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Community and Q&A

Exterior foam board insulation & metal or Hardie siding

paula_builds | Posted in General Questions on

Maybe I just need to find an energy efficiency expert.  But..just got done reading a series of articles & threads on two topics:  metal siding (specifically vertically oriented corrugated) and exterior foam board.

I’m in zone 4, off the coast but on the milder side of Oregon.  Winters are mild and very wet.  Our summers are very dry and the threat of wildfire increases each year.  My house will be in the woods and has firebreaks but I’m interested in either metal or hardie board siding for the fire resistance.

I insulated my last building with rockwool insulation between the 2x6s, and am happy with the thermal performance as a result.  However, some articles I read tempt me to go further and add exterior foam board insulation in addition to whatever is placed in the wall cavity.

1.  Difficulty.  I’m a fairly inexperienced builder (2 cabins so far).  I do wonder if getting all the details right with the foam board around my windows will be incredibly time consuming.  The other two buildings I did just had cavity insulation.    I’m a teacher so I only have a summer to get this thing ready for weather.  Foundation has been poured.

2.  How much foam? I read that if I do add foam, adding just a little could be more problematic than skipping it (moisture reasons).  How much foam would I need to significantly reduce thermal bridging and reduce heating cost/effort (heating with wood).  I have my Oregon Residential Code book.  R23 meets code for the wall insulation, and I have achieved that with the rockwool.  So this would be about going above the requirements.

edit:  Lstiburek seems to indicate here that the ratio of continuous to cavity insulation in zone 4 is 0.2.  I interpret that to mean that my original R23 would have about R5 exterior.  That’s 1 inch of foam board, or else 1.5 or so inches of mineral fiber board.

3.  Sprayed Foam? Putting sprayed in foam between studs would be simpler than attaching foam (or other) outside (and add significantly to performance), but what are the greenhouse gases/blowing agents (just asked about this in another thread)?  If horrible, then perhaps not.  Also it’s very expensive.

4.  Mineral Fiber Insulation board for continuous insulation might be a good alternative to foam board…reading a bunch of Lstiburek articles.

5.  Heavy siding sagging on screws through exterior foam board.  If I go with the exterior foam board, I am concerned about hanging heavy hardie board siding 2-4 inches away from the studs.  As a result I’m looking at metal siding as an option.  (I guess depending on the gauge I select it could be fairly heavy also.)

6.  Drainage with corrugated.  If I go with lap siding I know how to install a rain screen detail under it.  If I went with corrugated metal, would the rain screen be a simpler detail as it’s basically built-in?  So could I possibly add exterior foam, then either tape it or add housewrap, then furring strips (horizontal??) and then fasten on the corrugated metal?  It seems like horizontal furring strips would catch falling moisture, but there would still be open air channels for drying to the exterior.

Or–If I were skipping foam board, would the corrugated metal just be screwed directly through house wrap and sheathing into studs (no furring strips required)?

7.  Perfect Seam Seal?
Also with corrugated metal (assuming I find some that I like the look of), do I glue between panels and attempt to make it pretty air tight except for at top & bottom of walls, and below & above windows, where some sort of mesh for insects might be a needed detail?  If the install is imperfect, then there might be less of a seal between panels, as I discovered recently when putting corrugated metal on a woodshed.

Well I just put a lot out there so any input is appreciated.  Thank you!


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Paula.

    I'll try to address some of your concerns, and point you to some articles with more in depth answers.

    First of all, yes, detailing exterior continuous insulation and ventilated rainscreens properly is difficult (compared to not doing either of these things) and time consuming. The details are fussy and that doesn't change whether you are an inexperienced builder or a seasoned professional. It is the nature of this work. Both are also often the best thing to do for the performance and durability of the building.

    In Climate Zones 4, in the 2018 International Residential Code, the minimum requirement for cavity insulation is R-20, or R-13 cavity insulation plus R-5 continuous insulation. I've never talked to a code writer about this, but my assumption has always been that these are two different ways of meeting the same total wall R-value (of course assuming the same windows and doors in both walls).

    The benefit of the continuous exterior insulation over cavity insulation alone is durability. It keeps the framing more inside of the thermal envelope, reducing temperature swings and potential for condensation problems, etc. You can increase the R-value of the exterior continuous insulation as much as you'd like for better thermal performance (if you were to increase the R-value of the cavity insulation, though, you'd need to proportionately increase the R-value of the exterior continuous insulation). Check out this article: Walls that Work.

    Closed-cell spray foam insulation in the wall cavities is not cost effective or environmentally friendly. Open cell spray foam may be a little better, but I'm not a fan (personally). I'd rather see you detail the sheathing really well as an air barrier and use cellulose, which is cost-effective and is an environmentally-friendly option, as insulation goes.

    Mineral wool is a good choice for exterior continuous insulation. I've never installed it, but from what I know, it is even more fussy that rigid foam because it compresses more easily.

    You don't need to worry about sagging siding, you just need to use the correct fasteners for your furring strips. Check out this article for more on this: Fastening Furring Strips to a Foam Sheathed Wall.

    If you go with metal siding over foam, and if the foam is thick enough, you'll likely need horizontal furring strips. I think Coravent makes corrugated furring strips that allow drainage in a horizontal installation. You should install the metal according to manufacturer's instructions when it comes to fasteners and sealants, but yes, if there is an opening into the ventilation channels or a gap behind the siding at the top or the bottom, you should fins a way to keep critters out. I believe you'll find those types of details here: All About Rainscreens

  2. paula_builds | | #2

    Thank you for the detailed reply. I appreciate it.

    Regarding the details: I am beyond impressed with the level of detail provided in the Rockwool mineral fiber insulation board installation guide

    They go over how to detail from sheathing outward for not just windows & doors but around decks, balconies, water spigot, dryer vent, on and on.

    Follow up question:
    would there be a problem with having R21 in walls and R8 outside for continuous? Since that exceeds the ratio of exterior to total?

    The metal siding manufacturer agreed with horizontal furring strips. Where I'm unlikely to find resources are in the unique combination of rigid mineral fiber board with vertically oriented corrugated metal. I guess if I can get in touch with Rockwool and they are ok with horizontal furring strips to secure the board, then I'd feel ready to proceed.

    Follow up question #2.
    The detailing with mineral fiber board shows the windows being taped and sealed to the house wrap, and then the fiber board on top of that . If I go with 2 inches of fiber board, and my windows only are about 1 inch out from sheathing, then I have to include returns with my window trim since the windows will be sunk below the siding?

    I guess this is a question for my window person, but is there the option to request windows that stick out further? Or can I just furring strips to pop the windows out from the sheathing more? is a 2 inch thick furring strip excessive, or is that an option to solve the situation? It seems better to have the windows out for catching light.

    1. GBA Editor
      Brian Pontolilo | | #6

      Hey Paula.

      I believe that your proposed ratio of exterior to cavity R-value is fine. In fact, when using mineral wool as continuous exterior insulation, you have less concern as your walls will be able to dry in both directions should the sheathing get wet (assuming vapor-open cavity insulation as well and no class I or class II interior vapor retarder in your climate).

      I can't see any problem with the combo of the exterior mineral wool and steel siding. The important thing is the right spacing of furring strips and fasteners, which the siding manufacturer can probable spec for you.

      As far as the window install goes, you have options. Installing the window at the sheathing plane makes it straightforward to integrate flashings with the water-resistive barrier. Yes', you'll need extension jambs for the trim. Using a window buck, manufacturers or site-built, relieves the need for extension jambs, but makes flashing a bit trickier. You can make this decision based on your level of comfort with the work to be done. My opinion is that the first option may be a better bet. I'd rather see you get the flashing right, even if the trim work is a little more tedious.

  3. RILEYG | | #4

    Hi Paula
    I just finished an installation similar to what you are thinking of doing. Vertical corrugated metal, horizontal 1" x 4" furring with a beveled edge to deflect any moisture that may enter in behind the metal, and 1.5" exterior mineral wool. The mineral wool is easy to install. I used 5 cap nails on each 2'x 4' piece to hold it in place before the furring was installed. The furring is not difficult either. I went with 4" GRK R4 structural wood screws. To monitor the compression of the mineral wool I just used a 3.5 " nail with a mark on it set at 2.25 " to allow for the wool plus the depth of the 1 x 4. Push the nail through the mineral wool, screw the 1 x 4 furring onto the wall go a little extra, then just back the screw off until the mark on the nail is flush with the outer edge of the 1 x 4. Screw in a alternating top and bottom pattern with the screws 16" apart to catch the studs. Install 2 screws on the ends of each 1 x 4, I pre-drilled the holes on the ends to avoid any splitting. I used this process on a 38 x 44 x 12 building. Zero issues with the metal installation. 2 people makes the job much easier also.

    1. paula_builds | | #7

      Thanks Riley, that is very helpful. Do you recall any specific solutions you came up with for detailing the windows?

  4. Expert Member
    Akos | | #5

    I would price out the rigid mineral wool boards before you go much further. The 1.5" board is basically an R25 batt compressed to 4x the density, so you are paying for R25 of material but getting only R6.

    If you want to go with mineral wool insulation, you can go with horizontal 2x3 on edge with 2.5" Rockwool AFB (all commercial drywall places can get this) in between. This gets you a much higher R value assembly less money. You also don't have to deal with the compression issues the above poster mentioned. The 2x3 would have to be on 17.5" OC to fit the AFB batts.

    Going with 1.5" of polyiso is probably even cheaper. Polyiso doesn't burn like most foam products, it chars like wood. Not as fire resistant as mineral wool, but I think if the insulation under your metal siding is burning, you have much bigger problems.

  5. paula_builds | | #8

    Thank you Akos. Are you saying that the 2x3's went between the mineral wood boards? Can you clarify why this assembly costs less? Or is it just more R value per $ since I can step up to 2.5 inches. I would imagine that there is a tiny bit more thermal bridging with this solution, or that my effective R value is diminished by the boards between batts. The exterior insulation is no longer truly continuous. still it doesn't feel like a bad choice.

    Yes, I do need to price out the different options, but some materials don't cost more themselves, and end up costing more in the various hardware or other options they then force you to use.

    For example I'm wondering what the Thermal buck product I linked above would cost for all the windows in my project. One of those add-on expenses you don't think of immediately when you estimate the cost of continuous exterior insulation.

    Does anyone have experience with thermal buck and how straightforward it is to install, what the long term performance is?

    Thank you.

    1. Expert Member
      Akos | | #10

      AFB is fluffy insulation, stuff that goes into walls, the 2x3 need to go to the sheathing to support siding. The AFB is much cheaper $/R value than comfort board. The assembly I suggest is less because of the lower cost of the AFB plus the strapping install is much quicker. The R value boost comes from the extra thickness.

      Might be worth while to do a energy use calculation. In zone 4, I can't see a walls above an R20 assembly saving you much money on the yearly basis. You most likely better off to go with a 2x8 on 24" wall with R30 batts than deal with exterior insulation and all the extra details that involves. Instead of exterior mineral wool, for an extra layer of fire protection, you can sheath the house with dense glass instead of OSB.

  6. Zdesign | | #9

    The thermal buck is a great idea and solution for exterior insulations but the cost is very prohibitive for what you get. I ended up using 2xs ripped to size to match up with the 2" foam I was utilizing on the exterior of my house. Pricing for the thermal buck was well over $2k for the 25 windows and 3 doors on my house. For the exterior insulation look at the Kool Therm K12 Framing board from King Span. It is fire proof and has the highest R value per inch of any foam.

  7. paula_builds | | #11

    That's good information, Zdesign. Thanks.

    Regarding comparing different insulation, here's an article about the relative greenhouse gas impacts of each choice. I'm surprised to see polyisocyanurate looking just as good as rigid mineral wool. As someone else pointed out to me here on a recent thread, XPS and closed cell spray foam are far worse by comparison.

  8. ERIC WHETZEL | | #12

    Hi Paula, on our house we used 4" of Rockwool Comfortboard 80 over Zip sheathing. We also went with innie windows.

    I have a lot of photos posted here:

    Hopefully the photos help you visualize how all the elements come together, especially around windows and doors.

    There are also links to videos showing how to detail continuous insulation and innie windows.

    I had no previous experience building a house, and my siding guys had no experience detailing 4" of continuous insulation or innie windows, so it's definitely doable if you're detail oriented.

    Once you think you've settled on a game plan on how to proceed, I would highly recommend doing a mock wall assembly that includes a window opening. This way you can practice air/water sealing and bringing all of the components together. It's a safe way to practice before having to do it for real. You'll also figure out if you need to make any changes to your wall assembly or your flashing details.

    Good luck with your project!

    1. Expert Member
      MALCOLM TAYLOR | | #13

      "so it's definitely doable if you're detail oriented."

      Ah - but there's the rub Eric. You may not have built a house before, but you sure aren't your average DIYer either.

      1. ERIC WHETZEL | | #14

        I appreciate the compliment, Malcolm --- Thank you!

    2. paula_builds | | #15

      your website is a fantastic resource. Thank you for the meticulous documentation of many resources. I look forward to tapping into that.

      I'm curious what let you to select the comfortboard instead of the other option (that I'd consider), polycyanurate foam board. the foam board actually is rated a little bit lower than the roxul (for its greenhouse gas impact) on the chart I posted above, although both are very low & close.

      Aesthetically i do like the breathability of the comfortboard rigid mineral wool and prefer to avoid foam!

      Thanks Eric for chiming in and I look forward to reading much more about everything on your site. The mock wall assembly sounds like a good idea. I did do a rainscreen under cedar siding for my shed/studio and the trickiest part was getting the bug screen around the rainscreen strips. I certainly didn't do that perfectly but might come up with a better plan for next time. The wall would give me a chance to figure some stuff out in advance prior to ordering everything.

      1. Expert Member
        MALCOLM TAYLOR | | #16


        Rather than insect screen consider either perforated flashing like this: or Cor-A-Vent strips.

        1. paula_builds | | #19

          Hi Malcolm,
          that product looks great. I'm curious about how it could be installed with my corrugated metal. would a) the J hook around the furring strips, but still be tucked under the corrugation? Or b) would the J go under the furring strips and then below both them and the corrugations and then come up outside the siding?

          I think the second option would look funny but the first option would leave some openings for bugs.

          Or maybe I can get an L shaped flashing that will cover the bottom openings of the corrugations but not stick up outside and in front of the siding?

          1. Expert Member
            Deleted | | #20


          2. Expert Member
            MALCOLM TAYLOR | | #21


            Sorry - I should have been clearer. I just linked to the flashing to show an example of the perforated material. You should use an L-shaped flashing extending out to block the bottom of the insulation, strapped cavity and the corrugated siding.

      2. ERIC WHETZEL | | #17

        We went with the Rockwool for its mix of characteristics: won't hold onto water if it gets wet, fire and insect resistant, vapor permeable, and its R-value won't be affected by colder temperatures like the polyiso.

  9. AJ__ | | #18

    Hi Paula

    I think you'll find this guide very helpful, it covers a lot of your concerns

  10. paula_builds | | #22

    I've started looking at this guide. I was also looking through Rockwool's comprehensive installation guide. I'm leaning at this point towards the rockwool comfortboard (rigid mineral wool insulation sheathing board). However whether I use that or a foam board, it looks like I'll need a cement protection board for the portion of the insulation that is exposed above grade (see image from page 43 of the bchousing document which Alex P sent, thanks!!).
    what exactly is this cement protection board? Could it be hardie panel, or hardie backer? My lumber yard doesn't seem to know what it is.

    If I install the stuff ontop of the mineral wool do I hold it in place before the backfill?? screws going into the masonry, through the insulation? or just glue it. I don't have a sense of what this stuff is or how heavy.

    Will it require a parge coat or paint in order to be considered finished?


    1. Expert Member
      MALCOLM TAYLOR | | #23
  11. ERIC WHETZEL | | #24

    We used 5" of Rockwool on the outside of our foundation. The first layer was 3" thick, which we attached with construction adhesive (Rockwool told me this was ok). The second layer of 2" was attached using Rodenhouse fasteners through the 2 layers of insulation and into the foundation. The fasteners required pre-drilling with a hammer drill, but they worked great and I wouldn't hesitate to use them again. I have more info on the installation here:

    The area you're talking about --- protecting the insulation on the foundation where it's exposed above grade --- was easily one of the most frustrating parts of our build to detail, mainly because there isn't really a great solution. The 2 main options are the cement board with a parge coating, or else metal flashing.

    We ended up using mainly the metal flashing, and so far it's holding up well, but it's definitely not perfect. In our basement window wells we instead opted for a 3-coat stucco. That too, so far, is holding up well.

    Whichever option you pursue, if you can incorporate a gravel border around the perimeter of your house, it should also help, whether in terms of durability or if/when you have to repair or replace the protective material.

    I talk about our gravel border here:

    I include a video showing the Tuff II being directly applied to rigid foam (after sticky mesh tape has been applied to the foam). The Rockwool Comfortboard 80 is rigid, but not nearly as much as EPS/XPS, so I doubt this would work with the Rockwool.

  12. paula_builds | | #25


    Your house is so beautiful! I'm very impressed. Thanks for the two links. I've looked at them a bit, and don't see pictures of the metal flashing you mention that you used.

    I read the article Malcolm linked to, and hardie backer board or metal are the two options that stood out to me. I've seen this detail fail when people put an adhesive or some thin plastic on top of the insulation, above grade. It looks pretty bad to have peeling layers.

    For my application, a metal flashing band wouldn't work all the way around, as the width of the exposed insulation will increase as the hillside drops away from the house. I'm inclined to go with the hardiebacker or something similar with a stucco or similar coating for the larger areas, and perhaps a strip of metal flashing for the narrower exposure on the uphill side.

    The idea of having gravel next to the wall makes sense. I'm curious --in your pictures, Eric it appears that you backfilled directly to the comfortboard insulation. Did you add the flashing later? How did you fasten it to the wall?

    Thanks for the detailed photos. Even the q&a on your post was helpful.


    1. ERIC WHETZEL | | #26

      Thanks for the compliment, Paula!

      I'll attach a photo of the metal flashing covering the face of the Rockwool on the foundation.

      This metal flashing is really just acting like a traditional Z-flashing that's continued down the face of the Rockwool insulation on the foundation. It keeps things watertight transitioning from the Zip sheathing down over the face of the Rockwool.

      To attach it to the Zip we tried using the blue Tescon Vana tape and then Prosoco's Joint and Seam, but eventually ended up using the Prosoco Fast Flash to embed the metal flashing in this area, as well as for our head flashings above windows and doors.

      We ran a bead of Fast Flash, then pushed the metal flashing into this bead. Once it was in position, we then 'counter flashed' it by applying another bead over the connection between the Zip and the metal flashing. This helps ensure the metal flashing is fully adhered to the Zip sheathing, and that this connection is completely watertight.

      To establish our gravel border we first asked our excavator, when he was backfilling, to hold back on the last 2', essentially creating a trench around the perimeter of the house. At that point, after adding some landscape fabric, it was relatively straightforward to fill this trench with gravel.

      Having the gravel border in place meant it was easy to pull the stone away from the Rockwool several inches down while the Z-flashing was installed. Once it was installed, I pushed the stone back into place, covering the area where the metal flashing terminated on the face of the Rockwool in order to hide this area and to make it look continuous.

      The attached photo is showing the west side of our house, which had the largest area of Rockwool insulation exposed at grade level. In order to try and keep the seams of the metal flashing tight together we used a rivet gun to join pieces together.

      The stucco will be more expensive, but will probably end up looking much better if you can work the cost into your budget, especially if you have a larger surface area to cover. I would only do the stucco, though, if you will have the gravel border. Pushing any kind of soil or mulch up against the stucco finish is probably not a good idea for its long term durability.

      Inside our two basement window wells where the Rockwool is covered in stucco we filled the bottom of the window well with about 6" of gravel first. Before starting the stucco, we pulled this gravel back from the Rockwool. Once the stucco was finished, like with the metal flashing, we just pushed the gravel back into position, up against the stucco, to hide the point at which the stucoo terminated on the face of the Rockwool. The window wells don't see much moisture, but even when they have, whether it's rain or snow, because of the gravel this moisture is able to freely drain away from the stucco.

      In addition to GBA as an ongoing resource, both for info and the ability to ask questions, you may want to check out Hammer and Hand's Best Practices Manual. I even printed out a copy to have on site during our build just to have it handy as a reference guide for myself.

      It has a wealth of information on how to properly detail all the elements of a roof, wall, and foundation. I repeatedly used it as a resource throughout our build.

      1. ERIC WHETZEL | | #27

        Here's another photo.

        This one showing the first couple pieces of flashing as we experiment with the Joint and Seam and Tescon Vana, along with trying to decide how far down the face of the Rockwool we should go.

  13. paula_builds | | #28

    The photos really help! Did you stucco right onto the rockwool, or did you use some sort of lathe? When you say stucco is expensive, are you talking about labor or materials? I have a friend who needs help with stucco so if I help him & learn then I could DIY that and potentially be ok cost-wise. I love the gravel around the house and the japanese feel it gives. I will definitely go that route. I'm in rainy Oregon but did live in Illinois for a while back in the 90's.

    That Hammer & Hand manual looks like a treasure trove. I'll dig in. Thank you so much for shsaring, and I hope you are enjoying your build. My number one goal with my project is to enjoy myself and not get too stressed! Doing fairly well on goal that most days :)

  14. ERIC WHETZEL | | #29

    My stucco guy installed lathe directly onto the Rockwool with flat head concrete screws that were embedded into the foundation through the 5" of insulation.

    Rockwool recommends a drainage plane behind the lathe, typically established with some kind of sheet material made for this, but since this Rockwool was on our foundation and wasn't likely to see a lot of moisture (there's a substantial roof overhang above), we opted to stucco directly to the Rockwool.

    When the stucco does get wet any moisture that reaches the Rockwool can freely drain down into the gravel bed inside the window wells.

    I was thinking in terms of labor as far as the cost associated with the stucco. If you can DIY the stucco, then it could save you quite a bit of money I would think. You might want to check out this YouTube video channel for more info:

    I assume he knows what he's talking about, but it can't hurt to ask a couple of local installers for tips on the best products/procedures used locally. They could point you to the best product suppliers as well, as opposed to depending on just local big box stores, which can be hit or miss in terms of the quality of trade specific supplies.

    We really like the border as well. Not only does it work decoratively, it helps keep water away from the top of the foundation area. We used construction grade 3/4" washed gravel for the border. It's not as flashy as some of the other more colorful stone options, but it's cheaper and we personally think it looks nice.

    As stressful as our build was, being on the job site everyday making progress was always exciting and rewarding.

    Good luck with your project! And don't be afraid to ask questions on GBA --- it's an invaluable resource. If I had it to do over, I definitely would've asked more questions as issues came up on our build.

    1. AJ__ | | #30

      Your build was great Eric, rare that a week goes by without thinking "I wonder how Eric did that".

  15. Expert Member
    Peter Engle | | #31

    You need to be careful about a stone border like that. Eric described leaving a "trench" that he then filled with stone. That's not right, as a "trench" would hold water against the foundation. You want the soil near the foundation to be as impermeable as possible, and graded away from the foundation so that water falling near the foundation flows away. Many published details for this recommend capping the backfill with clay or even placing plastic or rubber sheet goods on or near the surface to keep the backfill as dry as possible. Once you do this, you are welcome to add gravel to the surface for all of the reasons Eric describes, as well as to control weeds so you don't have to use a string trimmer against your foundation protection board.

    Yes, the Rockwool is self-draining, and Eric's foundation has a waterproofing layer and wide roof overhangs. That's all wonderful. But still, we don't want to make that system handle any more water than is absolutely necessary. By diverting water at the surface, the underground stuff lasts longer and performs better.

  16. paula_builds | | #32

    Thanks Peter. That makes sense too. Trench may describe the method of construction, but the end product should shed water, not hold water. I'll keep that in mind.

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