Ideal wall assembly for unconditioned shed
Most GBA discussions focus on conditioned buildings, but I’m curious about the performance of unconditioned structures.
AKA: What’s the cheapest way to build a 100-year bike shed? = )
Say we’re in climate zone 4c: lots of rain, but only below freezing for a few months.
Since the shed isn’t heated, the sheathing will always be cold.
Cold OSB can be trouble (https://www.greenbuildingadvisor.com/blogs/dept/musings/how-risky-cold-osb-wall-sheathing), so maybe we should go with plywood?
But if it’s raised off the ground via piers and no one is cooking/showering, there isn’t any vapor being generated inside.
Any wetness must be coming in via air from the outside — so lets tape the joints to make it an airtight box.
Now does the sheathing matter?
What about insulation?
Temperature swings means relative humidity swings, so to prevent our unpainted steel bikes from rusting, we should insulate the shed.
If we cover with foam or use a vapor-closed WRB, the shed will never be able to dry — better hope the roof never leaks.
So lets stick with tar paper, Tyvek, or a fancy vapor-open, fully-adhered WRB.
For vapor-open insulation, with a fixed budget should we go for less on the outside or more on the inside?
E.g., at $1/sqft we can do either 1.5″ Roxul ComfortBoard on the outside (R-6) or 5.5″ Roxul ComfortBatt on the inside (R-23 nominal, R-19 w/ thermal bridging).
For my stackup, I’d do (inside to outside):
No interior finish — studs exposed on the interior
2×6 @ 24″ OC with ComfortBatts
1/2″ plywood sheathing, joints taped
cheap, vapor-open tar paper or Tyvek WRB
1×4 furring strips
corrugated through-fastened metal siding
I’d do this for both the walls and a simple 3:12 shed roof (perhaps using 2x4s as the “furring strips” on the roof and overhanging 2′ on each direction.
That said, I’d love to hear how ya’ll would build a 100-year bike shed.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part