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Polyiso versus XPS Foam for a roof??

workman67 | Posted in Construction Techniques on December 30, 2019 06:08pm

So I need to replace a section of a low slope roof on our ranch (1925 built) immediately. We are going to add on to the back of the house and have a cathedral ceiling kitchen in a few years so i want to do a test on this part that will not be disturbed. 
I have read and read for years about the possible applications and have old articles that i can adhere to. However i do not truly understand the difference between the 2 products and if it really does not matter on the roof. It will not get wet after i do the job correctly so does it matter that i use some XPS in conjuntion with the polyiso?
I will probably go with the suggested 6 inches of material but am not sure if i will incorporate the venting or not between the last layer of plywood and the roofing material.
Mainly because i am just not sure how i am going to attack fastening the material and furring strips just yet.
Anyone have any thoughts?? I see the polyiso has a little more r value but i am thinking it may be worth saving some money and only using 1 layer of that. We live in north jersey so i am thinking we are in Zone 6 or on the edge where i see it should only require 2 inches of closed cell foam to stop the  dew point. 

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  1. ktkcad | Dec 30, 2019 07:00pm | #1

    You don't stop the dew point, you 'move' it. And it will move on its own as the temperature and humidity levels change. Where it lands during the coldest months is what you design the insulation for, because that is when it sits and rots things because it never dries out. Check out this youtube video https://www.youtube.com/watch?v=Ld8pzIu45F8 for the building science and why you will still want to vent your rigid insulated roof.
    XPF is closed-cell - per Owens Corning: 'The homogeneous and hydrophobic closed-cell structure of XPS makes it resistant to water intrusion.'
    XPF also has been tested to gain R-value as it gets colder. This may offset the cost difference with Polyiso because it has been tested to lose R-value in colder weather.
    So stick with just one product and attach it according to the manufacturer's (and Dr. Joseph Lstibuerk's) instructions.

    1. workman67 | Dec 31, 2019 06:39pm | #2

      thanks for input!
      i have watched some of this video months back. Watched it again now.My situation is a bit complicated but will figure out a method after i tear it all up. There is old vinyl coated aluminum shake style siding and aluminum covered soffitt material now so i will be rebuilding everything anyway.
      The issue with the different styles of foam is still perplexing to me. I am somewhat limited to what i can purchase at these stores.

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