Duct leakage is invisible until the commissioning test, and by then it is expensive. SMACNA seal classes define the scope of sealing work, and the class in your specification determines both the sealant quantity you buy and the leakage figure you must hit.

The three classes

Class A. All transverse joints, longitudinal seams and duct wall penetrations sealed. The most demanding scope, associated with total system leakage typically in the region of 2 to 5 percent.

Class B. All transverse joints and longitudinal seams sealed, penetrations excluded. Associated leakage typically around 3 to 10 percent.

Class C. Transverse joints only. Associated leakage typically 5 to 20 percent.

The percentages are indicative of the class intent; the project specification and the applicable leakage test class govern acceptance.

Why Class A dominates in the Gulf

Cooling energy is the largest operating cost in most UAE buildings. Air leaking from supply ductwork into ceiling voids is cooling paid for and not delivered, so the payback on comprehensive sealing is short. Green building requirements and energy modelling assumptions also push specifications towards the tighter classes, and commissioning regimes increasingly test rather than assume.

Choosing and applying sealant

Water based butyl elastomer sealants such as Foster 95-44 suit low velocity duct systems, insulation joints and the laps of metal jacketing where an air tight and water tight seal is required. Application is by trowel, putty knife, caulking gun or power extrusion. On insulation joints, manufacturer guidance calls for roughly 1.6 to 3.2 millimetres wet film on abutting edges, pressed together firmly to squeeze out air and achieve full contact.

Practical points that decide test results

Surfaces must be clean and dry before sealing, since dust on new galvanised duct is the most common cause of poor adhesion. Seal before insulation goes on, because access afterwards is poor and rework is destructive. Allow proper cure before pressure testing, as testing a green sealant produces failures that would have passed days later. And do not neglect penetrations under Class A, since a handful of unsealed hanger and service penetrations can swamp otherwise careful joint work.

MENA Industrial Supplies stocks Foster 95-44 duct sealant in project quantities with delivery across Dubai and the UAE.