Hybrid Insulation for Metal Roof
I’m researching roof details for an insulated 8×12′ shed/workshop and came across this diagram from this site that I like: 
However, this article on hybrid roof assemblies calls for a minimum of R10 exterior insulation with R-49(!) interior insulation: https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-100-hybrid-assemblies#t01
Questions:
1. Can I halve the exterior insulation and interior insulation to keep the same ratio for condensation control?
2. Will this roof assembly be able to support solar panels?
3. Can I use caulk over the 5/8″ layer of sheathing to create the air barrier the detailing calls for?
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Replies
For a shed you don't need to meet code. For such a small space R49 doesn't make much sense. Depending on where you are located, something between R20 to R30 is more than good enough. If you reduce the R value, as long as you keep the ratio from the BSI article the roof will work great.
I would keep it simple, either insulate only on the inside (vented roof) or only on the outside(unvented roof). A hybrid assembly only makes sense when you have a lot of sqft to cover.
For something like a shed, I would insulate only on the outside. This way you don't even to finish the interior ceiling which saves time and cost plus makes the space feel taller. Roofing polyiso (sold at commercial roofing places) is much cheaper than rigid sold at the box stores and you can sometimes find it reclaimed for even less.
The air barrier should be the sheathing over the rafters. You can tape the seams of the OSB/CDX with a quality tape, much more reliable and less work than caulk. Make sure to figure out how to tie this to your wall sheathing to maintain air barrier continuity.
If you want to save a bit of cost you can skip the 2nd roof deck and install horizontal 1x4 strapping over the rigid and fasten the metal roof to that.
Solar adds very little extra dead load, for such a small structure I would not worry about it. Best is to go for a snap lock or seamed metal roof so you can use something like the S5 clips to install the panels.
Yeah that was my thought as well since it's just a shed / workshop. I'm going to go with the unvented roof assembly now. It's going to be a skillion roof so that should make things even simpler. I'm browsing local stores and classifieds to see if I can find low prices on recycled EPS / XPS / polyiso.
It looks like the images are suggesting house wrap for the air barrier over the sheathing, so I might just go with Typar or something like that to be on the safe side. They also suggest roofing paper above 2nd roof deck. Would I skip that since I'm using metal roofing? Or is it still good to use it?
Horizontal strapping does sound cheaper. The S5 clips look fantastic as well. Lots of good thoughts to consider here :) Thanks!
I prefer taped seams on sheathing as the air barrier but a well detailed house wrap can work as well. Again make sure to figure out how to tie the roof house wrap into the wall air barrier. It is a small space, but still take some care, most house wrap is extremely slippery to walk on.
You always need an underlayment under a metal roof to deal with night time condensation on the back side. Felt is fine if under the strapping, synthetic underlayment is easier to work with. Thinking about it, if you need to buy a a full roll of synthetic underlayment, get one of the permeable ones and you can use it as house wrap for the whole structure.
Good point about the house wrap being slippery. I will keep in mind and be sure to use some safety set-ups if I go that route. When you mention felt or synthetic underlayment under the strapping, does that mean the synthetic underlayment would go directly on top of the top layer of rigid foam?
I'll see if I can find any permeable underlayment. The ones I'm seeing right now all have super low perm values.
Saying, "For a shed you don't need to meet code" isn't quite accurate. If you're required to follow the IRC, there are exceptions: https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency#IRC2018_Pt04_Ch11_SecN1102. But if the space is going to be heated, it doesn't matter how big it is or what you name it.
Michael, the shed will have a volume of 960 cubic feet. With a ductless mini-split the efficiency gains of extreme insulation don't seem worth it for that size, especially since my state's building code doesn't require permits for a 96 sq ft shed.
I still plan to build at 16" OC, use 2x8" rafters, proper sheathing and nailing patterns, etc.
These small accessory buildings have become a it of a regulatory mine-field around here. Our area has ended up full 0f non-conforming structures that can't easily be brought into compliance.
Although generally our building code doesn't apply to those under 10 sq m, (100 sq ft) that exemption relies on their use as storage buildings, not living space. Quite often they are used as, or later converted to, home offices or B&Bs, where all the same code requirements for the similar spaces in a house apply.
Local bylaws also often limit their location, height, and use.
While it may not make sense to insulate to R-49 initially, it's worth thinking through how that, and other code conformity, can easily be achieved if the use changes.
That makes sense about achieving code conformity if needed. I think if I go with approaches mentioned here where I insulate the exterior walls and exterior roof, I'll have room on the interior to insulate the wall cavities and rafter cavities if I need to bring things up to code.
I would probably use 2" or 2.5" (R13 or R16) of polyiso on the walls. I'd probably go with 3-4 inches of (R19 to R26 or so) on the roof. With such a small structure, the overall energy loss won't be all that much since you just don't have much total area to "lose" through, and you won't be using much energy to condition such a small space to begin with. It will be a lot easier to use the thinner material, especially on the roof. I would use no interior insulation at all, although you could use some if you maintain the same inside:outside ratio. It is the RATIO that is important for moisture control, not the absolute R value.
Regarding solar panels, that's mostly determined by the structure, which isn't really detailed in your drawing. Maybe go up a size or two of rafter from what the tables show you need for snow load to allow some extra strength to support the weight of the solar panels. The only part of the sepecific insulation stackup that would concern me at all for the solar panels would be the shear strength of the long fasteners. I would probably built a few hard points into the roof (rigid details going from a rafter or two all the way through the rigid foam to provide an exposed attachement point on top of the roof), then use those to support the solar panel frame assembly. I'd use either strut (steel channel) for the solar panel frame, or a purpose-made system that will work with your solar panels.
You could also tape the seams of your rigid foam as your air barrier, then caulk the perimeter of the roof. You can detail the sheathing airtight too. I'd probably do it one of those two ways rather than add the additional WRB layer to use only as an air barrier here.
Remember also that these sheds are often only ocassionally occupied, so you can use a setback thermostat to let them run extra cool in the winter (or extra hot in the summer), as an additional way to save energy regardless of how you end up insulating the place. There is no need to fully condition the space if you're not there, aside from keeping things inside from freezing or getting so hot that they get messed up (think melted chocolate bars, short-lived cans of paint, that sort of thing). If you let it drop down to 50F in the winter when you're not there, for example, that's a pretty good energy savings over keeping it at your usual "I'm home" temperature when you're not there.
Bill
Bill
If I use no interior insulation, the ratio becomes irrelevant right? I like the idea of no interior insulation to add more 'space' on the interior, especially since it's such a small space to begin with. Although adding 2.5" to each side will increase the building footprint, which I'll have to take into account in my site plans.
I like the idea of rigid details from rafter through the roof. Would something like a 2x4 be what I want for those?
If I tape both the sheathing and the rigid foam (for extra peace of mind in case I missed spots with taping the sheathing), would I want to use house wrap or roofing underlayment over top of the top layer of foam?
A setback thermostat sounds perfect. I appreciate all of the advice :)