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Greg Smith
Senior Project Manager, AIA, CPHC
Asked a question 4 months ago

I'm curious about people's thoughts around mineral wool as exterior insulation - specifically having reduced insulation value when wet. I have heard some differing opinions from manufacturers.

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Mariana Pickering
Co-Founder of Emu | Passive House Training, Services, Systems

Hey Greg. We use reliable data from Fraunhofer22 to address this in our CPHT class. I'll attach the graph here specifically for mineral wool. Part of our field data gathering with our Pilot Program26 is to get a better idea of actual moisture content in Passive House constructions, and we are starting to get numbers back on that47 (will get some data published soon as we can). Short version - keep it dry. Dry, it varies from 3.9 to 4.4 R per inch, depending on density (similar to fiberglass). Only concerns are in the products that use formaldehyde as the bonding agent; otherwise it's a great product with high recycled content (up to 75%) and domestically produced. 

I'm curious about people's thoughts around mineral wool as exterior insulation - specifically having reduced insulation value when wet. I have heard some differing opinions from manufacturers.
Adam Cohen
Regenerative Explorer

Mineral wool is an excellent exterior insulator.  Like any product in the building assembly, it must be used correctly and should be detailed to avoid bulk water leakage.  It does have considerable embodied energy, but not much different then many foam plastic insulation.  Also the system used for attachment is important for a good insulation.  I am a big fan of fiberglass attachment systems.  Hope that helps.