Switching from Rockwool to polyiso or Halo Exterra
I’m in climate zone 5A (Northwest Kansas).
Original plan wall assembly was going to be (from interior to exterior):
– drywall
– 2×6 framing with rockwool insulation
– zip sheathing
– 3” of Rockwool Comfortboard 80
– vapor open wind barriers
– furring strips to make a vented cavity
– hardie cement board for siding
Of note is that I plan to keep humidity 30%+ even in the winter if I can imagine it. Average temps are around 30° F in the winter but we spend a fair amount of time around -5° T and a few hours every year between -15° F and -25° F.
Rockwool availability seems quite poor and replies to other comments on my other posts have convinced me that polyiso is probably what I should be looking at instead of conforboard.
How will that affect my wall assembly? It’s a vapor barrier, right? I need it to breathe to the inside I suppose…
How do I attach it to the wall? Same as I would have done with Rockwool?
Probably need 2.5” to keep condensation out of the wall cavities in all situations?
Where do I source it? Any brand names to look for? Is some faced and unfaced? I guess I’d prefer whatever is most readily available. Needing it soon.
My builder would kinda like to try Halo Exterra I think. That’s GPS? What would be the cons of going that route?
How do I keep insects and especially rodents out of this stuff?
Right now I’m feeling like I have planned for 2 years and enlisted people like EMU and now due to availability issues I’m having to change paths and I just don’t know if I can do it quickly enough and still avoid building an unreliable wall system. Really want to keep rodent and moisture issues at bay as we make this transition.
Thanks.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Zoro or Grainger is where you source it. Zoro is free shipping and 15% off regularly. Rockwool allows multiple stacks to create the depth you want.
Transitioning your wall assembly in climate zone 5A from Rockwool Comfortboard 80 to polyiso due to availability issues necessitates careful consideration of moisture dynamics and thermal performance. Let's address your concerns step-by-step:
1. Impact of Polyiso on Wall Assembly
Polyiso is an excellent insulation material with a high R-value per inch, making it suitable for your climate. However, it is semi-impermeable and can act as a vapor barrier depending on its facing. This change means you'll need to ensure proper moisture management within your wall assembly to prevent condensation risks, especially during the colder months.
2. Vapor Permeability and Installation
Vapor Permeability: If maintaining a breathable wall assembly is crucial, consider using unfaced or foil-faced polyiso in a manner that does not compromise the wall's ability to dry to the interior. The foil-facing has low vapor permeability, so placement and the overall wall strategy need careful planning.
Installation: Polyiso can be attached to the wall similarly to Rockwool, using mechanical fasteners with or without adhesive, depending on the system design and manufacturer recommendations.
3. Thickness and Condensation Risk
A thickness of 2.5 inches of polyiso is a good starting point for preventing condensation within the wall cavities, but this should be verified through dew-point calculations for your specific climate and interior humidity levels.
4. Sourcing and Brands
Polyiso is widely available through major building material suppliers. Brands like Johns Manville, Rmax, and Kingspan are reputable. Availability can vary, so contacting local suppliers early is advisable. Both faced and unfaced options are available; your choice should align with your wall assembly's vapor strategy.
5. Considering Halo Exterra (GPS)
GPS (graphite polystyrene) insulation like Halo Exterra offers good thermal performance and is easier to handle than polyiso. However, it's important to consider its vapor permeability and how it integrates with your wall's moisture management strategy. GPS is generally more vapor permeable than foil-faced polyiso, which might be beneficial for your wall assembly.
6. Pest Management
To prevent insects and rodents, ensure all gaps and penetrations are sealed. Using insect and rodent-resistant sealants or barriers at potential entry points is also effective. For the vented cavity behind the siding, make sure the furring strips and siding installation do not leave open paths for pests.
7. Addressing Availability and System Reliability
It's understandable to feel challenged by material availability and the need to adapt your plans. Working with your builder and consulting with building science professionals or companies like EMU can help you navigate these changes confidently. Ensuring that any substitutions maintain the wall assembly's thermal performance, moisture management, and durability is crucial.
Final Thoughts
Adapting your wall assembly to different materials requires careful consideration of thermal and moisture management principles. Engaging with a building science expert or a knowledgeable architect can provide you with peace of mind and ensure that your wall assembly remains effective and durable despite the changes. Keep communication open with your builder and suppliers to navigate availability issues while prioritizing the integrity of your building envelope.