SIPs are dangerous when applied to tiny homes on wheels. Make sure they aren’t used in your project.
Every now and again we get asked a question:
Yes you can, but you probably shouldn’t.
Often these questions come from well meaning, actively involved clients who like to do their research and dig around. I love that. Often, they get a bum steer from some individuals in the industry who build this way because it suits them, and as the saying goes: To a man with a hammer, everything looks like a nail. More on that later.
SIPs is the abbreviation of Structural Insulated Panels. At their heart, they are off the shelf panel sandwiches that promise to combine the structure of a building with the insulation of a building wall. They come in many forms. In Australia typically there are two kinds of SIPs: OSB SIPs, and Metal or ‘cold room’ SIPs:
OSB SIP panel cutaway |
Metal SIP ‘Bondor Insulwall’ |
OSB SIP being installed in a Garden |
Metal SIP ‘Bondor Insulwall’ being installed |
The thinking goes: If the structure of a building is expensive because it takes a lot of time, and insulating a building is expensive because it takes a lot of time, let’s combine both in one pre-assembled part, and the building can be assembled like lego!
Except it’s not that simple.
While the advantages of SIPs can be strategically used in some instances in traditional construction, they also arrive with a set of serious downsides. Several of these downsides make them downright irresponsible to use in tiny homes on wheels, and as always, the devil is hidden in the details:
By their nature, SIPs panels have an inaccessible core filled with insulation. This is great for the insulation, but really bad if you want to run, say, power and water within this wall. The insulation in SIPs panels also forms part of the structure (more on that later) so you can’t just run around taking out all the insulation to run your power and water. You’ve got to be a little more careful than that.
Some people get around this by running power and water on the outside of the tiny home wall. I think this is a really bad idea, because then you’ve got power and water in the zone of the external cladding, which is far more likely to get damaged during construction or throughout the life of the building. To fix any damage, you would have a very arduous task ahead of you – dismantling the weathertight cladding so the burst pipe or punctured electrical wire can be repaired.
Some SIPs panels have channels designed within them, so that services can be run more easily as they panels are erected. The massive downside here is that you now need both your sparky and plumber to be with you as you assemble your SIPs. Any advantage in labour saving due to the speed of installation is now gone, as rather than needing 2-3 carpenters to assemble your frames, the roughing-in of services now has to be done in each panel, as each panel is erected. Talk about inefficient.
This isn’t to mention the fact that not all services can be run together. Wired internet connections, for example, need to be run away from regular electrical wires to avoid interference. If you try to run the internet, power, water and gas through the same tight spaces, I wish you luck. In reality nobody does this for tiny homes, and services are run on the outside of the wall. As we’ve discussed, this is a bad idea. This kind of ‘highly optimised for one thing at the expense of everything else’ will become a bit of a theme…
OSB boards can be engineered, and reliably strong. They can be pre-cut on CNC machines, or cut by hand, and can be strengthened with steel strapping, tie downs, and connections.
OSB SIPs panels being installed on a tiny home on wheels
OSB SIPs panels almost completed on a tiny home on wheels
The problem is that OSB Sips by themselves are not completed, and the planning and cutting of the panels does not come for free.
Once you’ve installed your OSB SIP panels, including running services through the walls as you go, you still need to externally clad your tiny home with a waterproof cladding, typically using colorbond steel. On the inside, small gaps in the corners, floor and ceiling need to be covered up with strips, by hand. The cold reality is that the OSB Sips are only replacing the steel frame, and the insulation. Both of these are inexpensive, when done properly. If your tiny home builder has strong design skills and can order bespoke steel frames, OSB SIP solutions are far more expensive.
If your tiny home builder is opting to use OSB SIP panels, it’s a good sign of two things:
If your tiny home builder has industrial CNC equipment, and strong design skills, it is far less expensive to order strong, bespoke frames for each project, and line them internally with whatever finishing board you like!
The bespoke steel frame for a tiny home. Structure exactly where you need it, and nowhere that you don’t! Great fabricators can easily create custom steel frames for each tiny home.
On top of all that, OSB SIPs are a lot heavier than their lightweight steel frame counterparts. This eats into our ever-present, ever-important weight budget!
But don’t steel frames have bad thermal bridging characteristics? We’ll discuss that below.
But what about Metal SIPs?
When considering the forces that a tiny home should be reasonably expected to resist during reasonable use, forces on the road must be considered. As a key design criteria, a reasonable yard stick could be a force equivalent to a declaration from 80km/hr to 0 within 2 seconds. To better visualise it, this scenario would have a stopping distance of 22m.
Making some calculations:
Great, now we have a design load to test the walls against!
According to the engineering design guide v5 for metal SIPs, when these SIPs are built as bracing walls (with detailing requirements I’ll get to in a moment) we get the following bracing capacities:
On the floor plan of a large tiny home, we might have the following set of continuous walls, without the interruption of windows or other openings:
Left Wall
Right Wall
All Shear Walls in Plan
Calculating the bracing capacity of each wall following the above table:
Total bracing capacity = 26.6kN
This is only 67% of the required capacity. This means that if this tiny home was built like this, and detailed as per the engineering drawings, it would fall apart if it experienced a deceleration from 80 km/hr to 0 in 2 seconds.
If you run the numbers backward, you’ll find that to prevent the tiny home from falling apart, you’d need to 33m of stopping distance.
It gets even worse when we consider cross winds, because the front and rear walls are so short, and full of nice big windows!
So you can see, metal SIP panels are structurally inadequate for most tiny homes. For small tiny homes, such as the Deua model, SIPs panels can work structurally, with some appropriate detailing and strengthening. Be sure to ask your tiny home builder what structural provisions are being built to ensure the safety and longevity of your SIP tiny home.
Some people ask about SIPs panels because they think that the insulation performance is better than a steel frame. This logic makes some sense at a surface level: The SIP literally has ‘insulation’ in its name, and we all know steel conducts heat really well! However, the devil is in the details, and when we take a close look at both systems, we find that the opposite is true.
Let’s compare the thermal bridges of the bottom wall details for metal SIPs and Steel Frames:
Metal SIPs Base Channel – bolt required every 600mm
Steel Frame Detail
Metal SIPs base channel detail, photo:
This thick steel plate will transmit a LOT of thermal energy, and become a point of condensation on both the inside and outside of the wall, depending on the season.
If we assume a 2mm thick plate (which seems like a low assumption to me), then we can compare the thermal bridging of the two details:
Or said in plain english, the base channel is over 20 times more thermally leaky than the cold rolled steel channel when detailed in this way. This is also assuming:
The effect of this is that in times of differential temperature and humidity, water will condense on the base channel 20 times more readily than on the cold rolled steel channel. The situation repeats for the top detail, but is not quite as bad here.
But, you might ask, the base channel and top cap are only in those spots, but the steel channel is everywhere! Isn’t this a problem?
Moisture problems are threshold problems. It doesn’t matter if it’s one spot, or the whole building. If you get water condensation, you will get mould, fungi, rot and other problems of decay that will impact your health and wellbeing.
This is an image of a steel frame, viewed from the inside:
This is an image of the parts of the frame that can conduct heat between the inside, and the outside:
That red square? That’s an equivalent thermal bridge if they were all combined together. It is a square 324mm on each side. When the frames and bridges are dispersed in a frame, the energy transmitted in any part is so low, that any risk of condensation is greatly reduced.
When SIPs are detailed as shown above, it is a very unbalanced solution that will result in condensation at these very hot/cold spots. We are not allowed to detail it any other way without doing our own engineering testing and certification. Also, this detailing method is fundamental to achieving the rapid speed of assembly and hence cheaper construction that everyone is so interested in, in the first place!
In traditional construction, these details can be dealt with because there is more physical space to apply insulation and membranes etc. With tiny homes on trailers, this is not the case.
Further, the SIPs panels have other detailing issues that affect lofts, windows, and other areas of concentrated loads. As per the design guide, each spot that has a concentrated load requires internal welded structural steel, which is at once expensive, thermally leaky, and heavy. In a typical tiny home design, this would mean that:
I began this structural and design investigation hoping, like many, to find a new and innovative way to build tiny homes faster, with better thermal properties, less expensively, than ever before. Instead what I found was a ‘solution’ that appears excellent on the surface, but like many architectural fads such as shipping container homes, turns out to arrive with a menagerie of fundamental problems that make such solutions far worse. For tiny homes on wheels, SIPs panels are at best more expensive, and at worst downright dangerous. For anyone pursuing a SIP panel solution for their tiny home, please make sure to ask your builder lots of tough questions about the structure – how it is being designed, and what details they are using to hold your tiny home together.
Until we have a tiny home design standard, it is up to the consumer to do their due diligence.
Be safe out there!