Advanced Framing Method
Hello experts, I am looking for guidance around an advanced framing method with integral air barrier and siding. The principal is as follows: Framing is done with studs or posts ≥24″ OC, wood sheathing is eliminated entirely and replaced with 24″ OC horizontal framing members (girts) on the outside of the studs, and metal siding attached. SPUF is applied in the cavity directly on the backside of the painted metal siding, to 3″ depth. This framing method is used in commercial and agricultural construction in my area but I haven’t yet seen it widely discussed for residential applications. There is evidence on YouTube of people using these methods for housing but quality, thoughtfulness and code compliance vary widely. I haven’t seen this method analysed or mentioned in any Building Sciences Corporation publications or other leading edge publications. Can anyone direct me to resources where this method is discussed or documented?
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If you substitute "pole barn" for "Advanced framing method with integral air barrier and siding", you'll find many discussions and details. You've mentioned one component of the system. How it all works together, and the benefits/issues of the entire envelope require analysis. My neighbor had his metal walls spray foamed, but in order to use the space as desired, he built interior stud walls with additional insulation, which used as much material as a conventionally framed wall.
Mostly if I look for "pole barn" discussion on this site I don't find a lot of thorough or thoughtful discussion of specifics. I'm looking for analysis of what could be termed pole barn or post and beam framing, using metal siding as stressed diaphragm and SPUF as adhesive/air barrier/insulation/ vap.barrier
ep,
What you describe has a few downsides.
- Eliminating sheathing makes the structure much weaker structurally, especially in shear.
- Most building codes require a WRB in between whatever cladding you use and the insulation behind it.
- Many of the intersections in metal cladding (head-flashing, around windows and corners) are not entirely water-proof the way they are in other claddings, and rely on sealants and underlayments for their integrity. Without a solid substrate those are more difficult to include. You would be relying of the foam to perform that function.
- Metal cladding shrinks and expands, meaning that the foam may not function well as primary air barrier - especially over time.
- The 2"x girths mean the depth of foam will vary widely.
- While foam is effective in many situations, many consider it to be an unsuitable choice for new construction when other alternatives exist.
All that said - as you have seen, people do use pole barn framing as the basis for their structures. I built two 900 sf workshops last summer that way. It can be done successfully, it just demands a lot more thought and attention to the details.
You're discussing some of the concerns I have, namely how the system will act over time when water makes its way to the foam behind the metal. Eliminating the WRB membrane is a big deviation from current typical practice in building codes. Anecdotal evidence suggests this system performs well over a 20 year timeframe; I'm curious to find more detailed analysis of cases or reasons it might not perform well. 3lb SPUF is used as roofing material so in higher densities at least, it is proven to be up to the task as a water sealant. One weakness I could see is if water could accumulate in tiny areas between the metal and foam, perhaps it could push hard enough to damage the system over time during freeze-thaw cycles, as it does in concrete. But with the flexibility of foam being higher than that of concrete, maybe this won't be an issue. I haven't found evidence yet that this actually causes problems in the real world.
Metal shrinking and expanding and causing the foam to debond was a concern of mine, but I have not yet found evidence of this happening to anybody in the real world.
In a 3" foam application, if the girts are 1.5" thick, the foam covering them will be only 1.5" thick over the girts. Still, the perpendicular layout means that nearly all thermal bridges (>96% coverage) are covered in R10 foam thereby essentially replicating the thermal performance of exterior XPS sheeting over a stud wall.
With regards to losing shear strength without sheeting, I believe the system I'm describing is actually much stiffer than a sheeted stud wall. The foam glues all members together including the metal which takes the place of OSB as the structural diaphragm member. My own tests and observations have shown this to be true. I'm looking for evidence to the contrary I just haven't found any yet. The only published test I've found so far is here: https://www.sprayfoam.com/content/spf-strengthens-buildings/35
where in their test a wall built with a drywall diaphragm and spray foamed is much stronger in shear than a wall built with an OSB diaphragm and no foam.
The only place I have seen this type of system in place for a long time is in apple storage facilities. The ones I have been in are 40 ft.+/- high and have spray foam applied directly to everything. The foam is relied on as insulation and as an air barrier as these warehouses are sealed with cooler doors and filled with nitrogen and other gases to retard spoilage. These tall buildings flex quite a bit, and when the foam develops cracks, foam can be sprayed in to plug up the gaps.
The other application I've used is foamed double-skin panels. I don't think there's any delamination or moisture problems with them.
So I believe there's no real issue with the system, I just question the value over other systems. The main advantage of post-framed buildings is the ability to be very tall and enclose a lot of volume for cheap. BTW, those apple storage buildings were in violation of code; all that exposed foam was a fire waiting to happen. Good thing it was full of nitrogen 9 months a year.
ep,
Just to be clear: I'm not dismissing this as an idea - just pointing out what I think are the issues that need addressing. I have a small recording studio to design and build this spring and may use a similar wall design.
What is the advantage over conventional framing that y0u see this method offers? For my project it is simply about reducing the cost of exterior sheathing on the walls and roof.
Edit: I'm going move to your other thread to keep the discussion in one place.
In this area of Maine the Amish community builds many of their structures in a similar way.
One exception being the addition of a WRB between the girts and metal siding on the vertical walls and a roof membrane under the siding on the roofs.
Yes, here the Mennonites and Hutterites and other agricultural people use this framing often. The sprayfoam is not their preferred method, but I see synergies that could make it a good value.
You will be interested in tomorrow's post from Randy Williams. He has experience constructing post-and-frame structures, which are most often seen in agricultural contexts. They lack sheathing, rely on horizontal girts, and are often clad in corrugated metal. Check out the post tomorrow and share your question in that thread. Randy is a great resource for this subject, and he will be writing at length about how to make such structures energy efficient, comfortable, and durable--with an emphasis on the four control layers.
Stellar