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Community and Q&A

Exterior Insulation on Cinder Block Walls

jwt02180 | Posted in Green Building Techniques on

I have been through many articles talking about exterior insulation techniques, and insulating the exterior of foundations. My house is a little unique, and I’m hoping that someone can offer some insights/guidelines on how to handle things.

First of all, I live in mid-Vermont (climate zone 6A). I bought my first house last year, and the siding needs some work in addition to not being my favorite style. A project I’d like to do at some point within the next couple years would be to remove the old siding, insulate the exterior, and then install new siding. The new siding I’m looking into is a product from Shakertown, which are convenient cedar shingle panels with a built in rainscreen as part of the backing.

The house is built into a hill, so the hillside (rear) half is only one story tall, with the front half being a 2-story walk out basement style. It was built in 1950 and it’s exterior walls are entirely cinder block (the unique/unusual part) from the base of the foundation all the way up to meet the roofing system.

This is where my grey-area begins, since all the information I’ve been able to find about exterior insulation combined with siding assume a stud wall + sheathing construction as the exterior wall.

I’d like to use Rockwool (formerly Roxul) insulation as I’m against using foam. I have read this so I at least know how to determine the amount of insulation I should use.

I’m hoping to find suggestions on how the system should be laid out, starting at the cinder blocks and working outwards. Some specific questions I have include:

  • I’ve read the house wrap can go in front or behind the insulation. In this case though, where does house wrap get applied? I assume on top of the insulation, because how would you attach it to the cinder blocks…but then how do you attach house wrap directly to insulation?
  • Given the age of the house, and from what I’ve seen I’m inclined to assume the cinder blocks are hollow. I’m concerned about using a fastener that is “shot” in. Are there suggestions for a “gentler” style fastener?
  • After finding the appropriate anchors to attach the insulation, I’d like to apply it in a continuous fashion, which leaves attaching strapping over the insulation as my option for how to attach the siding. Keeping in mind the “gentle” fasteners, how would I attach the strapping to the house?

Thanks again for taking the time to read my post, and I appreciate any suggestions or feedback I might receive. Any musings about how the layers should stack up in this wall system are welcome.


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  1. Expert Member
    Dana Dorsett | | #1

    If it was built in 1950 it's almost certainly concrete block, not cinder block. But either type would still be consider a "mass wall" in IRC terms.

    To hit IRC code min on a mass wall such as CMU in VT (zone 6) takes R15 (or 4" of continuous rock wool), which COULD be trapped behind exterior grade 1.5" steel studs to hang the siding onto. As long as at least half the continuous insulation is on the exterior, R15 is fine. But 3.5-4" of continuous exterior rock wool could be pretty awkward if it's not rigid rock wool.

    But since the below-grade interior also has to be insulated to a continuous R15, it's probably easier to do the bulk of the insulation on the interior, then install rigid rock wool on the exterior sufficient to bring the U-factor under the code-max U 0.045,(R22 "whole wall", including the R-values of the siding, interior finish materials, the CMU, the interior & exterior air films, etc, and just ignore the thermal mass effects.

    Masonry screws work fine in CMU walls. It's slower to drill the holes than it is to use a ramset type fastener, but it's definitely "gentle" enough.

  2. jwt02180 | | #2

    Thanks for the correction, I was mildly aware that what I called "cinder blocks" is an old reference but wasn't sure if I should called the CMU's or not. I would like to add, that from my observations so far, it's probably safe to assume that this house was built by a "good ol' vermonter" homeowner himself back in the day. So I expect to find a lot of incorrect or unexpected things along the way.

    I'm not familiar with IRC code yet, though it will be my next step so I can ask the proper questions when it comes time to verify with local codes in my area for updates I have planned. Do you mean that code specifies R15 just on the exterior of a mass wall? Also, the below grade interior is unfinished, and not insulated on the interior at all.

    Since you mentioned the below grade interior, I've seen some signs of water penetration through the foundation, which is another project I'd like to do. The house is small, and the rear half of the house is built on top of an un-vented poured slab crawlspace, if it helps to picture it my basement area has a side profile similar to those foam sports #1 hands ==[ ]. There's probably only 6 feet of below grade foundation wall at full height. This makes it seem more feasible to excavate around the foundation to waterproof, dimplemat, and insulate the exterior surface of the below grade walls.

    Thanks again for the responding with your suggestions!

  3. Expert Member
    Dana Dorsett | | #3

    Vermont is entirely within US climate zone 6.

    The IRC/IECC calls out the minimum insulation for different assemblies by climate zone in TABLE N1102.1.2 (R402.1.2):

    Unless otherwise stated, the R-values are presumed to be center-cavity R of the insulation only, when installed between wood framing.

    With mass walls there are 2 values given, explained in the footnote:

    "i. The second R-value applies when more than half the insulation is on the interior of the mass wall."

    Compliance can also be done by U-factor, which requires a bit of calculation to determine the performance of all layers and thermal bridging structural elements, in TABLE N1102.1.4 (R402.1.4):

  4. GBA Editor
    Martin Holladay | | #4

    You have some big challenges ahead. The first problem you have to tackle, before you even begin thinking about insulation, is the water entry problem. Start by reading this article: Fixing a Wet Basement.

    If you decide to get a backhoe on site to excavate around the exterior of your house, down to the footings, in order to improve your footing drains and add an exterior waterproofing system (including a dimple mat) -- the best (although not the cheapest) way to proceed when a basement has a water entry problem -- then you should take the opportunity to consider exterior insulation.

    You might want to reconsider your distaste for rigid foam, especially if you are able to obtain some inexpensive recycled (reclaimed) rigid foam, generally available at 1/3 or 1/2 the cost of new foam. It's an environmentally responsible approach.

    I'm a little worried about whether you've done enough research. You cited one of my articles ("Calculating the Minimum Thickness of Rigid Foam") that has absolutely nothing to do with your situation. That article applies to wood-framed buildings, not buildings with CMU walls.

    Interior insulation is tricky, especially when it comes to thermal bridging at partition intersections and the transitional areas (where walls meet floors, and where walls meet ceilings or roofs). Think through your plan carefully as you decide whether you want to install exterior insulation or interior insulation. You need a plan that includes everything, from the slab to the roof.

  5. jwt02180 | | #5

    Thanks for the answers.

    Dana - That link is helpful, I'll definitely spend some time looking into codes for R-values. So it seems that depending on how much insulation is on the interior, it'd require either R-15 or R-20. I take that to mean the R-value for the whole wall, hypothetical being 60%(interior)/40%(exterior): R-12 on the inside + R-8 on the outside = R-20.

    Martin - I am planning to excavate around the house to take care of the exterior of the foundation (waterproofing, dimplemat, and exterior insulation). The water entry is very minimal at this point, and limited to one or two joints between the CMU blocks.

    Aside from the environmental concerns I have for foam, there's also a strong ant presence and I've seen enough reports on insects tunneling through foam that I'd like to use an alternative. This is what brought me to Rockwool's product that I mentioned, which is a rigid insulation panel.

    I'd like to point out I'm still very early in the research/information gathering phase for all of this. Which is precisely why I posted my question here. I haven't been able to find much information about how to insulate CMU walls properly (above & below grade), how to determine the ratio of external to internal amounts of R-value from insulation, or even the proper attachment methods when trying to build up a wall system on the external side of a CMU wall.

    If there are any articles you could point out that would offer some insight into this particular situation, and not ones that only seem to apply to wood-framed buildings I'd greatly appreciate it!

  6. GBA Editor
    Martin Holladay | | #6

    You don't have to worry about the ratio of interior to exterior rigid foam, because CMU walls aren't subject to rot. If you research ICF walls, you'll realize that some brands of ICF have unbalanced foam thickness -- for example, with thicker exterior foam than interior foam -- without any problems.

    You might also want to research EIFS, since EIFS installations all include continuous exterior insulation.

    Read this article: How to Insulate a Basement Wall. A section of that article discusses exterior basement insulation.

    You might also want to read this article (about installing above-grade exterior insulation on a brick building): Exterior Insulation for an Ugly Brick Building.

    When it comes to articles about continuous exterior mineral wool, see these three articles:

    Installing Mineral Wool Insulation Over Exterior Wall Sheathing

    Wrapping an Older House with Rock Wool Insulation

    Installing Roxul Mineral Wool on Exterior Walls

  7. Expert Member
    Dana Dorsett | | #7

    The irrelevance of the R-ratio for dew point control notwithstanding the ratio is still important relative to meeting IRC code minimums, which allows a reduced total-R if/when the exterior R is at least half the total R.

    The more R there is on the exterior, the warmer the structural wall is in winter, resulting in lower total moisture accumulation. As it happens having half or more of the total R offers HUGE dew point margin on whatever wall structure is in the middle, but with CMU it doesn't matter if there is seasonal moisture in the masonry. IRC's relaxed standard for total R on mass walls is all about the energy use benefits of thermal mass of the wall when half or more of the R is on the exterior, and nothing to do with moisture.

  8. jwt02180 | | #8

    Thanks again for the answers and links.

    There is one question still left from my original post, about a housewrap like tyvek. I had asked where it should be placed in my situation. From some further reading, it seems on a block wall it's not required? I found this mentioned in 703.1.1 as exception #1.

    I have one other question that is just out of curiosity. If I do verify that the blocks used to build this house are all indeed hollow, what are the implications of that? Any broad strokes outlining the major concerns that would bring up would be welcome.

    Thanks again for all the information so far!


  9. Expert Member
    Dana Dorsett | | #9

    Sometimes it's possible to see into the top of the CMU wall at the mudsill where the framed wall begins to determine whether the cores have been grouted full or not. Otherwise, a couple of exploratory 3/8"-1/2" holes drilled with a hammerdrill can tell you.

    While a WRB may not be strictly required on a CMU wall it's still not a bad idea. Using spray-applied WRB directly on the CMU can improve it's air tightness, if it has any obvious cracks.

  10. GBA Editor
    Martin Holladay | | #10

    Dana's giving you good advice. A liquid-applied WRB makes sense here. For more information, see Housewrap in a Can: Liquid-Applied WRBs.

    Filling all the cores of the concrete blocks is less important than blocking the tops (air sealing the tops) to limit air movement. If filling the cores completely is awkward, some contractors stuff a scrap of fiberglass batt into the top of each core hole at the top of the CMU wall -- assuming there is access -- and then seal the top few inches with canned spray foam.

  11. Expert Member
    Peter Engle | | #11

    Note that Jameson has already indicated that he will be applying exterior waterproofing below grade. Whether a sheet product or liquid product, that will provide both the WRB and air barrier below grade. I agree that liquid-applied WRB is a good solution for the above-grade portions of CMU wall. Probably the best solution.

    I would also second Martin's suggestions that you learn about EIFS and recycled foam insulation. Even though you intend to use vinyl siding products rather than the EIFS skin, you could still use EIFS installation techniques for the liquid-applied WRB, with the foam insulation bonded to the WRB. You can then fasten strapping through the foam to hang your siding.

    Finally, you don't mention it above, but if you are going to the trouble of excavating the foundations for installation of waterproofing and insulation, make sure to install new footing drains that slope out to daylight downhill.

  12. jwt02180 | | #12

    Thanks again for the answers. A liquid applied WRB certainly sounds much more palatable than trying to attach housewrap. My question about the CMU being hollow was related to concerns of structural integrity and insulation concerns. So I'll check the tops of the walls and see what's possible.

    Peter - I hadn't mentioned the footing drains just to try and keep the scope limited on my question. I've found that if a post is too broad it's hard to get focused responses sometimes. I would like to point out I'm not intending to use vinyl siding, I had linked a product from Shakertown in my original post. It's a plywood backed panel of cedar shingles that comes in 8 foot lengths, which solves the problem of me not being proficient in installing shingles one at a time manually.

    As long as we're back on the below-grade portion of the wall, are there any suggestions for an elastomeric liquid rubber product for the waterproofing layer?


  13. johnarch1 | | #13

    Hi Everyone,

    I have a similar set of conditions and some additional questions.

    We are in NJ - climate zone 5
    IRC mass wall insulation requirements is 13 exterior / 17 interior
    The existing walls are 12" cmu with Eifes on the exterior ( 3" = R15 +/-)
    The existing interior face of the wall is gyp. bd. on 1" furring strips on cmu.

    We have opened the walls in several location on the interior.
    We are considering adding 1" rigid foam insulation to the interior face of the wall between the existing furring strips.

    Here are my questions:
    1- Are there any reasons as to why adding 1" rigid foam (R-5+/-) to the interior face of the existing cmu wall would be a bad idea.

    2- Considering we already meet the R value code requirements with the Eifes insulation on the exterior. Would the cost benefit of additional 1" rigid foam on the interior of the house be worth it. Again we would not be doing this everywere only in the portion of the exterior walls in which the existing gyp. bd. has been removed about 50% of the existing cmu walls.

    Thanks in advance. John

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