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

²³Notes:

  1. Tom Wylot, "Energy saving performance of Tenmat Firefly 120 loft covers:, TRADA Technology Ltd.

  2. Fiona McClure, "Halogen Light Mitt - Thermal Efficiency Analysis", Vipac.

  3. "Test Report", Curtin University of Technology, Division of Science and Engineering.

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 0.45 K·m²/W covering downlights
===-=======-=======-==== Plaster + air film = 0.166 K·m²/W
  |_|  0.04 m² downlights covered with some insulation

Uncovered Downlights

~|   |~~~|   |~~~|   |~~ In roof air film = 0.11 K·m²/W
 | ^ |   | ^ |   | ^ |   Insulation not covering downlights
===-=======-=======-==== Plaster + air film = 0.166 K·m²/W
 |___| 0.2 m² uninsulated area due to downlights

Example: -

Instructions:

  1. 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".
  2. Look up in left hand column the "Ceiling System R-Value" from "What happens if joists are not covered".
  3. From the table "Reduction in Ceiling System R-value due to Uninsulated Downlights" obtain the R-value for ceiling element.
  4. 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
1 2 3 4 5 6 7 8 9 10
R 2.00 R0.9-1.1 R1.2-0.8 R1.4-0.6 R1.5-0.5 R1.6-0.4 R1.7-0.3 R1.7-0.3 R1.7-0.3 R1.8-0.2 R1.8-0.2 Open
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 R1.6-0.9 R1.8-0.7 R1.9-0.6 R2.0-0.5 R2.0-0.5 R2.1-0.4 R2.1-0.4 R2.2-0.3 Open
R2.3-0.2 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 R2.5-0.0 Covered
R 3.00 R1.0-2.0 R1.5-1.5 R1.8-1.2 R2.0-1.0 R2.2-0.8 R2.3-0.7 R2.3-0.7 R2.4-0.6 R2.5-0.5 R2.5-0.5 Open
R2.7-0.3 R2.8-0.2 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 R3.0-0.0 Covered
R 3.50 R1.0-2.5 R1.6-1.9 R2.0-1.5 R2.2-1.3 R2.4-1.1 R2.5-1.0 R2.6-0.9 R2.7-0.8 R2.8-0.7 R2.8-0.7 Open
R3.0-0.5 R3.3-0.2 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 R1.1-2.9 R1.7-2.3 R2.1-1.9 R2.4-1.6 R2.6-1.4 R2.8-1.2 R2.9-1.1 R3.0-1.0 R3.1-0.9 R3.1-0.9 Open
R3.4-0.6 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 R3.9-0.1 Covered
R 4.50 R1.1-3.4 R1.8-2.7 R2.2-2.3 R2.5-2.0 R2.8-1.7 R3.0-1.5 R3.1-1.4 R3.3-1.2 R3.4-1.1 R3.4-1.1 Open
R3.7-0.8 R4.1-0.4 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 R1.1-3.9 R1.8-3.2 R2.3-2.7 R2.7-2.3 R3.0-2.0 R3.2-1.8 R3.4-1.6 R3.5-1.5 R3.6-1.4 R3.7-1.3 Open
R4.0-1.0 R4.5-0.5 R4.6-0.4 R4.7-0.3 R4.8-0.2 R4.8-0.2 R4.8-0.2 R4.9-0.1 R4.9-0.1 R4.9-0.1 Covered
R 5.50 R1.1-4.4 R1.9-3.6 R2.4-3.1 R2.8-2.7 R3.1-2.4 R3.4-2.1 R3.6-1.9 R3.7-1.8 R3.9-1.6 R4.0-1.5 Open
R4.4-1.1 R4.9-0.6 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 R5.4-0.1 Covered
R 6.00 R1.2-4.8 R2.0-4.0 R2.5-3.5 R2.9-3.1 R3.3-2.7 R3.5-2.5 R3.8-2.2 R4.0-2.0 R4.1-1.9 R4.2-1.8 Open
R4.6-1.4 R5.2-0.8 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 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
4.00 2.00 1.33 1.00 0.80 0.67 0.57 0.50 0.44 0.40 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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