Downlights - ? sooo… last century darling…

13/05/2019

Last century – really?

If only that were the case! Because fitting them as an afterthought rather than designing for them initially can prove a real headache. There are issues regarding not only fire and electrical safety, but condensation as well – not to mention how to actually fit them in limited space. Ideally a false ceiling should be created, keeping the light fitting and electrical cables completely separate from the plasterboard and insulation above.

A cold ventilated loft would be relatively straightforward

With a cold ventilated loft with mineral wool insulation between the floor joists, things were relatively straightforward – push up the loft roll to accommodate the light fitting and fit an intumescent hood (that will seal off in the event of a fire), giving sufficient clearance around the back, and check that the current rating is suitable and does not cause overheating.

The advent of LED lights has changed things

With rafter-level insulation, things become much more difficult, even with modern low-profile LED downlights that output far less heat than halogen lamps. Not only do you have to consider overheating effects, but you must also guard against condensation risk. An insulation buildup should consist of a vapour barrier on the warm side (preventing water vapour produced within the house entering the rafter space), the main insulation, then either a breather membrane or a 50mm ventilated space on the cold side to allow any water vapour to escape.

What to look out for

In order to fit the downlight, the vapour barrier – and this could be a foil backing of the plasterboard, or TLX Silver - is cut through. This then allows water vapour to enter the rafter space – in fact the heat produced by the lamp acts like a chimney to carry it in - and condensation may then occur on the cold side of the insulation. If there is insufficient ventilation, or if a vapour barrier Multifoil has been used over the rafters, the situation can be even worse, and in severe cases the condensation can even drip down onto the light fitting and wiring.

How to fithow to fit tlx silver with downlights

To prevent this, a vapour barrier (e.g. 500g polythene) must be fitted behind the luminaire to maintain continuity of the vapour barrier, whilst observing the recommended clearance specified by the manufacturer. If using TLX Silver the cut edges should be taped and sealed to the vapour barrier patch. If this patching section is being made from the outer layer of TLX Silver, be sure to keep the shiny side facing away from the light fitting so that heat is not being reflected back onto it. If foil-backed plasterboard is being used to provide the vapour barrier, then tape the polythene patch to this. Be sure to keep the manufacturer’s recommended clearance around the back of the luminaire.

Rafter depth is important

Of course, unless sufficient rafter depth has been allowed, it might be necessary to cut voids in the rigid insulation board if this has been used above the downlights. If this reduces the insulation thickness significantly, then cold spots may be created, and Building Control or the SAP assessor may not be happy.

How do building control approach the subject?Using TLX Silver with downlights

There is quite a lot of choice when it comes to downlights. Modern ones can be obtained with a low profile, with LED lamps and some have a separate LED driver unit - though it should be noted that some Building Control inspectors will not permit downlights to be recessed into PIR board because of the danger of overheating. However, some downlights can now be obtained that are ‘insulation coverable’ and specifically designed to be recessed in PIR board. Many are fire-rated and can be obtained with various IP ratings.
Where an LED driver unit is to be fitted alongside with the cabling, and if TLX Silver is used for the insulation build up secured with a batten which creates an unventilated heat-reflecting airspace, then by having a deeper batten than usual the driver unit can be accommodated in this space whilst at the same time contributing the insulation build up. The manufacturer’s recommendations regarding clearance should be observed, and the maximum temperature TLX Silver should experience is 70°C.
Of course, there’s always flush-mounted ceiling lights instead…

TLX Technical hotline