross envelope area
*Non-combustible in this sense is construction that is not subject to mold and rot. This would mean no woodbased framing members or sheet goods, and no wood-based or paper-based insulation.
Gross envelope area is measured at the exterior of the thermal boundary, the same as for the energy model, and includes surfaces in contact with the ground.
Exception, for non-threatening air leakage: If the airtightness criterion is missed, and the extra leakage can be proven to be due to a non-assembly-threatening leakage element, certification staff may allow that element to be taped off for the purpose of passing the airtightness criterion. The untaped test result must be used for the energy model. Further details are subject to staff interpretation as detailed below.
Non-threatening leakage elements that may be taped upon PHIUS approval:
- Fire rated entry doors - leakage through operable components
- Entry doors with panic hardware requirements - leakage through operable components
- Entry door thresholds with universal accessibility requirements - leakage at threshold/sweep
- Elevator doors - leakage through operable components into shaft
- Dampers - leakage at seal
- Trash chutes and compactor systems, code required dampened openings like elevator shaft vents, gas meter room vents, coiling doors
- Direct vent gas fireplace – leakage through firebox
- Window mounted space conditioning unit – leakage through unit itself or through gasket inset in window glazing or window frame (some frames present durability risk and certifier has discretion to decline request for taping)
Non-threatening leakage elements already accounted for in the cfm/ft2 gross envelope limit. May not be taped:
- Standard casement/TT/awning windows - leakage through operable components
- Standard lift and slide doors - leakage through operable components
- Standard Balcony/French doors – leakage through operable components
- Duct/vent leakage if fully exposed and in conditioned space
Threatening leakage sources. May not be taped.
- Window or door installation - leakage at rough opening
- Duct/vent/wiring - leakage at penetration of exterior wall
- Duct or vent - leakage inside assembly
- Exterior wall mounted space conditioning unit installation– leakage through rough opening
For comparison to 0.6 ACH50, the “crossover” is at roughly 10,000 square feet of envelope area, that is, the air-tightness target for projects larger than this will be slightly tighter than the 0.6 ACH50 metric and projects smaller than this will be slightly looser than the 0.6 ACH50 metric
The higher limit for tall buildings should be considered temporary – more research is needed and the criterion might be tightened in the future.
The PHIUS Technical Committee goal was a clear standardized building enclosure metric for all buildings, large and small. ACH50 is not an equitable metric since the volume of a building does not scale at the same rate as surface area. PHIUS views the primary purpose of the airtightness threshold as the reduction the risk of building assembly damage due to air-leakage driven moisture in super insulated assemblies with minimized mechanical systems, and energy efficiency as the secondary purpose.
Establishing a maximum durability leakage rate recognizes that in many smaller buildings, while the conversion of a 0.060 cfm/ft2 leakage limit to ACH50 results in a threshold higher than 0.6 ACH50, the driver behind a project team’s leakage target for these structures will be achievement of energy performance thresholds, rather than durability. That is, the heating/cooling criteria may effectively require air-sealing tighter than the certification limit for the blower door test noted above, and in that case the assumed air-tightness in the energy model becomes the new certification limit for the blower door test.
Testing at 75 Pa aligns with commercial code (IBC, IECC) and U.S. Army Corps of Engineers (as well as U.S. General Services Administration Facilities Standards, ASTM E779, ASHRAE 90.1, and the National Building Code of Canada.)
- Acceptable Air Tightness of Walls in Passive Houses – Salonvaara and Karagiozis 2015
- PHIUS Technical Committee Airtightness Comparison
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