Posted: Jun 4, 2010
Topics: Efficiency and renewables > Lamps and luminaires

Swiss cheese and downlights - more in common than you think ...

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It’s hard to imagine the terms ‘Swiss cheese’ and ‘halogen downlights’ being used coherently in the same sentence - ordinarily the two have nothing in common with each other. But the subject of this article makes an exception. The common practice of mass installations of halogen downlights in homes and businesses is effectively turning ceilings into ‘Swiss cheese’.

Over the past couple of decades, halogen downlights have become the light fitting of choice in most new and renovated homes in Australia and, indeed, most of the world. However, despite their current popularity, they are among the most inefficient light fittings available for a number of reasons. From a lighting perspective, they are not as inefficient as the now-banned incandescent lights but there are a number of little-known facts about halogen downlights, and the environment they are installed in, that make them very environmentally unfriendly and highly dangerous.

But that doesn’t mean we have to immediately start looking for alternatives to replace them. There are steps that you, as an electrical contractor installing them or a lighting designer specifying them, can take to continue to deploy them and help keep the homes they are installed in ‘green’.

First the facts

There is no dispute that halogens lamps emit considerable heat. In fact, many commercially available heat lamps use halogen lamps to produce heat. And we are all aware of the recent spate of house fires that are directly attributed to the heat build-up from halogen downlights, under incorrectly installed insulation, causing nearby combustible materials to ignite. But the problem is not necessarily the light fitting itself - the problem is often the downlight installation. What is not known to many is that, in summer, halogen downlights can make a living area warmer and, in winter, they can make it cooler. This may seem back-to-front to you, but this is where the connection between ‘Swiss cheese’ and halogen downlights comes in.

The primary culprit for this is open-air connectivity and the subsequent lack of insulation between the living area and the roof. It is more than just a hole in the ceiling - it is a thermal passage between two different temperature zones. Temperature differentials always cause heat to flow from one area to another. So, in summer, when the roof cavity is heated by the sun and hot air outside, heat will flow down into the living areas through the thermal holes made by the downlights in the ceiling. And, in winter, the cooler ambient temperature in the roof space will leak into the living area, working against the home’s heating system. This is a big ‘no-no’ when it comes to achieving a high energy star rating for a home.

It is an established fact that good building design requires that these two areas be thermally separated as much as possible, because the roof is there to insulate from the elements, not join the home and its occupants to the elements. For example, in a recent evaluation conducted by Vipac Engineers, it was shown that a significant temperature differential between these two spaces does occur and that, without adequate insulation, the heat loss/gain can be substantial. The evaluation tested a 2 x 2 m room with insulation of 3.5 ‘R’ rating installed and adhering to Australian standards with 200 mm clearances around a standard downlight. The System R-values (R) and corresponding rates of energy loss (Q) for each temperature condition are summarised in Table 1.

 
Table 1: System R-values and rates of energy loss.

On a per-downlight basis, energy loss (Q) outlined in these calculations from the Vipac report shows considerable energy loss in thermal bridging between the attic and the living area.

‘Swiss CHEESE’

The detrimental thermal effect and fire safety risks that halogen downlight installations can have on a home’s thermal insulation can be best summarised by an acronym that should be easy to remember: ‘CHEESE’.

  • Convection currents - When lights are turned on, heat from the halogen lamp being pushed both downwards and upwards creates a passage of warm air that ‘shoots’ up into the roof area through the fittings.
  • Heated air pressure - During cold days, openings around the downlights and gaps in the insulation enable hot air from the heating system to freely pass into the roof space from the living area. Hot air always wants to go up and it will find any path it can to do so.
  • Expansion of heated air - During hot days, the sun creates extremely hot air in the roof area (70°+C). This air expands and pushes down into the living area through ventilated downlight fittings.This effect is especially undesirable when LED recessed light fittings are installed, because LED lamps can be damaged by operating in temperatures of 70°C (more on the right way to install LEDs later in this article).
  • Excessive air drop - During cold nights, downlight fittings enable cold air to freely drop into a house. The house gets cooler quicker, especially with ventilated downlight fittings.
  • Spread of draughts - During windy days, the roof area can become quite windy, these draughts freely move into the living area with ventilated downlight fittings installed.
  • Exposure to fire damage - Anything can get stuck in recessed downlight fittings that could ignite into a flame.Should insulation or debris get too close to a standard halogen lamp (300°C+), a fire can propagate very easily.

Technically, why do downlights make our ceilings into ‘Swiss cheese’? The root cause of these inefficiencies is the way standard dichroic/standard IRC (cool beam) lamps operate:

  • They produce most of their light downwards; and
  • They try to force the majority of their heat and a good portion of the light backwards up into the roof.

Their design inherently creates a connectivity between the living area and the roof area.

Options to solve the ‘Swiss cheese’ effect

There are a number of potential solutions for downlights available in the market right now, but what do they actually fix?

CFL fitting replacement with ‘tin can’ cover

Pros:

  • Low wattage;
  • Reduce draught; and
  • Less clearances for insulation and load-bearing structures.

Cons:

  • Poor light output;
  • Expensive install;
  • Hard to make dimmable;
  • Poor-quality light colour; and
  • Problems with insulation around and over the fitting.

LED lamp retrofit with ‘tin-can’ or plastic cover

Pros:

  • Extremely low wattage;
  • Flexible coloured lighting;
  • Excellent light output;
  • Retrofit existing fittings; and
  • Less clearances for insulation and load-bearing structures.

Cons:

  • Extremely expensive;
  • Lamp is susceptible to damage in high roof temperatures;
  • May not work with all electronic transformers in a MR16 retrofit situation; and
  • Problem with insulation around and over the fitting, because the fitting could easily be retrofitted with halogen lamps when the LED has been replaced (recommended operating temperature for most LEDs is 60°C or less).

IRC dichroic halogen retrofit with ‘tin-can’ or plastic cover (with fixed-head fitting)

Pros:

  • Cheap retrofit;
  • Re-use existing fittings;
  • Bright light output;
  • Desirable light colour;
  • Fixed head fitting reduces draughts; and
  • Less clearance for insulation and load-bearing structures.

Cons:

  • Ventilate into roof area;
  • Higher energy consumption;
  • Still susceptible to fire risk;
  • Fixed head fitting requires a conversion ring when covering a gimble hole; and
  • Problem with insulation around and over the fitting.

A common problem that emerges from all of these approaches is that there is no real mainstream solution to stop multiple single points of failure in home insulation, while also providing fire safety. While each of these options provides a solution to some of the CHEESE challenges, none of them adequately address all of them collectively. Some provide low energy consumption; some provide low heat output; some are low cost; and some reduce the risk of fire.

Expanding on the requirements outlined above in the CHEESE challenges, the ideal downlight fitting solution should have the following attributes:

  1. Reutilise existing fittings to maintain the décor of the home and recycle old fittings to reduce waste;
  2. Doesn’t produce much heat and pushes most of its heat downwards away from the ceiling area;
  3. Insulation at the back of the fitting to stop draughts, improve consistency of insulation in the loft area (not plastic, cardboard or tin boxes that provide no insulation);
  4. No gaps in the downlight cover that can allow attic debris to enter the fitting and thereby pose a threat of fire next to the halogen lamp, which is a good ignition source;
  5. Compatible with LED lamps when they become more affordable, while still providing protection for the LED lamp fitting from extreme heat in summer;
  6. Quick installation to reduce the cost of re-fit;
  7. Maintain light output;
  8. ‘Fire-rated’ not ‘fire-resistant’ covers, to contain a fire before it becomes a real problem;
  9. Resolve minimum clearances of load-bearing structures to help old downlight fitting installations that were installed with old regulation clearances;
  10. Insulation should be able to be installed all over the fitting, without any real safety issues;
  11. A poorly installed downlight cover installed right up to a standard 50 W lamp should be able to operate without melting and becoming a safety issue; and
  12. Create more uniform light to reduce unnecessary bright spots and install lamps that aren’t too bright in areas where it doesn’t need to be bright, for example corridors and hallways.

It should be clearly understood that more and more safety and environmental regulations are progressively being introduced to make houses safer and more energy efficient, while Energy Star ratings are becoming more important to homeowners. In spite of these trends, halogen downlights remain increasingly popular in homes, so a solution to this lighting conundrum is critical.

‘Gimble’, or adjustable, downlight fittings have clearances around the lamp and lamp holder, allowing the light to be directed as required. These lamps are our biggest enemy when it comes to efficiencies in the home, due to the fact that air flows easily between the roof and living areas through the clearances. Solving this problem is not a simple matter of light fitting design. Rather, the problem should be addressed by the thermal shield. Downlight covers are now emerging on the Australian market with an R-value and they turn this negative into a positive especially where LED lighting is concerned. LED lighting is sensitive to high temperatures, so insulating the fitting from the roof air and maintaining a connection to cooler air in the living area becomes a very valuable feature. LEDs can be permanently damaged when operating in extreme heat, which can be a very costly exercise especially in the northern parts of Australia. When an attic temperature reaches 71°C on an extremely hot summer’s day, placing a downlight cover with an R-value can reduce the temperature within the fitting to 48°C, which can make a difference of life or death for an LED lamp in operation.

LED lighting isn’t quite up to scratch and, right now, halogen lighting is the biggest problem in the equation because the halogen lights ventilate into the roof. New ‘silverback’ style and ALU-type halogen lamps eliminate light output towards the back of the fitting and force the majority of the heat downwards. In the 60° lamps (FNV), the beam-angle spread is enhanced compared to a standard dichroic lamp and bright spots are also substantially reduced, creating a more uniform light spread in the area below, using lower wattage (20 or 35 W) rather than conventional 50 W lamps.

The LED or silverback halogen lamps, used in conjunction with a quality downlight cover that includes an R-value and a fire rating, in most cases, are a solution that can be implemented while preparing households for the up and coming LED lamp revolution, which is destined to touch down in the not too distant future.

Remember - a light fitting is either in the living room or in the roof area. There can be no in-between.

By John Konstantakopoulos, Efficiency Matrix



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