What happens if downlights are not covered?


Dimension	Incandescent lamp	Halogen lamp
A - Clearance above luminaire	50 mm	200 mm
B - Side clearance to structural member	100 mm	200 mm
C - Clearance to thermal insulation	50 mm	200 mm
D - Clearance to supply transformer	50 mm

Important Clarification of AS/NZS-3000:2007¹ Requirements is Here

¹AS/NZS 60598.1 and AS/NZS 60598.2.2 detail the test method and the maximum surface temperatures permitted for recessed luminaires. These maximum temperature limits must be satisfied to permit any reduction in the default values ...

Open area		m²
Reduced area	clause 4.5.2.3.b¹	m²
Insulation		K·m²/W
Plaster board		K·m²/W
Ceiling air films		K·m²/W
Area per downlight		m²

This is what happens if downlights are not covered

Cutaway view of fire rated Tenmat FF120 Loft Cover

Covered Downlights
   _       _       _
~~| |~~~~~| |~~~~~| |~~~ In roof air film = 0.11 K·m²/W
  |^|     |^|     |^|    R 0.5 K·m²/W covering downlights
===-=======-=======-==== Plaster + air film = 0.189 K·m²/W
  |_|  0.05 m² downlights covered with some insulation

Uncovered Downlights

~|   |~~~|   |~~~|   |~~ In roof air film = 0.11 K·m²/W
 | ^ |   | ^ |   | ^ |   Insulation not covering downlights
===-=======-=======-==== Plaster + air film = 0.189 K·m²/W
 |___| 0.2 m² uninsulated area due to downlights

Example: -

We have a room 4 m × 4 m = 16 m²
In this room we have 4 down lights.
That means that each downlight serves an area of 16 ÷ 4 = 4 m².
If each downlight has a cut out of 100 mm Ø (5 cm radius) and 200 mm (20 cm) default clearance as required by AS 3000. Our uninsulated area for each downlight is then π·(0.05+0.20)² = 0.20 m².
This is an uninsulated gap of 5%.

Instructions:

Divide the total ceiling area that requires insulation by the number of downlights in that area to get "Ceiling area in m² for each 0.2 m² uncovered downlight".
Look up in left hand column the "Ceiling System R-Value" from "What happens if joists are not covered".
From the table "Reduction in Ceiling System R-value due to Uninsulated Downlights" obtain the R-value for ceiling element.
Return to "What happens if joists are not covered" find matching "Ceiling System R-Value" and corresponding "Roof System" for the value just matched.

Reduction in Ceiling System R-value due to Uninsulated Downlights
Without	Ceiling area in m² for each downlight: -																				Treatment
Without	1		2		3		4		5		6		7		8		9		10		Treatment
R 2.00	R0.9	-1.1	R1.3	-0.7	R1.5	-0.5	R1.6	-0.4	R1.6	-0.4	R1.7	-0.3	R1.7	-0.3	R1.8	-0.2	R1.8	-0.2	R1.8	-0.2	Open
R 2.00	R1.9	-0.1	R1.9	-0.1	R2.0	-0.0	R2.0	-0.0	R2.0	-0.0	R2.0	-0.0	R2.0	-0.0	R2.0	-0.0	R2.0	-0.0	R2.0	-0.0	Covered
R 2.50	R1.0	-1.5	R1.4	-1.1	R1.7	-0.8	R1.8	-0.7	R1.9	-0.6	R2.0	-0.5	R2.1	-0.4	R2.1	-0.4	R2.1	-0.4	R2.2	-0.3	Open
R 2.50	R2.3	-0.2	R2.4	-0.1	R2.4	-0.1	R2.4	-0.1	R2.4	-0.1	R2.5	-0.0	R2.5	-0.0	R2.5	-0.0	R2.5	-0.0	R2.5	-0.0	Covered
R 3.00	R1.1	-1.9	R1.6	-1.4	R1.9	-1.1	R2.1	-0.9	R2.2	-0.8	R2.3	-0.7	R2.4	-0.6	R2.4	-0.6	R2.5	-0.5	R2.5	-0.5	Open
R 3.00	R2.6	-0.4	R2.8	-0.2	R2.9	-0.1	R2.9	-0.1	R2.9	-0.1	R2.9	-0.1	R2.9	-0.1	R2.9	-0.1	R3.0	-0.0	R3.0	-0.0	Covered
R 3.50	R1.1	-2.4	R1.7	-1.8	R2.0	-1.5	R2.3	-1.2	R2.5	-1.0	R2.6	-0.9	R2.7	-0.8	R2.8	-0.7	R2.8	-0.7	R2.9	-0.6	Open
R 3.50	R3.0	-0.5	R3.2	-0.3	R3.3	-0.2	R3.4	-0.1	R3.4	-0.1	R3.4	-0.1	R3.4	-0.1	R3.4	-0.1	R3.4	-0.1	R3.4	-0.1	Covered
R 4.00	R1.2	-2.8	R1.8	-2.2	R2.2	-1.8	R2.5	-1.5	R2.7	-1.3	R2.8	-1.2	R3.0	-1.0	R3.1	-0.9	R3.1	-0.9	R3.2	-0.8	Open
R 4.00	R3.3	-0.7	R3.6	-0.4	R3.7	-0.3	R3.8	-0.2	R3.8	-0.2	R3.9	-0.1	R3.9	-0.1	R3.9	-0.1	R3.9	-0.1	R3.9	-0.1	Covered
R 4.50	R1.2	-3.3	R1.9	-2.6	R2.3	-2.2	R2.6	-1.9	R2.9	-1.6	R3.1	-1.4	R3.2	-1.3	R3.3	-1.2	R3.4	-1.1	R3.5	-1.0	Open
R 4.50	R3.7	-0.8	R4.0	-0.5	R4.2	-0.3	R4.3	-0.2	R4.3	-0.2	R4.3	-0.2	R4.4	-0.1	R4.4	-0.1	R4.4	-0.1	R4.4	-0.1	Covered
R 5.00	R1.2	-3.8	R1.9	-3.1	R2.4	-2.6	R2.8	-2.2	R3.1	-1.9	R3.3	-1.7	R3.5	-1.5	R3.6	-1.4	R3.7	-1.3	R3.8	-1.2	Open
R 5.00	R4.0	-1.0	R4.4	-0.6	R4.6	-0.4	R4.7	-0.3	R4.8	-0.2	R4.8	-0.2	R4.8	-0.2	R4.8	-0.2	R4.9	-0.1	R4.9	-0.1	Covered
R 5.50	R1.2	-4.3	R2.0	-3.5	R2.5	-3.0	R2.9	-2.6	R3.2	-2.3	R3.5	-2.0	R3.7	-1.8	R3.8	-1.7	R4.0	-1.5	R4.1	-1.4	Open
R 5.50	R4.2	-1.3	R4.8	-0.7	R5.0	-0.5	R5.1	-0.4	R5.2	-0.3	R5.2	-0.3	R5.3	-0.2	R5.3	-0.2	R5.3	-0.2	R5.3	-0.2	Covered
R 6.00	R1.2	-4.8	R2.1	-3.9	R2.6	-3.4	R3.1	-2.9	R3.4	-2.6	R3.7	-2.3	R3.9	-2.1	R4.1	-1.9	R4.2	-1.8	R4.3	-1.7	Open
R 6.00	R4.5	-1.5	R5.2	-0.8	R5.4	-0.6	R5.5	-0.5	R5.6	-0.4	R5.7	-0.3	R5.7	-0.3	R5.8	-0.2	R5.8	-0.2	R5.8	-0.2	Covered
Gap%:	20.00		10.00		6.67		5.00		4.00		3.33		2.86		2.50		2.22		2.00		Open
Gap%:	5.00		2.50		1.67		1.25		1.00		0.83		0.71		0.62		0.56		0.50		Covered

It Gets Worse!

Note that no allowance has been made for loss of conditioned air. Vented downlights such as the gimbal type suffer the combined effect of loss of conditioned air in addition to reduced system R-value. These are the most common low voltage MR16 downlights in use!

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