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Old 06-25-2018, 05:50 PM   #21
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What's all this about application of foam in multiple passes providing multiple vapor barriers? The suggestion that multiple applications creates multiple barriers implies that the surface or skin of the foam is the barrier. Is there any research to support this view? Everything I recall reading used phrases like "foam thicker than X inches ... provides a class Y vapor retarder/barrier." This phrasing suggests it's not the skin, but the material as a whole through its full thickness that retards vapor flow.

There's an article at Green Building Advisor that digs into the question of air barrier vs thickness and type (open or closed cell). One has to have a membership to read the article... or use the "view page source" function of your web browser. There's a table showing thickness needed of a handful of different products in order to achieve the "air barrier" leakage rate of 0.02 liters/sec-m˛ @75 Pa (0.004 cfm/sf @ 1.57 psf), which happens to be the performance of a sheet of 1/2 inch drywall.

Unfortunately, that article mostly avoided vapor barrier -- it mentioned only that "Air barriers may or may not act as a vapor barrier." Still, it's clear that a vapor barrier must also be an air barrier, and I propose that if air leakage rate is a function of thickness (not application passes) then vapor leakage probably is too.


The industry organization Spray Polyurethane Foam Alliance gives an explanation for limiting the thickness in each pass:
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What is a chemical exotherm, and how can it cause damage?

The chemical reaction used to create polyurethane and polyurethane foams give off heat, which is called an exothermic reaction. Closed-cell SPF products, when installed at pass thicknesses exceeding manufacturer’s recommendations, can generate excessive heat that can be trapped inside the foam. This heat can results in poorly formed foams that dramatically reduce coverage rates and diminish product performance, causing loss of R-value or shrinkage. In extreme cases, where closed-cell foam is applied at thicknesses several times the manufacturer’s limits, can generate enough heat to self-ignite the foam. The SPF contractor should always follow manufacturer’s installation instructions regarding pass thickness and times between passes to eliminate damage caused by exothermic reactions.

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Old 06-25-2018, 05:51 PM   #22
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What's all this about application of foam in multiple passes providing multiple vapor barriers? The suggestion that multiple applications creates multiple barriers implies that the surface or skin of the foam is the barrier. Is there any research to support this view? Everything I recall reading used phrases like "foam thicker than X inches ... provides a class Y vapor retarder/barrier." This phrasing suggests it's not the skin, but the material as a whole through its full thickness that retards vapor flow.

There's an article at Green Building Advisor that digs into the question of air barrier vs thickness and type (open or closed cell). One has to have a membership to read the article... or use the "view page source" function of your web browser. There's a table showing thickness needed of a handful of different products in order to achieve the "air barrier" leakage rate of 0.02 liters/sec-m˛ @75 Pa (0.004 cfm/sf @ 1.57 psf), which happens to be the performance of a sheet of 1/2 inch drywall.

Unfortunately, that article mostly avoided vapor barrier -- it mentioned only that "Air barriers may or may not act as a vapor barrier." Still, it's clear that a vapor barrier must also be an air barrier, and I propose that if air leakage rate is a function of thickness (not application passes) then vapor leakage probably is too.


The industry organization Spray Polyurethane Foam Alliance gives an explanation for limiting the thickness in each pass:
I wasn't implying that about the vapor barriers. Only that a few folks have done foam and said they wished they'd done a quick, light first coat.
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Old 06-25-2018, 06:14 PM   #23
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As I understand it talking to a manufacturers rep, The layering of coats for added vapor barrier protection yields minimal results. Layering however is important if you want more than 2" final thickness. Anything thicker in single coats cause the issues fw brought up.
I imagine , looking at a cut away of the foam it is many cells, but the skin is sealed. This has to help as a VB, even though minimal.
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Old 06-27-2018, 01:42 PM   #24
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GREAT response!!
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Old 06-27-2018, 02:07 PM   #25
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How close are you to spraying Marc?
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Old 06-27-2018, 03:07 PM   #26
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How close are you to spraying Marc?
Not. Do you want to be the guinea pig?
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Old 06-27-2018, 03:12 PM   #27
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I wasn't implying that about the vapor barriers. Only that a few folks have done foam and said they wished they'd done a quick, light first coat.
I totally agree with that view. For the complete amateur DIY applicator (such as myself) it makes sense to lay the first coat on thin. There's less risk of over-heating and oil canning the sheet metal, less risk of over-filling the cavity and wasting expensive material, etc.
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Old 06-27-2018, 03:16 PM   #28
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i totally agree with that view. For the complete amateur diy applicator (such as myself) it makes sense to lay the first coat on thin. There's less risk of over-heating and oil canning the sheet metal, less risk of over-filling the cavity and wasting expensive material, etc.
exactly!!!
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Old 06-27-2018, 03:56 PM   #29
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Not. Do you want to be the guinea pig?
I guess i can be! I still have some ceiling panels to remove and finish removing some my interlock\unneeded wiring. I was hoping to get tips after you did yours.
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Old 06-27-2018, 03:58 PM   #30
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I guess i can be! I still have some ceiling panels to remove and finish removing some my interlock\unneeded wiring. I was hoping to get tips after you did yours.
I need to set up getting the equipment here and where to buy material and start to practice on something.
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Old 06-27-2018, 04:37 PM   #31
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10-4 i guess like anything else there is a learning curve.
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Old 06-27-2018, 04:44 PM   #32
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10-4 i guess like anything else there is a learning curve.
For someone who's never done it before, you bet. Shouldn't take but a few minutes to get the hang of it.
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Old 06-27-2018, 04:45 PM   #33
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Maybe i can come supervise, lol.
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Old 06-27-2018, 04:47 PM   #34
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Maybe i can come supervise, lol.
Where in Ga. were you?
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Old 06-28-2018, 12:04 AM   #35
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Is that the big kit that you buy large liquid drums?
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Old 06-28-2018, 07:20 AM   #36
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Where in Ga. were you?
I am in Monticello.
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Old 06-28-2018, 09:55 AM   #37
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I’m not too far from you either o1marc, maybe I could make the trip from Augusta if you get the hang of this spray foam stuff. Keep us posted—could be a little side business for you!
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Old 06-28-2018, 10:22 AM   #38
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I'm less than 700 miles from him. Could be worth a trip. My mom lives in Trenton. 10 internet points to everyone that knows where that is without looking it up.


On second thought, that's 200 gallons of diesel fuel. Meh, still ... if I was headed that way for something else anyway.
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Old 06-28-2018, 10:32 AM   #39
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I'm less than 700 miles from him. Could be worth a trip. My mom lives in Trenton. 10 internet points to everyone that knows where that is without looking it up.


On second thought, that's 200 gallons of diesel fuel. Meh, still ... if I was headed that way for something else anyway.
I have a client in Trenton.
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Old 06-28-2018, 10:38 AM   #40
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I have a client in Trenton.
A client of what; there's not hing out there!! She's on Lookout Mountain. It'll work out well when I need somewhere to park the bus while hiking the Appalachian Trail.
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