GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Rock wool vs. EPS exterior wall insulation

davorradman | Posted in General Questions on

I am trying to decide between these two exterior insulation types for a house:
– 200 m2, flat roof
– built with Thermo brick (like f.e. Porotherm)
– ETICS/EIFS insulation system
– location: Croatia, the climate is corresponding to Philadelphia

Trying to achieve at least ~35 kwh/m2/a. Probably will end up better, since ERV is also planned (Zehnder ComfoAir Q350), and most glass will be on an unobstructed south side.

Proponents for rock wool are really passionate. But I am wondering if its advantages would mean anything for me. Well, other than fire resistance. Sound is not a problem. But vapor diffusion is something that, no matter how much I read, I can’t really get it.
Since exterior insulation will be thick (8inches), there should be no condensation whatever I choose?

From what I’ve gathered: vapor moves from hot to cold. During summer, this means vapor would be trying to move into the house. In which case EPS sounds better? And during winter, it would move the opposite direction? Do I need, and where to put vapor barrier or vapor retardant? On the inside of the wall, preventing outdoor vapor diffusion, but letting inside vapor to dry to the outside?

So far, I’ve been able to surmise the following regarding the rock wool:
+ fire resistant
+ better sound insulation
+ vapor permeable
– if not done right, can lead to problems
– 50% more expensive
– fewer installers, and consequently, fewer reliable installers

I have read this article and comments, but I’m still in doubt on some things, since my house would be brick and concrete, with stucco, flat roof.

The price difference amounts to 2% of the whole house. Which is not negligible. Is there a strong argument fo rock wool for someone who does appreciate indoor air quality?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Your choice of insulation will not affect your indoor air quality in any way.

    I had never heard of Porotherm, so I looked it up. It is a brand of clay blocks used for walls. I've seen similar blocks used in older buildings in the U.S., but that type of clay block hasn't be used in the U.S. for 80 or 100 years.

    Clay block walls aren't susceptible to moisture damage, and I imagine that your walls will dry readily to the interior.

    When it comes to your roof assembly, you won't have any outward drying (your roofing will stop outward drying), so your choice of insulation is irrelevant.

    If I were you, I would choose the less expensive option -- especially if that meant a higher R-value was possible.

  2. Anon3 | | #2

    Do you have ant problems?

  3. davorradman | | #3

    No ant and termite problems in the area.

    Thanks for the advice Martin. Reading all the articles here and elsewhere, this was my thinking as well.

    Curious about the clay blocks.. In fact, there "thermo clay blocks" are relatively new here. They are a little bit improved versions of regular clay blocks. What do you make your brick walls from, if not from these?

  4. GBA Editor
    Martin Holladay | | #4

    In the U.S., masonry walls are almost all made with concrete blocks (also known as concrete masonry units, or CMUs). These walls are then faced with stucco or one wythe of brick venner.

    Of course there are other systems, like ICFs, autoclaved aerated concrete blocks, etc., but these other systems represent a small percentage of the market.

    I assume that the reason that U.S. builders prefer concrete blocks to clay blocks is simple: They are cheaper.

  5. brendanalbano | | #5

    Regarding condensation, you're right that with 8" of exterior insulation, you're fine with both EPS and mineral wool. Even in the summer, with the permiable exterior mineral wool on a wall, if moisture from the outside moves inwards through the mineral wool, and condenses on the outside of your clay blocks, it's not a big problem, as that's still on the outside of your home, the clay blocks are not damaged by moisture, and it can dry easily. Grain of salt, I'm not familiar with clay block construction, so there may be nuances I'm missing, but that's the general idea.

    No additional advice, it sounds like Marin's got you covered, but I find it fascinating that even in our very global world, there can be pretty radical differences in standard constructions practices between the US and Europe, such as these hollow clay blocks!

    Are the relative prices of clay and concrete different in the US vs Europe? Are the machines that extrude these complex clay bricks only available in Europe? Is it just cultural? Fascinating!

  6. Expert Member
    Dana Dorsett | | #6

    Clay block is susceptible to freeze/thaw spalling in much colder climates, but that won't be an issue in Croatia. In VERY humid climates the fairly high vapor permeance can potentially become an interior paint failure problem in air conditioned buildings, but the Adriatic region isn't particularly humid in summer.

    The thermal performance is 3-5x better than the simple concrete CMU used in the US, but doesn't quite match CMU for structural capacity. It will still need some amount of other insulation to make a low energy use building in that climate.

    In earthquake zones (such as Croatia) it's important to pay attention to the amount & type of reinforcement needed for earthquake resilience. I'm not quite sure what it will take for this type of block, but there may be information on that available from clay block manufacturers.

    Rock wool is far more fireproof than EPS, won't shrink with age or heat. Even if it is higher cost, it's a greener product, and should outlast most other building materials going into the house.

  7. davorradman | | #7

    I think it's just scale. And regulations that force that scale.
    We do have concrete blocks, but they are today, AFAI can see browsing some stores, more expensive where I live, ~10$/m2, while clay block go as low as 6$/m2. This is for 10 inch thick blocks, which is the minimum you can use for a house.
    But, that is relatively recent. There are houses built with concrete blocks, though there are much more barns built with it, because they were once cheaper.
    Local concrete factory has just started making them, so they are a bit cheaper, but they are also of lower quality and can only be used for making smaller objects, like sheds and garages.

    It really is fascinating, the differences. I am veering of subject now, but when we set out making the house, we leaned towards timber construction (we call them "prefabricated houses" because prefab is the only way to make them here). But they are ~20-30% more expensive.

    In any case, I appreciate your opinions. It is not something I will base my final decisions on (because after all, our traditional systems of building are so different), but it gives me valuable talking points when I discuss this with project managers and contractors.

    I really ought to pay for the membership, as there are also many articles and CAD details that can be applied here, regarding f.e. water saving measures and such. I only wish I had more time :)

  8. davorradman | | #8

    One other pain I have is also wood cladding. I so so much would want that. Nice charred clapboard cladding. But because so few people do it, it's multiple times more expensive than stucco, and I simply can not justify it.
    So is the case brick cladding as well. In Belgium, it's cheaper than stucco. Here, it's probably 2-3x as expensive.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |