sisalation on top of ceiling

I have a late 1980's brick veneer home with a concrete tiled roof, no sarking under the tiles, but batt insulation on the ceiling.

I'm about to replace the 12v halogen downlights with 240v compact fluorescents, and hope to seal over the top of the light fitting to prevent warm air leakage up through the fitting. This made me think about the insulation in the roof - I'll be able to put the batts back around the light fittings (the previous owner installed the lights and made a joke of the insulation, gaps everywhere - I'm assuming the fluoro's won't heat up like halogens so I can fill the gaps in?).

I'm wondering what effect laying a sealed blanket of sisalation over the batts would have? I'm thinking it will fully seal the ceiling and so reduce heat loss into the roof space, but would the gain be worth it? It wouldn't be hard to do and should be relatively cheap...

Am I missing something, cos I've never seen this done but it seems like it should help?

I'm no expert but I'm pretty sure sisolation has an insulation value of .5 so I guess you'll get an extra .5 insulation value on top of your batts' value!?!

OK, I should have looked that up myself first, but thanks anyway. Was thinking the gain would come from sealing air gaps in the insulating layer of batts (creating a sealed envelope between the gyprock and sisalation), but on reflection would be better off making sure the bats themselves were properly fitting together.

Herman, you'd be better off putting another layer of poly batts over the top of the exisitng. (I hate fibreglass)
That's what I'm about to do in one of my ceiling areas. I'm going to put a new layer of R3.5 poly over the top of the exisitng F/G batts 0 that are at a guess around R2.5.to R3 fibreglass that is now 20 years old (so maybe R1.5 to R2 in performance and all powdery and crumbly.

I also have a roll of sislation, but I'm using that to hold up the R3.5 batts that I am putting onto the vertical areas of exposed gyrock where some internal walls back onto the ceiling space that I've just discovered were never done.
I'm using sisalation for batt retention, not thermal as such.
Steve

Read this first:

http://www.yourhome.gov.au/technical/pubs/fs48.pdf

http://forums.envirotalk.com.au/Insulat ... t7876.html

Once foil gets dirty it loses its effectiveness. It also needs an air gap to reflect radiant heat. Best to line the ceiling with fiol. Its usefulness is more to prevent summer (radiant) heat coming in than winter (convective) heat going out.

Halogens downlights do get hot and although very popula now there are intermittent reports that poor installation has caused fires. Fluoros are cooler but still can get warm. Probably not enough data to suggest that packing insulation around them is safe. You can get some special covers over the top nowadays.

Downlights are intrinsically not home energy efficient. They are portals for draughts and nevertheless difficult to insulate well.

With ridiculous ideas such as these, its no wonder the insulation industry has had so many house fires.
Down-lights must breath, allowing hot air to escape. If you block the escape route you will find the lights will burn out bulbs, and even worse, could start a fire from heating up the nearby wood trusses. Also risk being electrocuted when trying to staple down the reflective insulation to your joists. Theres a high chance of death from this practice.
Best thing to do is find a professional to install your insulation to avoid these pitfalls.