Commercial Enclosure Air tightness testing Volume vs Surface Area Targets
Airtightness on large enclosures can get tricky and complicated. We see some pretty questionable things in specifications for the testing of these types of facilities, which in most circumstances are pretty airtight considering the type of construction that they are but the metric they are measuring to makes no sense what so ever.
Targeting ACH is NQR (Not Quite Right)
For example, using ACH@50Pa(Volume) or ACH@Ambient-4Pa(Volume) instead of a permeability rate(Surface area) to understand how leaky a commercial/industrial facility/Oil Rig refuge actually is, means that the larger the enclosure, the more airtight it is. So a uniform standard leakage rate cannot be specified in this specification. As an example. Compare the ACH for these 3 examples.
A standard single story house
Floor Area: 125m2
Surface area: 389.5m2
A building at 5 m3/h/m2@50Pa would be 6.5ACH@50Pa
Not so much of a problem, the permeability rate and Air change rate are very similar, due to these types of buildings usually being quite small. Volume and Surface area numbers are largely similar. (However, McMansions or Large homes, can enjoy the benefits of ACH)
Small Commercial Building
Volume: 20m x 20m x 12m = 4800m3
Surface area: 1760m2
A building at 5 m3/h/m2@50Pa would be 1.83 ACH@50Pa
Identical Build quality as the home but it enjoys over THREE times lower ACH Rate
Large Commercial Building
20m x 20m x 260m = 104,000m3
Surface area: 21600m2
A building at 5 m3/h/m2@50Pa would be 1.03ACH@50Pa
Identical Build quality as the home but it enjoys a FIVE times lower ACH Rate. This building could be Passive house air tight if passive house didn’t have the rule that buildings larger than 4000m3 in volume need to use a permeability rate.
Using ACH on Commercial buildings, Oil & Gas rig refuges or any type of large enclosure larger than 3000m3 in volume, means the larger the enclosure, the easier it is to pass a particular target inadvertently. For build quality, a permeability rate which uses the surface area of the building, requires a target based on air flow through a square meter of building envelope with simulated airspeeds of 32km/h winds. That way it is a standard number for all facilities of that type, and the build quality can then actually be compared meaningfully and reproducibly no matter the dimensions and volume of the building.