The Skinny on SIPs

27 Jun

As rivers rise and workloads increase, I’m reaching into the mail bag again;

Recently, I was asked (again) about my choice of roof systems for an ISBU Home.

In fact, I get asked this question about 20 times a week.

It’s usually a two parter;

  1. Do I REALLY NEED a roof? and then…
  2. If so… what roof do you recommend?


Even ARCHITECTS ask me this question. There are those who feel like the weather-resistant corten steel ISBU roof is suitable.

They’re WRONG.

First, in spite of what some may say;

That pretty corrugated steel up over your head ISN’T structural. It’s just part of the envelope built to hold boxes and keep them from flying out. Walmart wants their TVs in one piece… so their own dock handlers can break them! 🙂

Not really… I’m sure Walmart frowns on ” TV Bowling”…  😉

Put a couple of big guys up there on your ISBU “roof” and then tell them to jump up and down. Standing anywhere but underneath them… watch what happens to that steel.  I promise you that you’ll quickly see what I mean. You’ll end up with an ISBU top that looks like it barely survived a rocket attack.

It’s for this same reason that YOU CANNOT BURY AN ISBU CONTAINER without extensive modifications. The corrugated steel alone just isn’t going to do the job. Trust me on this one.

Okay, now that we’ve established that… again… but I’m not bitter because it seems no one is listening to me… mumble, grumble, snort…

If I can build a roof any way that I want to… It’s going to be SIPs (Structural Insulated Panels) based every single time.

Yes, you can build an entire house out of them… but it’s kinda pricey!

A SIP is just an insulation sandwich, made of OSB (Oriented Strand Board) and EPS,  XPS or Polyurethane  foam.

EPS foam SIPs will get you about r3.8 per inch on average.

XPS foam SIPs will bump you up to about r5 per inch of thickness.

Polyurethane is the big winner here (and the most expensive, granted) with r values in the high 6’s per inch of panel thickness.

Whatever flavor you choose them in… smash them all together and you have the base for the perfect roof system. It’s light, it’s strong and it’s easy to work with.

And you can get SIPs to cover virtually any roof you need to build, whether you’re in the Mohave Desert or Manitoba…

It’s all about “R values”…

SIP R-Values

R-value is defined as a measure of the capacity of a material – such as insulation – to resist heat flow. More is better. Increasing values indicates a greater insulative capacity.

The higher the R-value of a material, the greater its insulating capacity.

SIPs R-values range from approx. R-16 to R-57…

… depending upon the type of foam core and its thickness. Most industrial R-value testing is conducted at a mean temperature of 75 degrees.

And with SIPs – what you get – is what you KEEP.

SIPs don’t degrade, decay or rot. The OSB used as a sheath on both sides of that wonderful foam core doesn’t de-laminate like conventional plywood.

Note: You CAN MELT them, however.

SIPs will start to get “gooey” at sustained temperatures over 165 degrees. So, it’s important to note that you need an air gap between your SIP panel and that SSMR. That metal is like a hot skillet… and that foam is the egg it’ll be frying if you’re not careful.

Design accordingly.

And because they are rigid by their very nature (unlike fiberglass batt insulation),  the foam core in a SIP doesn’t sag, shift, settle or compress, or otherwise compromise the integrity of the original R-value rating.

R-Values for SIPs

Thickness R-value
4-1/2″ 16
6-1/2″ 23
8-1/4″ 30
10-1/4″ 38
12-1/4″ 45

Polyurethane SIPs

Thickness R-value
4-1/2″ 26
5-1/2″ 33
6-1/2″ 40
Thickness R-value
4-1/2″ 19
6-1/2″ 29
8-1/4″ 37
10-1/4″ 47
12-1/4″ 57

Here’s where it gets good;

SIPs are basically “pop and drop”. Two guys can easily handle a 4×8 SIP panel. Even when dealing with really thick panels (like 12 inch ones), you’re averaging a little over a hundred thirty pounds per panel.

Want more SIP goodness?

Depending on how long your roof run is and how you use them… SIPs will support themselves without trusses or framing.  That saves materials and labor.

Top that with a waterproof membrane and a little air gap and then slap a Standing Seam Metal Roof on it… and you have a roof system that is rarin’ to go, even when the weather is rarin’ right down on you.

Are there other roof systems that will work?

Of course there are. However, if you want a system that’s easy to apply, really energy efficient, really low maintenance and achievable by your “idiot brother in law and his drunk friends”… this IS the way to go…

Or better still, hire a pro crew and let them do all the work. Contractors are starving for work right now. You may just find that they can accomplish your goals for less than you could. And, they’ll get the job done faster, safer and with less worry about falling off the roof and cracking your skull.

Until later…

Be safe.

8 Responses to “The Skinny on SIPs”

  1. Penny Nelson June 27, 2011 at 11:57 am #

    Great article – have been trying to explain to my other half why we need a SIP roof and he can’t wrap his brain around the structural part.

    But really why I’m posting is… can you send some of those contractors who are desperate for work up here to BC? We seem to have too few of ’em and those that we do have are so busy they treat all of us paying customers like dirt (aaarrgghh).

    Seriously – is there a way for your guys to work in Canada?? Not clued up on visas etc but know a great immigration attorney who also organises work permits.

    • Renaissance Ronin June 29, 2011 at 10:24 am #

      Um… would we have to learn to speak “Canadian?” 🙂

      That could be a deal-breaker…;)

  2. Alton June 27, 2011 at 10:11 pm #

    Does the use of a SIP roof system mean that you are not required to use insulation in the attic, of either a batt or blow in type? I’m thinking more in regular house construction than in ISBU’s specifically.

    Have a house with a flat roof, that I seriously want (need? lot’s of large trees, poor drainage) to replace with a trussed roof. But if SIP panels can do the job without insulating, they may make more sense all around.

    • Renaissance Ronin June 27, 2011 at 10:47 pm #

      Hi Alton,

      Depending on the thickness of the SIP panel, using SIPs means that your roof is fully insulated.

      Remember also that SIPs will support themselves without trusses, in carefully thought out designs. That helps save money to pay for ’em! 😉

      Hope this helps.

  3. Madrigorne June 28, 2011 at 6:06 am #

    That SIP looks like an ice cream sandwich!
    Okay now that I have that out of my system…how do you attach these puppies? Do you have to nail all the way through or can you nail into one layer of OSB and through the miracle of modern adhesion the sandwich stays sandwichy? What is the compression factor on these, are they rigid or squidgy? I am all curious!

    • Renaissance Ronin June 29, 2011 at 10:22 am #

      Yeah, a SIP does look like a frosty cold confection, doesn’t it? 😉

      Attaching SIPs is pretty simple.

      When attaching SIPs to ISBUs for roof systems:

      Note – This is the LOW TECH way:

      Apply a wood Sill Plate to the top of your ISBU to provide an attachment point.. I weld angle iron to the top of the ISBU to receive the sill plate. You can seal this seat using foam or sill tape under your sill plate. Anchor the sill plate to your ISBU (in the “sill seat” that you just created) using bolts. At every lumber to metal and lumber to lumber connection, you want to use sealant to keep things tight. You are building a superinsulated roof. Why punch holes in it? Duh! 🙂

      Screw thru the SIP down into the Sill Plate. Screw spacing and quantity requirements will be based on your required performance based on weather and seismic activity in your region.

      Apply hurricane straps as necessary to combat uplift in severe weather.

      Then, finish your roof system. This will include a waterproof membrane of some sort and then your roofing surface of choice.

      The drawings for this are in my new book – The Nuts and Bolts of ISBU Building.

      If it’s me, it’s probably going to be Standing Seam Metal Roofing in a reflective “color” built up just high enough to let air pass under it, cooling the SIP…

      My panels (Solar and Photovoltaic) will be mounted on stand-offs designed to use the SSMR SEAMS so that I don’t have to punch any holes in that perfectly sealed roof. This allows your panels to create a plenum under them, assisting the panels in staying cool (they work more efficiently that way) and helping to take the heat load off your roof.

      And unless you overheat a SIP (sustained temps over 165 degrees), a SIP doesn’t get “squishy”. 🙂

      Hope this helps.

  4. Alex Gerrits July 1, 2011 at 2:19 pm #

    I contacted a SIP factory near me and asked about green roofs. They said that I would need to support the SIP every 4′. How would you suggest doing the extra framing?

    Also, I’m sure you’re really busy but any idea when “The Nuts and Bolts of ISBU Buildings” will be available.


    • Renaissance Ronin July 1, 2011 at 3:45 pm #


      Green Roofs DO INDEED require additional support, as you have to carry not only roof and skin, you have to carry the load of the soil (remember that soil gets far heavier when you water it!), plant materials, terraces, knee walls, railings, deckings. ect…

      Depending on the size of the green roof, the easiest/strongest way is to create a collar using steel around the perimeters of your container top. WELD it directly to your container.

      This can be something as simple as using angle iron, depending on the thickness of your joist. I’ve used 3/8th” (thick) 4″ x 6″ angle to do this a few times with pretty good success.

      And before you ask, we even compensated for insulation thickness by applying SPF to the container top FIRST. DO ALL of your welding before you apply the foam.

      So you have (in layers) corrugated steel, SPF (closed cell), steel collar (made from angle iron), joist hangers and joist “system” (usually dimensional lumber), SIP, waterproof membrane and then your “Green Roof” materials.

      Yes, it’s tricky (there is a minor “thermal bridging” issue that results), but it’s a problem easily solved if you think it thru.

      Once your “steel collar” is in place, tack weld in your floor joist hangars and use dimensional lumber to create a deck seat. It’s pretty much like building a conventional floor, except that your joists are going to be much further apart.

      The size of the joists will be dictated by the requirements for your roof to carry the load you’re creating.

      NOW, apply your SIPs to the joists and glue/screw thru them down into your joists – to secure them. Observe the spacing requirements for your load. It’s pretty easy to compute.

      Don’t forget to add the hurricane straps! 🙂

      This will give you a secure SUPPORTED SIP roof that will carry your “Green Roof” load.

      You still have to deal with the exposed ends of your SIP panels and roofing, just like any other application.


      And barring any unforeseen events (it’s in final review now), we expect “Nuts and Bolts” to start shipping the end of July.

      Hope this helps.

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