Ratios for Mineral Wool Exterior Insulation
I am looking to utilize a 2×6 or 2×8 stud wall assembly and a 2×12 rafter assembly for a detached 1.5 story shop/garage. Advantech or plywood sheathing, liquid applied waterproofing/air seal membrane from prosoco or henry, 3/4″ rainscreen and either a manufactured wood or fiber cement cladding. Interior would be gypsum wall board.
Its being built as an unheated garage first due to budget with plans to moderately heat and cool the main garage space (heat to 55-60f winter time, cool to 78-80f summer time). The upstairs loft will eventually be an office space for both my wife and myself. It would be nice to forgo any exterior insulation to allow drying in and siding to proceed quicker but adding moderate amounts that wouldn’t complicate conventional siding details would be a consideration.
It doesn’t need to meet energy codes as an accessory structure in my area. I am located in ZONE 4A.
I’ve read through many articles on here recently and through out the last few years as a hobby of sorts. I am a commercial electrician by trade, so all aspects of construction are interesting to me.
I understand the external/internal insulation ratio is to prevent condensation from forming on the sheathing. In a deep wall assembly, I assume this is already occurring infrequently already without any external insulation.
I cannot afford to insulate the exterior with a rigid mineral board in thicknesses that would be required for Zone 4a. However I would like to utilize some external insulation as it would bring the entire wall assembly up and help thermal bridging.
Is adding 1″ or 1.5″ of roxul comfortboard or thermafiber hc product dangerous on a r-23 or r-30 cavity filled wall?
I was planning on utilizing air tight drywall details due to costs and simplicity of the building structure (big empty rectangle).
All shop outlets, switches, panels, equipment and lights will be surface mounted with wire ran in conduit to eliminate almost all exterior wall drywall disturbance.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
sayn3ver,
There is no minimum. If the wall assembly would be safe without exterior insulation, any amount of mineral wool simply makes it safer. It's different with foam, where you have to stick to the ratios, due to foam's impermeability.
Thanks. I've seen conflicting mention of mineral wool board lumped in with with foam regarding that.
sayn3ver,
Mineral wool boards are around the same permeability as house-wrap, so adding them doesn't impede drying to the outside.
The only caveat is that a thin layer of mineral wool may not cure a wall that is having problems with moisture in the sheathing. A thin layer of mineral wool won't move the dew point outside the sheathing the way sticking to the ratios will, but if the wall is safe without exterior insulation, adding mineral wool of any thickness makes it more so.
If I understand this correctly I can add as much mineral wool (ComfortBoard 80) to the exterior in climate zone 5/6 as I want regardless of how much insulation I have between the studs?
Mixed_Beans,
Yes but with several more caveats.
- The mineral wool will allow the wall to dry to the outside in a way that exterior foam won't. Walls that allow drying that way benefit from having a more permeable sheathing, so it's safer to choose fiberboard or plywood over Zip.
- Walls with permeable exterior insulation can be susceptible to inward solar vapour drive, so it's also a good idea to include a rain-screen gap.
https://www.greenbuildingadvisor.com/article/installing-mineral-wool-insulation-over-exterior-wall-sheathing
Hmmm. I'm not sure I understand this statement.
"Walls that allow drying that way benefit from having a more permeable sheathing, so I would choose fiberboard or plywood over Zip."
I've never seen an assembly, that uses exterior mineral wool, have an inward sheathing that is not ZIP or covered with a WRB. Are you talking about an assembly similar to this video where they are using plywood covered by a vapor permeable barrier inward of the mineral wool? https://www.youtube.com/watch?v=p_oWGZq5bc8
Speaking of that video, they use plywood outward of the mineral wool too sandwiching it in between. I'm not sure why
Mixed_Beans,
The explanation is in the link I provided under the sub-heading "Damp sheathing and drying to the exterior". It's the low permeance of Zip that could cause a problem, WRBs are almost all very high perm.
Ok. Still trying to wrap my head around this. If I understand this right the concern is that if the interior of the wall gets wet it can't dry to the outside with ZIP correct? What if the home is being built air-tight? With sheet rock, and dense packed cellulose in the wall cavity and enough mineral wool on the exterior (plus rainscreen) wouldn't that make the low permeability of the ZIP a moot point? In that scenario any water that got into the wall cavity dries to the inside and any water on the outboard side of the ZIP dries to the outside. I think.
Mixed_Beans,
See my post #3 above. It's not that with ZIP it can't dry to the outside, it's that if you want to rely on that, some types of sheathing help more than others. The permeability isn't a moot point, but in most assemblies isn't that important. That's why I added the caveats. Walls with different ratios of mineral wool to cavity insulation rely on the drying capacity of the sheathing to different extents.
> It doesn’t need to meet energy codes as an accessory structure in my area. I am located in ZONE 4A.
I recently posted a question related to this (https://www.greenbuildingadvisor.com/question/irc-energy-requirements-applicability-to-accessory-structures).
My further reading of the code seemed to suggest that it does apply to an accessory building if it's heated. Did your conclusion about not needing to meet energy codes come from a code or other legal reference, or was it the building department? If the latter, I'm hoping my building department will feel similarly. :-)
Now, as to the manner at hand... given that the bulk of the space will be only "modestly" conditioned, I would consider running some energy modeling (e.g. BEOpt) to see what the payback is on the additional insulation. Everything will just be more work to detail and it will take longer to build.
Since you're already considering 2x8 studs, 24" stud spacing and R-30 cavity insulation gets you to a similar overall wall performance level of current CZ4A code (R-5 continuous and R-20 cavity).