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.
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Replies
Insulation doesn't dehumidify - so I didn't follow you there. Added heat will. Or a dehumidifier.
Jon, my thinking is that insulation will reduce temperature swings, which reduces the time that the bikes are below the dew point.
(There was an earlier discussion that touched on this: https://www.greenbuildingadvisor.com/community/forum/general-questions/97031/woodshop-super-ventilation-any-reason-air-seal-insulate).
That said, I'm not 100% convinced either way, hence the question = )
Perhaps it'd be more effective to divert the entire insulation budget into the air-sealing budget?
Or is it impossible to prevent rust without active heating / dehumidification?
Kevin,
I think that it's impossible to prevent rust completely. The risk occurs when cold weather is followed by warm humid weather in the spring. The cold weather makes metal cold. When warm humid air contacts cold metal, condensation forms.
Insulating the shell helps, but it doesn't eliminate the chance of rust. If you add a south-facing fixed window, your shed will stay a little dryer, on average, than if your shed doesn't have a south-facing window.
When you have a cold interior/bike, warm air outside and a well insulated building, you are only extending the time that the bike stays cold/damp. Even on a daily basis.
I have found that a very air and water vapor tight building plus dehumidification is very effective at preventing rust - and it costs little to run.
If I didn't have power, I'd add a Trombe wall, smart controls, insulation and thermal mass to maximize heat gain and then retain it. Or maybe just add PV solar, a dehumidifier and some re-chargeable desiccant (eg, bentonite clay).
Martin, good point about the window --- free conditioning from the sun.
Jon, it sounds like a few layers of foam insulation with staggered, taped seams would work for vapor tightness and insulation.
But then the sheathing would only be able to dry to the inside.
So if any moisture gets inside (roof/window leak), you're relying on the dehumidifier/desiccant to keep the structure going until someone notices.
It's not obvious to me that approach will be more reliable than an vapor open (but airtight) wall assembly that can dry to the outside.