Enclosure Air Tightness Testing Volume vs Surface Area Targets ACH vs PERM

Enclosure Air tightness testing Volume vs Surface Area Targets (ACH@50 vs Permeability rate)

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

Volume: 300m3

Surface area: 389.5m2

A building at 5 m3/h/m2@50Pa would be 6.5ACH@50Pa

OR

A standard single story Passive house

Floor Area: 125m2

Volume: 300m3

Surface area: 389.5m2

A building that achieves passive house .6ACH@50Pa needs to achieve .462 m3/h/m2@50Pa 

Smaller volumes or components of buildings trying to achieve .6ACH@50Pa can be next to IMPOSSIBLE to achieve when services penetrations and windows are a part of the building fabric.

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.