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Smart membrane or vapor retarder for mineral wool

Nicholas C | Posted in Green Products and Materials on

I hope this question can help others who are working with mineral wool in the future so feel free to edit my question so it’s easier to read through.

Back in 2015 I was educated on using Mineral Wool thanks to GBA.  I had to special order the product and every tradesmen working with me questioned me wondering what the heck the product was and why I would spend so much more on it.  I tried to explain the pros and why it was a better product but I truly don’t think it registered with most besides the HVAC contractor. At that time the cost of it was double the price of paper faced fiberglass insulation.

Today, now four years later, the only product available *Still through special ordering only* is actually more than double the cost by a tiny amount; it comes out to be about 103% (as in 100% = 2x the cost of fiberglass) more expensive per square foot to purchase R23 mineral wool vs R21 paper-faced fiberglass.  That’s a hard pill to swallow, folks.  When most home builders only use the R value as a determining factor, they see that they will never get their money’s worth with that much of a cost difference. It seems they think it only would ever make sense for the homeowner to be the one requesting it because they have done their research and know it’s a superior product.

Now this is a bit disheartening but just provides a little background.  I’ve yet to see it used by any home builder in our area besides myself.  So when I go to try and discuss the use of a smart vapor retarder, you can imagine the looks I get.  Anyways, back to the point of this question.  What is the suggested moisture blocking, or vapor retarding method now in 2019 and moving on when using a mineral wool insulation?

I personally used CertainTeed Smart Membrane, but I have noticed that is getting more expensive by the day.  I have also read articles online that are arguing it’s not worth the added cost or time (to properly install and seal things up).

GBA did an article on smart membranes back in 2013 but those links are now invalid and there are more products out there.

Where should home builders look for proper methods in this day? Are smart membranes fading out like “vapor barriers” did in the 80s and 90s?  If a builder is not air-sealing with something like Zip Sheathing, and/or just taping regular 7/16″ OSB seams, what is the suggested product or technique of sealing when using mineral wool for the exterior wall insulation? I would like to see GBA do an article similar to the one in 2013 but perhaps with current products and various methods.  Seeing some test results of those rather new products in 2013 would also be enlightening I think but that is always an idea for the future testers out there.  For now I’d just like to discuss the options with those who are actually familiar with mineral wool unlike the local builders or local home building material stores.

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  1. User avatar
    Jon R | | #1

    You should still follow the recommendations here. And if considering a smart retarder vs something else, review here (it does quite well).

  2. User avatar GBA Editor
    Peter Yost | | #2

    Hi Nicholas -

    It's not easy to determine the suitability or need for smart vapor retarders without knowing the full context: all the other components or layers to the building assembly in question AND the climate.

    And certainly for any air-permeable cavity insulation--including mineral wool--a continuous air barrier or control layer is substantially more important than which vapor retarder you use.


    1. Nicholas C | | #3


      Sorry I got too caught up in the discussion of the options and not the presentable climate region. I am in zone 5. It almost seems worse to be here in the middle between both extremes because we have to build and insulate walls in a hybrid mindset so that they can handle both very hot and humid and very cold and dry. We get spikes of 105 to 110 days that may be 80, 90,95% humidity and then we get days that are -15 with 10-15% humidity.

      Almost 3/4 maybe even 5/6 of builders use paper faced fiberglass here in their 2x6 stud wall cavities. I'm trying to convert others over to mineral wool and I also want to perfect my design with using mineral wool.

      The standard wall assembly built here for both cost efficiency and because of the contractor's experience is: Siding - Vinyl or LP Smart Side | House Wrap - I use Kimberly Clark brand while most use the cheap builder house brand | 7/16" OSB | 2x6 Studs with PF FG or Mineral Wool | ??? | 1/2" Drywall | 1 coat primer, 1 coat Benjamin Moore latex paint.

      In the above example, what is the most time-effective method to increase the reliability and comfort of the wall and home? I have two mindsets; either I focus on the interior side with smart membrane where I try to also use it to make an air barrier which seems like a bit of a fool's game due to staples in it to the studs OR I buy 8 rolls of some 3m 2" 8067 tape and tape the outside seams of the OSB prior to house wrap.

      Am I wrong to assume that one or the other can be done? Do you have a better third or fourth suggestion?

  3. User avatar
    Jon R | | #4

    It is beneficial to use both interior and exterior air barriers. For a better wall, consider some insulation outside the sheathing (mineral wool or EPS).

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