Posted By Dana1 on 08 Dec 2010 02:09 PM
If you (or the next owner) is going to run incandescent bulbs in the fixture you could exceed the operating temp of the foam for some types.

Posted By Dana1 on 09 Dec 2010 04:03 PM
I took you at your word that it wasn't a pot light, but I didn't make the presumption that they were pendant- lights either. Many surface-mount fixtures transfer significant amounts of heat to the box when there'rs 100-300W of incandescents burning away there.

But feel free to repeat it if you like, eh? :-)

Posted By greentree on 10 Dec 2010 08:45 AM
A foam distributor said the yellow and orange foams are the same, besides the fact that some of the yellow foams are low expansion. The foam (orange or yellow) is not a firebreak and cannot be used around hi-temp applications like chimneys, A or B vent pipe this from the mouth of a guy who sells the foam to distributors. You can foam around boxes but not inside the box.

Posted By greencleaning on 09 Dec 2010 10:33 PM
The problem with putting the insulation outside of the roof sheathing, ( snip) An air barrier is useless without continuity.

http://www.youtube.com/v/2fBbZRx_mcI

Fortunately my ceiling fixtures appear to be contained inside thin plastic vapour barrier boxes used for electrical fixtures on outside walls, so the foaming seems to work well.
 
Here is the first one taped up and foamed. I injected the foam between the plastic box and the steel electrical box. The holes in the steel electrical box are all taped up with duct tape, which will be removed when the foam is trimmed.


Foam should fill the space between the plastic tray and the electrical box and also leak out any hole in the plastic tray or gaps where the tray meets the poly sheet. There is only one ceiling penetration per room, so sealing each fixture in this manner should restore the integrity of the vapour barrier and end any air leakage, with a more resiliant fix than other solutions. 
 
You can see the taping better here, before it is removed. (note: protective tape has already been removed from the fixture mounting screw holes.)

This is a few hours after the foaming was done. The foam between the plastic and steel will take more time to cure. The tape will be left on for 24 hours, before a test bit is removed, in order to allow the foam more time to cure. After the duct tape tape is removed, any penetrating foam will be cleaned up, and the edge of the electrical box and the ceiling will be bridged with foil tape, covering the foam, adding an additional seal, and providing heat and spark shielding for the foam. Theoretically, trimming the excess foam early should allow the deeper foam to cure better overnight, since it is then not  so far removed from the air. This is my first one so I will find out how long it takes to cure when I pull off a bit of tape tomorrow.

So, I found that moving the wires compresses the foam on one side and opens a gap on the other. The foam does not stick to the insulation of the electrical wires.


The solution was to position the wire and then caulk the gap, to ensure a good seal.



The wires were clamped in position and when the caulk dried, the clamps were removed to inspect that the seal was good, before restoring the clamps and taping over the foam.

In new construction Energy Star homes we use caulk for that per the HERS rater's requirements.  If the hole around the box is that big we get the drywall guys to close it up with hot mud.  Some of this can be done from the topside if necessary.    The good thing about doing it from the top is you can see light coming through where most of the leaks are.  Bath fan boxes are often another big leak.

We use a lot of spray foam behind the drywall, just not for electrical boxes.  The problem with regular spray foam is it gives off poisonous gasses when ignited.  I don't know about the orange foam, but we only use that were required for fireblock purposes because of the extra cost. 

Those thin plastic box shrouds you (or someone else) pictured are quite interesting.  I'd have to wonder though of the DW guys would tear them up with their rotozips.   

For new construction there are better quality electrical boxes that have a gasketed flange above the drywall and flexible grommets where the wires are pulled through.