Gang - can a recessed light box burn/ignite MDF?

Since we're doing GG/DD/RSIC, we need to enclose can lights for soundproofing. So, we used the MDF/Hardibacker (cement board) approach to build a "fireproof" box to enclose each light.

Some of the Hardibacker inside panels (that line the box) are a bit "short" - up to a half an inch. So, there is a half inch or so "perimeter" around the boxes of MDF, where there is no cement board.

I looked up the "autoignition temperature" (the point at which a material will burn, even if not exposed to fire), and it's around 450 degrees for MDF (iirc). Does a recessed light or the enclosure it's contained in get this hot? (I used IC-rated, which I know has a thermal shutdown switch that turns the light off if it gets above a certain temp, but I can't find anything definitive on what that temperature is - tho I think I saw 185 degrees mentioned on AVS as a shutoff point..

I can't easily cut 1/2'' or 1/4'' strips of Hardibacker (man, that stuff is NASTY to work with) to "patch" the open spots of MDF on the box.

Is this a problem? Do I need to find room to put another layer of Hardibacker or some other material in the box to "protect" the 1/2'' or so of exposed MDF?

I know that a guy on the "lightbox" thread used MDF ONLY for his boxes and apparently hasn't burned down the house..

Also - I'd like to caulk the inside of the box. GE Silicone II caulk says to keep below 450 degrees. 3M Firecaulk (oddly enough) says to keep below 120 degrees [?!] I know some guys DID caulk the inside of the box with the GE stuff, which is also flammable - or so I'd think? This makes me think the lights must NOT get into the 400 degree range..

Thanks for any/all help. I've already re-built these boxes once (long story) and don't want to do it again if I can help it..

Does a recessed light or the enclosure it's contained in get this hot?

It better not! Standard Romex (NMB temp. rated) is only rated to 90C (194F). If you hit 450F the jacket will have melted.

-drin

Is this a problem?

NO

Thanks, guys..

What about the "bottom" of the light assembly (by the bulb?)

I can understand that the metal ENCLOSURE (box) containing the IC light never gets above 194 deg or it would melt the Romex on top, but what about by the bulb, where all the heat is generated?

This is what's near the exposed MDF..

TIA..

I have 6 inch cans with 65 watt reflector bulbs in my office. They've been on all day. I just stood on a chair and I can stick my fingers on the reflector near the bulb and keep it there without burning.

This is not going to be an issue.

Next Problem?

I'm guessing I'm "THAT" guy who used MDF for his back boxes... :rolleyes: I did use the metal back boxes inside the MDF boxes though. (And the bottom of my trim rings are open.)

No issues to report, other then perhaps reduced bulb life. Currently the 50 watt PAR 20s are still running full wattage (not on the GE yet), and see a lot of use per day.

I'm guessing I'm "THAT" guy who used MDF for his back boxes... :rolleyes: I did use the metal back boxes inside the MDF boxes though. (And the bottom of my trim rings are open.)
Thanks, Tedd..was actually thinking of someone else but definitely interested in your experience here as well.

What are "metal back boxes"? You mean the metal boxes enclosing an IC-rated light (all of which you get when you buy an IC light from HD, etc)?

Yes, the metal box used to keep insulation away from the retrofit style light fixtures I went with. It is enclosed inside a double wall MDF/GG box. There's maybe a 1/2" between the top of the metal box and the bottom of the mdf back box.

And yes, they came from Home Depot. :)

None of the six lights have never tripped off, and they have seen extensive use. Up to 16 hours' use per day, for at least six months.

Thanks, guys..

So, Tedd - sounds like you have your boxes made out of NOTHING but (2 layers) of MDF with GG between?

Man, I was really sweating thinking I had to remake those lightboxes - AGAIN!

Glad to hear I won't set the house on fire if I don't remake and get the Hardibacker "just" right..

BTW, how did you mount your boxes? I'm considering RSIC DC04 clips (two on each side) to hold them in place decoupled from the joists..

This is the best thread I have seen on back boxes:

http://archive2.avsforum.com/avs-vb/showthread.php?t=654719

I pulled my pictures out of there (response 22) and attached here.

The back boxes are double wall mdf, with Green Glue. I think I used a layer of 5/8" for the inner box and a layer of 3/8" mdf for the outer box, as I had only seven inches of joist cavity height. I painted the mdf with some leftover BBQ paint. I used some angle iron and polyurethane glue to mount the metal box in the interior of the mdf back box as I didn't want any rattling. The MDF back box is supported by some 2x4 braces, with a layer of GG between them. Two screws per end.

The two layers of mdf differ by 5/8" to build a flange that is sealed to the box. Only one layer of drywall covers the mdf back box.

The wire inside of a recessed fixture is higher temperature rated than Romex.

Code requires 1/2" clearance between recessed housing and combustables with non IC can. IC can housings can touch any material. IC fixtures are designed to be installed in an area that entraps heat.

Use IC rated airtight cans which are better for sound proofing anyway.