Insulation ?

[CxHxExVxY]

Looking to insulate and finish the inside of my shop. It has the bubble wrap style insulation (Astro armor) under the tin.

It will be studded out 16in oc and going to use r21 batts

So should I plan to use unfaced batts due to the possibility of faced batts causing a double vapor barrier or am I over thinking this and it doesn’t matter

Thanks

 

[jack stand]

I would say yes. (un faced)

 

[C-S-H]

You did not say where you are located. But since you have a vapor barrier on the outside of the wall assembly, you need some combination of impermeable board insulation and batt insulation (if any) to prevent condensation. Don't know if the bubble wrap has good enough R value to serve the same roll as the board insulation (XPS, ISO). Then the building will need humidity control (under 60% RH) since it cannot breathe, unless you design the ceiling/roof assembly for that function.

 

[C-S-H]

And the kraft paper on the batt insulation is not a vapor barrier. It is vapor semi-permeable to permeable depending on the relative humidity. It is used to throttle the vapor transport rate into a breathing wall assembly, and other uses.

 

[AC-WC]

OK, not intentionally stirring the pot but I'll throw a wrench into everything, I was taught you always have a vapor barrier on the heated side of the wall whether it's plastic sheeting, kraft paper insulation. I see where you're coming from being cautious of 'too much' barrier. My opinion I think with R21 you have enough distance to prevent vapor between the interior barrier and the bubble barrier not to cause an issue.
Owens doesn't say anything about too much barrier.

https://www2.owenscorning.com/literature/pdfs/10017858%20Install%20Guide%20Light%20Density%20Fiberglas%20Bldg%20Insul%20Tech%20Bulletin.pdf

 

[C-S-H]

I was taught you always have a vapor barrier on the heated side of the wall whether it's plastic sheeting

But that means a moldy death to walls in the South. If you partition the wall or floor or ceiling or roof assembly for vapor, then you must build-in prevention of condensation, or prevent it through other strategy like HVAC. The correct design depends on the climate. For structures like garages and cabins that aren't HVAC'ed 24/7/365 I am a big fan of breathing assemblies.

 

[billconner]

But that means a moldy death to walls in the South.

With the poly on the "heated" side, the exterior in the south?

 

[C-S-H]

With the poly on the "heated" side, the exterior in the south?

Poly is generally not used in wall and ceiling assemblies in zones 1-5. For locations with four seasons, these assemblies breathe all the way through. If a vapor barrier is used then you usually need exterior insulation (zones 1-8 ceilings, zones 4-8 walls) and some level of HVAC.

 

[u2slow]

The use of that bubble wrap under the tin has set you up for cooling-primary.

I dont suggest you add a second VB, otherwise the wall assembly will trap any leak/condensation that may occur.

Normally use a vapour retarder opposite the VB; like kraft paper, tar-paper, or tyvek. Jmho, tyvek is also too tight to use with poly in a humid climate.

 

[Kaizen]

rockwool and no barrier. Then plywood or whatever.

 

[ericm]

With just bubble wrap and insulation under the R panel, where's the exterior air sealing like you'd get with sheathing?

 

[dcg9381]

where's the exterior air sealing like you'd get with sheathing?

Tyvex is commonly used here. It's actually water tight and vapor permeable (it doesn't seal air).
I'd think the problem with bubble wrap and R panel is going to be a "gap" in many places between the R panel and wrap. If you can "press" it in with R21 insulation, that might work, but I'd want to see another shop in your geography done the same way.

Steel building companies like to sell "blanket" insulation here, I guess that works too, my guess is it doesn't leave an air gap though.

I just spray open cell foam straight on the R panel. Works great. But for stick and brick home construction, we wrap the outside of the house with Tyvex and then use spray foam or traditional bats. I see people saying that this can't possibly work and "it'll void your warranty" but I call hogwash.. Any place where your insulation traps water is going to void your steel warranty and LOTS of shops are built this way here, many standing for 10-20 years without a problem.

My favorite insulation is hybrid. 1" of closed cell and then do traditional fiberglass/rockwool or whatever. Most cost effective way to do a tightly insulated structure IMHO.

OP, it appears that a LOT of this depends on geography (climate).