Foam insulation

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
My daughter is involved in certifying Energy Star homes. The company she works for is now looking at ways to make existing structures compliant. Existing ceiling sheet rock and lath/plaster are difficult to seal so they would like to spray the top of this with a thin layer of foam and then add cellulose insulation on top of that. Her concern is the wiring that will be covered by the foam.

My first though is that it should all be at least 90c. Next would be no bundling of cables. Any comments would be appreciated.
 
ptonsparky said:
My daughter is involved in certifying Energy Star homes. The company she works for is now looking at ways to make existing structures compliant. Existing ceiling sheet rock and lath/plaster are difficult to seal so they would like to spray the top of this with a thin layer of foam and then add cellulose insulation on top of that. Her concern is the wiring that will be covered by the foam.

My first though is that it should all be at least 90c. Next would be no bundling of cables. Any comments would be appreciated.
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What calculation did you do to come up with the 90C idea?

IIRC, NM is rated to be completely buried inside insulation. In fact, i think all wiring methods can be buried in insulation. You can't bury light fixtures that are not rated IC though.
 
petersonra said:
...IIRC, NM is rated to be completely buried inside insulation. ...
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Does that include sprayed-on foam insulation?

I'm not familiar with IIRC. But I have seen installations where there was sprayed foam on NM, and the NM looked obviously overheated.

carl
 
coulter said:
I'm not familiar with IIRC.
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(If I Recall Correctly)

Don't feel bad, I had to ask him once too. :D

Unless the wiring is knob and tube, I don't see an issue with insulating over it - but I could be missing something, I reckon.

Edit to add: There, I just added it to Charlie's list. :D
 
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ptonsparky said:
Her concern is the wiring that will be covered by the foam.
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Congradulations, not only did you teach your daughter well about wires,
you've taught her to notice her surroundings.
 
georgestolz Unless the wiring is knob and tube said:
I think everone is missing something here. I am not worried about the nm cable but rather the recessed cans. The fact that they are ic rated has nothing to do with the fact that these fixtures are foamed into place covering the JB with the splices. On top of that if you had to access the J-box from the ceiling below you would not be able to take the can out to get to the JB. I have run into this on a few jobs and the inspectors are not even picking up on it. Here is a picture I posted in another thread.
This is not soft foam it hardens-- the stuff you see in boxes for shipping.

47b7db22b3127cce84165d5c6b9400000016100UaNmbRs1Ys
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petersonra said:
What calculation did you do to come up with the 90C idea?

IIRC, NM is rated to be completely buried inside insulation. In fact, i think all wiring methods can be buried in insulation. You can't bury light fixtures that are not rated IC though.
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More a process of elimination. K&T was out. I think most every fixture now requires 90 c wire to it and I don't even know if I have thw laying around for tie wire.

Dennis's picture is close to what I imagine. No cans in the ceiling. She made that clear up front. Legal or not they leak to much air. Good point on the J box accessibility.

She has helped pull enough wires in old homes to know that future accesibility is an issue and smurf tube may be part of the answer, along with routing of the wires.

It is a model home to give people ideas on how to deal with older classic construction during remodels.
 
My gut tells me that given sufficient insulation, NM will overheat even when used within NEC limits.

The temperature of the insulation is set by the rate of heat production and the thermal resistance to ambient. Increase the thermal resistance, and the same amount of heat results in a much higher temperature.

I don't think that fiberglass or loose fill has been a problem, but I'd want to see more evidence of the sort Carl provides above.

-Jon
 
Not to throw a boat anchor in the punchbowl or anything, but I'd be curious to see a code citation for someone to fail the proposed installation. :)
 
georgestolz said:
Not to throw a boat anchor in the punchbowl or anything, but I'd be curious to see a code citation for someone to fail the proposed installation. :)
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For the sake of augment how about 310.10? :)
 
ptonsparky said:
It is raining today. Looks like an apprentice will get a job that includes a can of foam insulation, a length of NM cable, and an electric space heater. He needs to learn how to use a non contact thermometer anyway.
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Great, let us know what you find out.
 
Fred Hartwell had a proposal a few cycles back to limit the ampacity of NM where it was within insulation. He cited some testing that showed excessive heating of the cables under that condition. I don't remember what code cycle or all of the details. The proposal was rejected.
Don
 
I can't imagine what it would be like on the wall where your panel box is with all the homeruns going up the wall. Yikes. I guess it may be no worse than being sleeved in conduit.
 
Please be aware that you need to understand the difference between NM and NM-B. NM has insulated rated at 60 deg c and was replaced by NM-b, 90C rated conductors, in the 1984 NEC. NM was overheating when covered with insulation.
Perhaps not all wiring metods can be insulated around.
Note the NEC article 334 is NM but the product is labeled NM-B
The spray foam insulation is thermally very efficient and could lead to overheating. How would you ever know?
 
The problems I recall were in the mid 80's, about the time the 90C conductors came out. So all of the overheated conductors I saw would have been 60C. Could be the 90C conductors don't have this problem - don't know.

I never heard anyone mention of this problem with fiberglass or loose fill insulation - just sprayed foam.

Interestingly, it was well known (in the 80's) that spraying foam on NM would eventually cause a cooked insulation issue. An excepted practice was to put the NM in EMT in the areas where the insulation would be sprayed foam. The thinking was the small airgap around would help dump the heat. I can't tell how well that worked, since I have never demoed any of those installations.

I can't tell you what the current excepted practices are today. I have not been part of any NM wiring crews for the last 20 years. However when I did my house last year, I routed the NM around any areas that would be eventually foamed. Necessay or needed? Don't know. Feel better about it? Oh yeah.

I'm very interested in ptonsparky's tests.

carl
 
I have been thinking about this alot lately and I am wondering about UF cable. UF cable encapsulates the conductors and does not seem to be a problem so why would the foam. I can see a possible issue with multiple cables being next to each other and then the foam being applied but I am not sure that the foam will affect one cable.

Again my concern is more for access to recessed cans and JB and were there are multiple cables in a stud space. I have no doubt that it is illegal when applied to thick around recessed cans.

The first house I worked in they actually sprayed the roof rafters and the attic itself was just an envelop of air sealed from the outside. There were no ridge or soffit vents at all. How that passes building code I am not usre.
 
I know it would be hard to do but you probably could get the can out and chisel out the insulation to get to the j box.

Getting the can out could be tough though.

The first house I worked in they actually sprayed the roof rafters and the attic itself was just an envelop of air sealed from the outside. There were no ridge or soffit vents at all. How that passes building code I am not usre.
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I would like to know how that passed. I'll bet the shingles don't last long with that much heat.
 
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