A Tight Box is a Happy Box :)

29 Sep

I know what you’re thinking… Ted! Stop it! This is a family show! 🙂

Readers of this blog know that this week, I started a series of posts on ISBU insulation.

It’s the hardest part of ISBU construction, to hear people tell it. Who tells it that way?

Um… well… people who are either “idiots”, people who have never done it, or people who are insulation professionals who want an unhealthy chunk of your building money, to send their kids to college or pay for that second honeymoon…

Okay, okay, maybe it’s not that bad… 🙂  but from the email I get, you wouldn’t believe the things that are happening out there.

As I wrote the first post, I knew in my gut that I was going to immediately get hammered by readers. And, I was right. By Monday night, I had 27 emails in my Inbox, all asking for “insulation help”.

I’ve found that using reader mail to address ISBU issues has been pretty productive, so I’m going to take one of the letters I got and share it with all of you, so you can see how this works.

Brad’s mission is to build a (2) container ISBU home, using a pair of 40′ High Cube containers as the shell. He’s going to do this in the Houston area of Texas. When you read his letter, you’ll see just how much thought he’s put into this and you can see that he’s intent on achieving his goals.

Let’s see if we can help him.

Here goes:

Brad says;

Hi Ronin,

Thanks for your response about insulation.  Since I’m new to your site I suppose I missed previous comments about the subject. (Editors note: READ the ARCHIVES!) 🙂  I appreciate the input and above all your belief that this is not rocket-science and can be done inexpensively.

Just yesterday I was looking into closed-cell foam and rigid board as options and then recieved your email backing up that idea.  I want to insulate the exterior.  Have you seen furring strips attached, then foam sprayed between strips and then siding attached to the furing strips.  My question here is how to attach furring strips to the metal container without penetrating the metal wall.  The interior wall I would like to remain corrugated metal and just paint, as it is.  How difficult is the metal to penetrate with screws and drills suited for the job?

You asked about my particular design so I’ll describe it for you.  (I have a degree in architecture, btw, and practiced residential + hospitality design 25 years.)  The shell is simply 2-40ft high cubes (9’6″high) with the 40 sides of each unit joined side by side to form a rectangle (16×40).  Currently I have two-8′ sliding glass doors joined and centered on the front 40′ wall.  Then within each of the 4 corners (left to right) is kitchen, office, bath/closet/storage and bedroom.  The plan is open with no doors but with 21 LF of interior partion (most of this partition forming closet and closing off bath area).  2 windows, one kitchen + one office.  The 2 doors at the end of each container would be in the bath (shower/toilet) area and bedroom area.  If money held up (did I mention 30-50k max) I would intall sliding glass doors just within clearance inside of the container metal doors.  Allowing what you have already seen done before, the metal doors opened and exposing the sliding glass door for light and air.  Basically the center area of the 2 units joined would be the living/work/open area.  This area is currently 15’8″ x 12’8″ which flows without walls into ajoining spaces excluding the bath/storage area which is entered around the corner partition from the bedroom space.  Confused?  More than you wanted right?  So the plan is 640 sq.ft. with about 950 sq.ft. of exterior wall to insulate.

I want a roof garden to do the insulating on the top of the units.  A feature in Dwell mag had a unit in San Antonio with a beautiful roof garden.  Do you know if 12″+ of soil will insulate the roof?  The Ford motor plant in Detroit uses a green roof for cooling and a variety of sedum grows there.

Sorry for the rambling I just had coffee.

thanks for your help, Ronin.


Hi Brad,

First, welcome to the fray.

Second, start hitting the archives. There’s a ton of stuff in there.

Third, you’re on the right track. Regular readers of the blog know that I ALWAYS insulate on the EXTERIOR of the ISBU when given that option. Sure, some people like the industrial look and want a “Mad Max” front yard, but most don’t. 😉

And, that insulation of choice is SPF, every time.

That leaves the trick;

How do you attach the insulation and the siding to an ISBU, to form your finished exterior?

The idea of drilling and then attaching fasteners, hence puncturing the sanctity of that weather resistant Corten Steel shell is crazy!

What is SPF? GLUE.

Your furring strips are made of WOOD, right?

How do you attach wood to metal? You can use fasteners, and make a mess of that waterproof box, or you can use construction adhesive to glue your furring strips into place. Sounds crazy, right?

Here’s what I often do;

I just fab a long steel “L” out of lightweight sheet steel (think gutter or flashing), that runs along the top and bottom of the container. Using light enough steel, you can literally do this using a “brake” made out of dimensional lumber. Seriously.  Just weld it into place, like a bizarre flashing. Top faces down, bottom faces up. Are you seeing it?

Now cut your furring strips to length so that they fit inside that “tray” up against the container corrugation. That added metal lip is just going to help hold everything in place.

Apply glue to the furring strip, and then stick it where it needs to be located. The “siding flashing” will help hold it in place. Once your furring strips are “glued” into position, drill your holes and run a fastener (screw) into each furring strip thru the flashing and into the furring strip.

Congrats! Your furring is now “glued and screwed”… with “ZERO Penetration” to the box.

NOW… insulate using SPF. SPF applied between the furring strips is going to glue the entire thing together in one huge monolithic mass. Think about this. The foam is going to expand to fill the cavity between the furring strips and the corrugated container, in effect gluing everything together. It’s what SPF does! The end result is going to be a vapor barrier, a moisture barrier,  an insect retarder, insulation, and even some structural assistance to help your home deal with racking and shearing.

Apply siding. Predrill, and use SCREWS! If you work fast, that insulation will even help GLUE the siding to the house…

Trust me, once the SPF sets up, that entire wall section isn’t going ANYWHERE. I’ve come back after the fact to add more containers (7 years later) and getting that “siding mass” off the house and out of the way was an ORDEAL. We inspected the removed panels for “insulation or furring strip failure”. Zip. Stuck like Chuck.

Would your typical Planning and Zoning Nazi like this? Nope. They’ll have kittens. But, the insulation and siding that you install in this fashion will be a part of that house until the houses demise.

Sounds simple, right? Too simple, maybe?  Build a dummy panel in your garage, and then test it, if you don’t believe me. Try to pull it apart in 24-48 hours after the SPF has set up. You’ll see what I mean.

Now, let’s address that  “Green Roof”:

First, how are you reinforcing your roof, to accommodate that “green roof” dirt load? And better still, what are you going to use as insulation, below it?

The corrugated steel roofing isn’t going to support the weight of a realistic Green Roof. That roof isn’t even “structural” in the traditional sense. It’s simply there to keep water out of the cargo. Any ideas about building “Green Roof Surfaces” has to include a structural support system.

The R-value of dirt always varies greatly depending on its composition, and how high the moisture content is.

We all know that in the construction world,  the term “R value” is used as the reciprocal of thermal conductivity…

A pile of dirt equals “thermal mass”, right?  Big pile, big mass.

But it gets tricky. “Thermal Mass” isn’t the same thing as “heat transfer resistance”. And, for those of us working in the biz, we figured out long ago that R-Value is just heat transfer resistance.

A check of the material tables will show you that (typically) R-value for earth is R0.25 per inch.

That means that the foot of dirt that you’re talking about isn’t going to go very far against the heat of Houston, Texas  Summers. You’re gonna get about R3, tops. And… the “thermal coefficients” aren’t “bankable”, they are affected by several  other factors, and the biggest killer is soil moisture content.

Granted, this can get pretty complicated and we’re not going to cover it here, suffice to say that soils are inherently “heterogeneous and nonlinear.” That’s a couple of three dollar words, huh? 😉

Translation: Get soil really wet, and it’s worth about zilch as “insulation”.

Really want that Green Roof?

Start with SIPs (Structural Insulated Panels) to get both your insulation and your roof support. Then top with a waterproof membrane system. Once that is applied, you can go as green as you want, without fear of a roof failure from wet soil due to that Corten Steel’s longterm exposure to moisture.

Third – I’ve sketched out your floorplan, from the notes you’ve left.

If I’m understanding it correctly, have you considered putting the bath and kitchen close enough to each other to save some plumbing costs? Say, back to back?  With the budget that you’re working with, every penny you save is going to count.

What are you doing for a foundation?  NOT an uninsulated slab, right? Readers here know that putting a container onto an uninsulated slab means that you’re just hemorrhaging energy directly into the ground. You’re insulating the bottom of the containers, right?

A tight, super-insulated box is a happy box.

I hope that I’ve given you some ideas, and some things to think about.

“Living in a box” means “thinking OUT of the Box”.

Some of the tasks that you’ll complete will be accomplished differently than in a “traditional” housing situation using conventional materials. But if you keep your wits about you, take your time, and exercise “care and caution”… soon, you’ll be living in a sturdy, affordable Corten Castle of your very own.

My Thanks to Brad, for sharing his project with us.

Keep us posted!



9 Responses to “A Tight Box is a Happy Box :)”

  1. Brad September 29, 2010 at 6:43 pm #


    Thanks for the reply. I will get to the archives for more juice. Promise.

    I like the idea of gluing the furring strips to the container exterior wall but if it will cause problems with the inspectors/city hall it doesn’t get me very far (at least working above the table). They like the seal of an engineer and especially on wierdo projects. I am familar enough with foam insulation to agree with you that it will indeed create a solid unit with whatever it sticks too. What dimensions of furring strips were you thinking?

    The roof garden structural support issue can’t be too out of line since most ISBU websites will state that the units can be stacked 8 or 9 high. Obviously there is adequate vertical support (at a minimum the 4 corners) of each unit. Simple to design from there. I’ll also look into the SIP and see what they offer.

    Your remarks about soil, R values, depth, moisture content….ALL helpful in considering this project scope.

    Yes Houston is Hot in the summer. Very hot and humid. However I’m not familar with an “insulated slab”. At least not a floor slab that is poured in place onto the earth. Please enlight me.


    • Renaissance Ronin September 30, 2010 at 7:40 am #


      Keep in mind that the strength that allows stacking an ISBU comes from “frame to frame” contact via those twistlocks. The weight load is NOT distributed over the entire surface skin.

      You’re talking about doing something ENTIRELY different. You’re talking about using the corrugated roof material to support a weight load. That corrugated material is essentially UNSUPPORTED itself. It’s just there to provide cover, not STRUCTURE.

      If you don’t believe me, crawl up on top of a container and then jump up and down. I guarantee you that after you have done that, you’ll understand the point I’m making.

      Applying those SIPs over the top will solve ALL your problems, trust me.

      IMHO – Insulating a container sidewall to R14 in Houston is just dumb. It gets HOT, HOT, HOT there. I wouldn’t go less than r21, and in that zone I usually opt for something closer to r25ish…

      The width of your furring strip will depend on the insulation depth required.

      Remember also that when buying SPF to do an entire home, you’re not going to go to Walmart or Home Depot for supplies. You’re going to order your foam components from a roofing supply company. This will dramatically reduce the “per square foot” application price.

      In our “little group” we actually have at least two SPF applicators floating around from job to job. Between them, they’ve insulated many, many containers. They’ve paid themselves off several times over.

      Stay tuned for a post on “insulated slabs”. I think it’s time we revisited “ground physics and conductivity”.

      Hope this helps. Keep us posted!


    • Brad September 30, 2010 at 10:18 am #

      Yes, thank you.
      I have realized from the start that the corrigated roof will not support a roof garden. I have years of construction experience, hands on, not only at a computer or desk. My intention was NEVER to use the corrigated roof for supporting the roof garden. My lineage is 5 generations of structural engineers.

      Now, what I am technically unaware of is the structural dead/live load capabilities of SIPS per a design suited to work over the existing roof of a container. Is this appropriate direction for a roof garden? Perhaps. I can easily research this topic as an option.

      Also to clarify my statement, I said that R14 value for walls would be a STARTING point depending on CLIMATE/LOCATION/ENVIRONMENT. Again I agree that R14 for Houston is entirely too low.

      I completed a 2500 sq.ft. home in FL 2 years ago for a client and we used a SPF on the underside of the roof rafters in a huge walk-in attic/storage space and it has worked beautifully in Florida. Client could not be happier with the reduced heat gain from the roof area. Its also an ICF house with 11 inch thick walls. Helps.

      I suppose I just wanted you to know I’m not a fool. Structural systems are familar to me.

      Thanks for your help.

      • Renaissance Ronin September 30, 2010 at 10:37 am #

        Hi Brad,

        Sorry if it sounded like I was “schooling you”. I have to keep telling myself that many other people will read these comments and they aren’t as “experienced” as you and I are, so I have to paint pretty exacting pictures sometimes so everyone knows what we’re talking about.

        (Otherwise, my email box explodes almost immediately afterward.) 😉

        So please accept my apology if you thought I was “talking down” to you. I understand your credentials.

        I’ve used SIPs many times to accomplish the task that you are targeting without mishap or failure.

        The boon here is that you not only have the ambition to build an ISBU home, you have the “expertise in the arch/engineering areas” that so many families lack.

        That is one huge blessing.

        And for what it’s worth, I never thought you a fool. 😉


  2. James September 29, 2010 at 9:07 pm #

    Hey Brad.

    Just a favor to ask that you DO keep this updated. I live just a jump, skip, and a hop away from you over here near the Woodlands. If you are able to get your project started, I would absolutely LOVE to help you out in any way, shape or form that I could.

    I have never seen an ISBU container home, and I want to learn the steps that go into assembling one.


  3. ted yrizarry September 30, 2010 at 5:27 am #

    Gee THANKS Ronin! LOL! I’ve been deployed almost a year and YOUR the one talking about tight boxes and getting “glued and screwed”! At least I blush at that! Hahaha!!
    So with an eye to the zoning nazis…What would there be to cause them concern? Lift? Wind shear?? What am I flying a plane here!?! Given the way you describe the application, you are essentually creating a giant SIP panel using the steel of the container as the inner and the siding as the outer skins. Other than the over kill of the furring strips (I don’t know if SIPs have these?) added…what else could they want?
    So maybe they look at it and think there is not enough (LOL) keeping the assembled mass attached to the container? Then they are idiots but we don’t need to dwell on the obvious. Maybe in order to placate them you could ad some tie ins along the way. Easiest way I can see doing this would be to weld the head of some bolts to the container, threaded side out. Once the furring strip is in place you could use a piece of angle to join the two together. The end of the welded on bolt goes through a hole in one face of the angle and the other face can be screwed into the furring strip.?. Sorry for the fantastic verbal description there…just trying to help. There is always bribary…a warm plate of home made cookies is always hard for an inspector to resist! Works like a charm when I’m getting inspected here! Haha!

    • Renaissance Ronin September 30, 2010 at 7:31 am #

      “Glued and Screwed?”

      What? I’m just talking about “construction techniques”… 😉

      I have no idea what you’re referring to. 🙂

      About furring attachment;

      P&Z Nazis will almost always require you to use fasteners to attach furring to your subsurface material.

      In this case, we’re talking about a weather-tight surface that has NO penetration. I just can’t see drilling and filling all those holes to attach something that is essentially going to be glued into one monolithic mas, once the SPF expands. It makes no sense, and after having done it many, many times with NO failures, I can’t see changing streams now. It’s fast, almost “laborless”, it works, and it’s “forever”.

      If you were really worried you could add additional strapping across the furring strips, but you’d need to be careful to insure that they didn’t get in the way of your actual siding application. Spacing would be everything. And with the wall surface areas that we’re talking about, it just seems like overkill.

      In the past, because we were dealing with “ma and pa novices” worried about everything including Armageddon, we experimented with using threaded rod welded to the corrugation to allow an additional attachment point for securing the furring strips. It was a LOT of work, and it took forever. Remember, that when you do that you have to also countersink the washer and bolt holding the assembly together. This required us to make our own furring strips out of cut down 2×4’s. It wasn’t cheap. And in the end, it didn’t really “feel” any different.

      I’m not even going to go into the “thermal bridging” nightmares it causes. Remember, if you do this, you have a piece of metal going all the way thru your wall system from inside to outside, creating a “conducting point” for heat and cold (ruining the sanctity and integrity of your wall). This means taking even more time to seal each and every countersunk bolt with more foam. Oy.

      I encourage you to think this thru. Draw that mental picture. Now that you’ve done that… are you gonna do it MY way, or that way?

      After considering it and evaluating it, we decided that we wouldn’t repeat the “nuts and bolts” process, unless we were dealing with VERY difficult siding or veneer circumstances. Would I do that if I knew we were going to hang stone or veneer off the exterior? Maybe. But siding? Nope. Nuh-uh.

      And bribing army inspectors with homemade cookies, when they’re stationed out “standing the wall” in the boonies might work, but here… they want brownies, at least… 🙂

  4. Curt Cs October 26, 2010 at 11:47 pm #

    Hi Ronin,

    What type of glue do you recommend for attaching the furring strips to the container?


    • Renaissance Ronin October 27, 2010 at 6:56 pm #

      Hi Curt,

      I’ve used everything from 2 part epoxys to the Heinz 57 of construction adhesives to adhere furring strips to corrugated steel, but you know what?

      While not a product endorsement, Liquid Nail (Hi Strength) works just fine. It’s easy to handle, it works fast, and although it’s rated with a 20 year “life” the furring strips, the adhesive, the metal (Corten Steel) that you’re bonding to, and the insulation all encapsulate to form one solid “slab”.

      According to the manufacturer – “LIQUID NAILS High Strength” is called LIQUID NAILS (SOLVENT) in some markets to differentiate it from the other Liquid Nails products.

      Like most Construction adhesives, remember that LIQUID NAILS High Strength shrinks slightly on curing.

      It’ll set up overnight. We usually use straps to “fix” the furring strips in place, to prevent movement while curing. The object is to NOT use any fasteners that puncture the Corten Corrugation.

      It’s really pretty low-tech. anyone can do it. And best of all, you can get it anywhere.

      Hope this helps.


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