Spray foam attic ceiling ! What is going on here?

(Roy Lewis, CMI - North Florida Inspector) #1

Frost on the roofs..
My neighbor home has spray foam.Image 1..
The other neighbors home does not..Image 2..
What is going on with the spray foam Image 1?
Thermal transference through the trusses?

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(Michael A. Clark, CMI) #2

My guess, from my own experience, is the first neighbor either did it himself, or the contractor didn't apply it properly. It looks like he sprayed between the rafters, instead of encapsulating them.

I did the same thing with my addition, metal roof on purlins. At first, I sprayed the metal parts of the roof. I was still getting some condensation anywhere I didn't cover the wood purlins with foam. So I went back and sprayed more, this time sealing the purlins. No more condensation.

The stuff is messy, and expensive. It shouldn't be applied by Harry Homeowner. I found out the hard way, but I learned something along the way.

Now I know how to apply it correctly, though I'll never, ever, tackle a job like that again!

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(Roy Lewis, CMI - North Florida Inspector) #3

No ! I watched the whole process.
He is a structural engineer and had it done by the pros..
It is fully sprayed.

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(Michael A. Clark, CMI) #4

Maybe the "pros" forgot to encapsulate the rafters?

Definitely looks like heat transfer, but I'm just a lowly home inspector, with an untrained eye, without IR camera or training... :p

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(Roy Lewis, CMI - North Florida Inspector) #5

It was sprayed a foot deep.

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(Dave Fetty, CMI) #6

Unless they mound it up at top chord areas it is still thinner than the rest of the insulation.

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(Roy Lewis, CMI - North Florida Inspector) #7

[quote="sfetty, post:6, topic:120537"]

Unless they mound it up at top chord areas it is still thinner than the rest of the insulation.
[/QUOTE]

2X4 trusses and the foam was a least a foot thick.

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(John Olson) #8

Could it be just surface temp difference and not heat loss?

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(Roy Lewis, CMI - North Florida Inspector) #9

[quote="jolson8, post:8, topic:120537"]

Could it be just surface temp difference and not heat loss?
[/QUOTE]

I'm sure it is surface temp difference, but why?
The trusses seem to be warmer than the rest of the roof.

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(John Olson) #10

I don't know why, just theory. Every material holds temperature at different rate and heats at different rate. 2x4 may heat up or cool down faster,from the outside air, than shingles. Material differences have an impact on heating and cooling as does heat or cool loss. Just a thought, can't prove. There is no foam on the roof side of the truss.

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(Erik Schmidt) #11

Even if the truss top chord is buried in foam, they are (probably) covered with less foam than the sheathing, the inner webs are connected to the chords, and they are exposed to attic temperatures, so maybe there is still some thermal bridging going on here, plus what John says about the exposed materials retaining different temperatures. The real story is being told inside the attic, where it matters most, my WAG is that a thermal image taken inside the attic when the outside pic was taken would show nothing to be concerned about.

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(Marcel R. Cyr, CMI) #12

Roy, was that home located in North Florida with a frost on the roof like that?

Did they use icynene Insulation Open Cell or Closed Cell?

Did they use polyisocyanurate insulation closed cell?
It makes a difference in hot humid gulf climates.:)

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(Roy Lewis, CMI - North Florida Inspector) #13

Yep! My next door neighbour...
I don't know.

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(Barry Adair, TREC#4563 EIFSTX#39) #14

simple thermal principles
solar heat gain, conduction, occurs heating the solid top chord
condensation/frost occurs at the cooler, not solid, insulated areas
thus the top chords telegraph the dissipating heat leaving their signature

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(Barry Adair, TREC#4563 EIFSTX#39) #15

simple thermal principles
solar heat gain, conduction, occurs heating the solid top chord
condensation/frost occurs at the cooler, not solid, insulated areas
thus the top chords telegraph the dissipating heat leaving their signature

it happened from the outside heat source in
not inside heat out
think thermal movement
hot to cold
wet to dry

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(Barry Adair, TREC#4563 EIFSTX#39) #16

simple thermal principles
solar heat gain, conduction, occurs heating the solid top chord
condensation/frost occurs at the cooler, not solid, insulated areas
thus the top chords telegraph the dissipating heat leaving their signature

it happened from the outside heat source in
not inside heat out
think thermal movement & dew point
hot to cold
wet to dry

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(John Olson) #17

Thanks Barry

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(David A. Andersen, TN HI# 40) #18

Thank you BA! . Close.

Roy, your looking at it as heat coming from the inside. It is not.

What causes frost?
The conditions in which frost forms is based upon temperature.
Which part of the roof has the greatest R-Value? That is the part of the roof that will "Store" the heat from the sun. Lower heat transfer effects the thickness of the frost or it's existence.

This is the same principal as IR Scanning. You must understand the conditions that are happening and why before you can say what your looking at in the scan.

What is the driving force of the heat transfer? The "view effect" (something you learn in that worthless Level III training...). Heat transfer here is about radiation transfer, not conduction (which is confusing you because you are discussing "Insulation").

Based on the pictures, I assume they were taken in the am before the sun came up.
The frost is not melting because solar radiation is melting it, it is just thicker because it is exposed to the -60F sky and the insulated area gave up it's heat faster. What would your T-Reflect setting be if you were trying to measure temperature?

What was the OA temp? I'm willing to bet that it was above 32F seeing your in Fla.
How does ice form when it is above 32F?

% Conduction + % Convection + % Radiation = 100% of the heat transfer.

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(Mike Pagozalski, CMI) #19

What David A said just to add a different angle ( same concept) just do the math......Roy you said 1 foot spray foam, right? Well like Fetty hinted at instead of 12 inches of spray foam uniformly across the roof sheathing each truss means it's just 8.5 inches .....do the math per inch, the difference equals your pic

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(David A. Andersen, TN HI# 40) #20

R-Value or mass?

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