Building Tightness Limit vs Minimum Ventilation (When Air Sealing Needs a Fan)

The logic runs backwards from the ventilation requirement rather than forwards from the test. A house needs a certain outdoor air rate; a leaky house gets some of that for free through infiltration; the building tightness limit is the CFM50 at which the free share exactly covers the requirement. Above the limit, air sealing is pure benefit -- less energy, no ventilation consequence. Below it, sealing without adding a fan trades energy for indoor air quality, and that is not a trade a contractor should make silently on a homeowner's behalf. The limit is the ventilation requirement times the LBL N factor, and N is what ties it to place and building rather than to the house alone. A windy cold exposed site drives far more natural infiltration through the same hole than a sheltered mild one, so the same house has a different tightness limit depending on where it stands -- which is the whole reason the limit cannot be a single number in a code table. The practical use is writing the scope of work. A house comfortably above the limit today can be below it after one afternoon of sealing can lights, top plates and the rim joist, and the moment to discover that is while the scope is being written rather than after the crew has left. That is why the planned reduction is an input here: the question is not only where the house is but where the job will put it. Natural infiltration itself is a seasonal average and is wrong on any given day by a wide margin -- still mild days give almost nothing, a windy cold night gives multiples -- so a house that meets its requirement on the annual mean can be badly under-ventilated for weeks at a time. Using an infiltration estimate to claim a house is adequately ventilated is the error this calculation is most often misused to commit. This does not measure anything, select the N factor (which depends on climate zone, storeys, shielding and leakiness), size or select a ventilation system, or address distribution, filtration and the source control that matter as much as the rate. ASHRAE 62.2, the BPI or RESNET protocol in force, and the rater's own judgment govern.

the ASHRAE 62.2 whole-house requirement 0.03 x floor area + 7.5 x (bedrooms + 1) in cfm, converted to a CFM50 tightness limit by multiplying by the LBL N-factor; the house is below the limit when its measured CFM50 falls under it, and the margin is the sealing that can be done before mechanical ventilation is required.

ASHRAE 62.2 whole-house ventilation by name, with the LBL N-factor conversion between natural and 50 Pa flows. The limit is a SCREEN on whether air sealing will drive a house below its ventilation requirement, not a substitute for the 62.2 calculation or for a measured ventilation rate.

One published rate expression and one divisor.

Estimate. AHJ and licensed professional govern.

Field names used by the API: floor_area_ft2, bedrooms, cfm50, n_factor, ceiling_height_ft, planned_cfm50_reduction, required_cfm, tightness_limit_cfm50, margin_cfm50, natural_cfm, after_cfm50

• Infiltration credit is a seasonal average wrong on any given day by a wide marginASHRAE 62.2 and the LBL model
• N-factor climate zone, height and shielding; a threefold rangethe LBL table
• Not a signed test report a screen, not a rater's certificationRESNET or BPI protocol