How do we achieve an air tight ceiling? Firstly we consider all the holes that we cut through the ceiling for light fittings, exhaust fans, air diffusers etc. Limiting the amount of these holes is a good start, then carefully applying low density expanding foam or silicone to air seal the resultant gaps is a good strategy.
One air leakage point in the ceiling that is often overlooked is the gap between the wall plasterboard and the top plate. The wall plasterboard is fixed to the studs with dobs of adhesive as well as mechanical fixings. The adhesive creates a slight gap between the plasterboard and the framework. This gap when multiplied over all of the walls in the house represents a large hole in the building fabric and an excellent passage for air to travel through.
Attention to detailing this air leakage point at the ceiling level will make a dramatic difference to the tightness of the house and the effectiveness of the ceiling insulation. As in the supplied image the gap can be addressed on top of the ceiling. Alternatively a fluid or applied gasket can be installed to the vertical face of the top plate, (and around window and external door openings) prior to the plasterboard being installed. The problem with air sealing from the top is the restriction of access under the truss bottom chord. Also trying to reach the junction at the external wall at the low end of the truss is very difficult.
When conducting a blower door test, power outlets throughout the house leak. This air leakage is most likely travelling through the top plate/plasterboard connection. If you want to address air leakage seriously, pay attention to this detail.
The provided image was obtained from an American blog about air tightness. Click here to read the article. Construction methods differ slightly to ours but the concepts are the same.