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#53435 06/24/05 08:00 PM
Joined: Mar 2005
Posts: 24
Was up in an attic today and couldnt help but notice that the air handler and every inch of duct work had been sealed with expanding foam. Has anyone else come accross this? It looked like a complete waste of money, and talk about a pain to work around. I was trying to add a 2nd switch leg to several switch boxes and had to tell the H.O. that it was impossible unless I could start tearing his foam apart which he said NO WAY. What on earth could covering the ducts and air handler in expanding foam accomplish? If it was to prevent leakage of conditioned air I think new ducts would have been alot cheaper than the $4000 the H.O. said he paid. Does anyone know if this foam is safe for NM cable as several lines of NM cable were sealed with the duct. I wish I had taken a pic and could post it here. If I go back next week I will, it was quite a site.

#53436 06/24/05 08:23 PM
Joined: Jul 2002
Posts: 680
I have NM sealed in foam in my house. Expanding foam is used all the time in colder climates and one can get there whole house done in foam. Its expensive but guaranteed to stop air leaks if done right.

#53437 06/25/05 12:07 PM
Joined: Jun 2003
Posts: 1,143
Like Walrus said - it's expensive, but it'll stop the air transfer.

I've only seen the hi-ex foam on tv being applied over NM [Linked Image] Coming from conduit country, I kept thinking "that'll suck to change"

No idea if any cable manufacturere has tested their outer jacket in the foam solvent; in addition, the foam heats as it sets... not sure what the temp is, but wonder it if could affect the insulation on the NM.

In addition, wouldn;t the complete encapsulation of the NM affect it's heat dissapation characteristics? The '05 Code addresses NM that's bundled or bunched to go through holes in framing... wonder if the NEC will address this in '08?

[This message has been edited by DougW (edited 06-25-2005).]

#53438 06/25/05 12:34 PM
Joined: Feb 2003
Posts: 27
When I build again the air conditioning will be in attic as cold air drops easier than raises and I plan to have the metal ducts sprayed with the foam that is now require for two by four walls to reach the R-23 value. But I can't imagine what the problem would cause the problem you are having on this job Is the duct in the way what is the problem. The only problem NM wire will have is if it were twisted tightly and has what looks like a knuckle and the circuit is fully loaded then the NM will begin to over heat and discolor outer jacket and compromise the insulation. Just give a reply why you have to take the insulation off the ducts.

William Runkle
#53439 06/26/05 12:00 AM
Joined: Mar 2005
Posts: 24
The reason I would have to tear the foam and the duct work out of my way is because the ducts lay right on top of the header that I need to feed my 14/3 romex down, and the foam has sealed the duct to the attic floor. There is no way to get under the duct without causing major damage to the foam installation. Does that answer your question Runkle?

#53440 06/26/05 01:19 PM
Joined: Jan 2005
Posts: 5,460
Likes: 3
Cat Servant
The expanding foam is a urethane (synthetic rubber). It does not have a 'solvent' in it, as much as it has two components that react with the moisture in the air to create the foam. Sure, it gives off heat, but I doubt the heat can get anwhere near the 200+ degrees necessary to start decomposing the PVC jacket of the NM.

Wonderful as insulation is, junction boxes still need to be accessible.

IMO, when something is insulated as cloesly, and as well, as happens with foam, I would have to say that de-rating runs where the wires are covered for more than two feet is warranted. I don't think the code requires it- as 'insulating' and 'bundling' are two different things. And- to throw a wrench in the works- if the wires are next to a cold duct, your 'de-rating' process might actually result in a HIGHER ampacity, once you correct for temperature!

Now I've had to add wires to foam-filled walls. I usually use a short piece of EMT to bore into the get some sense of feel as to whether you're hitting something. Just keep a stiff wire near, for pushing the foam plugs out of the EMT.

As for it being a 'pain to work around,' well, they call it "work" for a reason.
If you have to replace the foam you remove, then perhaps a layer of tin foil will allow for easier removal of the new foam on the next visit.

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