polyisocyanurate in walls?

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  #41  
Old 05-21-10, 04:56 PM
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Hey Bud-
Thanks for the quick reply and glad to hear the first scenario is the one you prefer. Would you mind saying more about how conduction has been broken and radiation effectively stopped in the 1/2" gaps? I really am trying to grasp all this...
Thanks,
Rob

Originally Posted by Bud9051 View Post
In your 1/2" gaps, conduction has been broken and radiation effectively stopped, plus the narrow gap slows convection, should all be a plus.
 
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  #42  
Old 05-21-10, 05:18 PM
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There are three modes of transport for heat, convection, conduction, and radiation. Block all three and you have no heat loss. A gap eliminates the physical contact, therefore, no conduction. A good foil surface can either reflect or not emit radiant heat, reflective surfaces do both, often in the 98% range. So the foil faced gap will have blocked the radiant transfer and the conduction transfer of energy.

Bud
 
  #43  
Old 05-22-10, 08:02 AM
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Thanks again, Bud. I guess I am all set then... I had originally intended to have no extra air gap and use a combination of 2" and 1.5" XPS followed by a layer of 2" PIC across the studs. I realized, however, that 1.5" XPS is about $4.00 more per sheet than 1" PIC, and there is only a difference of 1 in terms of the R value. So assuming that 1/2" gap isn't causing much harm, saving $4.00 a sheet seems like the way to go... Rob
 
  #44  
Old 05-30-10, 10:24 AM
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Double Vapor Barrier??

Sorry to dig this thread up, yet again. I've been reading about vapor barriers and issues with "double" vapor barriers. Does the scenario talked about end up creating a double vapor barrier between the sections of PIC?

1. 2" XPS cut, fit, and sealed between the studs with spray foam
2. 1" foil faced PIC cut, fit, glued, and fastened against the XPS (does this layer need sealed with spray at the edges?)
3. 2" foil faced PIC fastened over the studs
4. strapping
5. drywall
 
  #45  
Old 05-30-10, 10:34 AM
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Air is the primary transport mechanism for moisture, so none should be getting in. Diffusion through the wood occurs in both directions,so what it would let in, it should let out as well. I personally do like to eliminate multiple layers of foil, but people just like the sound of the slightly higher r-value. Since your last layer of 2" PIC will have foil on both sides, there is no need for the one inch piece in the cavity to be foil faced as well.

This layering of XPS foam and PIC is lacking in history and history is usually where we discover our mistakes.

Bud
 
  #46  
Old 05-30-10, 12:26 PM
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I think I see what you're saying. So I guess it would be more of an issue in cases where foil faced fiberglass is used along with foil faced PIC, since the fiberglass would potentially allow more air/vapor to pass through? So I take the condensation could lead to mold, etc?
 
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