As insulation can only limit the flow of heat it depends on specific and latent heat within the building to regulate temperature. The problem with air leakage is that we lose conditioned air and thus lose or gain heat irrespective of the insulation levels that are actually achieved.
The current Australian Standard for windows and doors has limits on air infiltration. The limit is 1 L/s·m² at 75 Pa. (I.e. 1 (Litre per second) per metre squared of opening or 1 (L/s)/m².)
One of the problems with recessed lights is that they may have high air infiltration which reduces the effectiveness of insulation and this can result in very high energy usage . In the US air leakage in downlights is limited to 2 ft³/min (1 L/s) at 75 Pa (0.3 in water). Typical values for non-rated downlights at 75 Pa vary between 10 to 30 ft³/min (5 to 14 L/s).
It does not make that much difference if the electrical consumption of a light bulb (or lamp) is 7 W or 70 W when we have a loss of 100's of Watts via air leakage through the light fitting (or luminaire) itself. It is this air leakage that is the major cause of energy consumption of non-conforming recessed lighting which in the past has been completely overlooked. This energy usage takes place around the clock and not just for the short time that recessed lights are switched on.
When we look at energy efficiency for a lighting system we must look at the total power to achieve a minimum lighting level (E in lx). For example if a luminaire has more uniform light then less will be required to achieve a minimum level and thus you will have less ceiling penetrations. Once a minimum lighting level has been achieved you can then look at the critical factors like thermal bridging  and air leakage  of the alternative luminaires being considered.
In relation to Vented Recessed Light Fittings Chapter 8 of the Energy Smart Housing Manual  has the following recommendations:
- In existing buildings, the only solution is
replacement with surface-mounted light fittings.
- Avoid use in new construction.
ACIMA has been advised that recessed lights must now meet AS/NZS-60598.1 and AS/NZS-60598.2.2 requirements for insulating ceilings. Suitable recessed lights can be identified by the capped F mark. For more information see this page. ACIMA members have a solution that can be retrofitted to non-conforming recessed lights.
In the United States there is now a requirement to use energy efficient 'Insulation-Cover' (IC)  rated recessed lights in the building envelope. For example Capri Lighting  have Type-IC  downlights.
Though you may desire to have flush recessed lights in your low ceiling you should consider comfort and energy costs for the life of your home. Consider alternatives that are rated for low energy if available or consider low profile surface mount down lights.
A surface mount light has opening equivalent to an area area of only 0.82 cm² [ASHRAE]. Furthermore these lights are completely covered with thermal insulation to the full depth.
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