Replies

1. 
   BSBot | Feb 16, 2024 09:49pm | #1

   Alright, Steve, let’s unpack this with a focus on the practical aspects and the potential chemical dance between materials, keeping in mind the principles of building science as we go.

   First off, your approach to installing Nichiha fiber-cement lap siding directly over 1/2" reflective foil-faced polyiso is not unheard of. The foil-faced polyiso acts as a continuous insulation layer, improving the thermal performance of your walls while also serving as a water-resistive barrier (WRB) when seams and perimeters are properly sealed. Your attention to detail in sealing is crucial, so that’s a solid move.

   Concerning Material Interaction:

   The concern about the foil reacting chemically with the fiber-cement siding isn't a prominent one in the grand scheme of building science. Fiber-cement is an inert material, meaning it doesn’t react with many substances, which is part of its appeal as an exterior cladding option. The foil on polyiso is typically aluminum or an aluminum composite, which also doesn't have a high reactivity, especially with cementitious materials. So, from a chemical standpoint, you’re looking at a low-risk scenario.
   Reflective Side Out:

   Following Johns Manville’s recommendation to have the reflective side facing out is about optimizing the thermal performance of the insulation. In your climate zone, the reflective surface can help in minimizing heat gain by reflecting radiant heat away from the building during the hot months. This is a smart strategy in climate zones where the battle is often against heat intrusion.
   Using Polyiso as a WRB:

   Your decision to use polyiso also as the WRB is viable, provided that all seams are meticulously taped and the perimeter sealed. This approach streamlines the wall assembly and reduces thermal bridging, but the execution needs to be spot-on to ensure the building envelope's integrity. Make sure your contractor is familiar with the specifics of this installation method to avoid any potential issues with moisture management.
   No Furring/Spacer Strips:

   Not using furring strips places a greater emphasis on ensuring that the polyiso is well-secured to the wall and that the siding is appropriately fastened through the insulation into the structural sheathing or framing. The absence of a ventilated rain screen (which furring strips would provide) means you'll want to be doubly sure about the WRB capabilities of your setup, especially in a mixed-humid climate.
   Final Thoughts:
   While the direct application of fiber-cement siding over foil-faced polyiso isn’t traditionally fraught with compatibility issues, the key to success lies in the precision of the installation. Ensuring that the WRB function of the polyiso is uncompromised and that the siding is properly secured are your main challenges. Given the shift in climate zones and the emphasis on managing moisture in a mixed-humid environment, maintaining airtightness and ensuring proper moisture management should be top priorities. Keep an eye on the long-term durability of the sealants and tapes used, as these will be critical in maintaining the integrity of your WRB over time.

2. 
   acrobaticnurse_Eli | Feb 18, 2024 08:07pm | #2

   Hello from nearby Durham County :-) I was looking at Manville's foil faced foam earlier today as well for a little project. When I check their website, given that they recommend it as a WRB and say the foil is non-corrosive I would expect it to be a nonissue to have fiber cement siding against it. If you need something more certain than that maybe their technical support line would be helpful. The number is listed in the linked document: https://www.jm.com/content/dam/jm/global/en/building-insulation/Files/BI%20Data%20Sheets/Resi%20and%20Commercial/BID-0151-AP-FoilFaced-DS.pdf
   "Johns Manville can provide technical information to assist in addressing questions regarding AP Foil-Faced Polyiso Continuous Insulation. Please call 800-654-3103 for technical assistance."

1. 
   SteveWill | Mar 15, 2024 08:20pm | #3

   Thanks for that info. I had gone back to them but had not updated my post. Johns Mansfield validated there are no interactions between polyiso and fiber-cement siding. Nichiha validated the same with regards to foil- backed insulation but did emphasize aluminium material such as flashing and nails should never make direct contact with fiber cement siding.
   I have read several posts about carpenter ants loving to nest in polyiso foam. I can tell you first hand that is true. When we demoed our existing polyiso there were sections intricately carved out by them. I've heard people try to buy borate infused polyiso but can't source it. Our contractor is using caulk (Quad Max, recommended by Nichiha for joints) to seal the polyiso exposed edges at the wall perimeter to keep the bugs out and caulking the perimeter foam to the framing to further create an air and bug seal. We're also still taping all seams.
   Good luck with your project!