Vapor retarders and plastered straw walls in zone 6 – GreenBuildingAdvisor

Hello,

I have just read the most recent Q&A spotlight article regarding a wall assembly in zone 6, midcoast maine. I am designing my own house in the same area and have come up with a similar assembly I would like to use for similar reasons. However, mine varies in one crucial respect. 

My assembly is as follows, ext to int: 

– sawn wood siding (horizontal or vertical TBD)

– 2x or double cross hatched 1x rainscreen depending on above

– timberboard. R value TBD, depending on concerns expressed below

– mento 1000 wrb (taped and detailed to be air barrier)

– diagonal pine board sheathing

– 18″ double stud wall with strawbale infill for R28

– ~2″ lime plaster interior finish applied directly to bales

The concern aroused by the Q&A spotlight was Michael Maines noting that a class 1 or 2 vapor retarder is required in zone 6 as of 2021 IRC, which Maine has adopted.

This presents some issues for me:

– 2″ Lime plaster is something like 9 or 10 perms, so that is not a vapor retarder.

– For the poster of the original Q&A, their solution seems to be to add a membrane behind their blueboard. I want to avoid products like blueboard and prefer a thick multi coat plaster system for thermal and moisture buffering properties, and want to use the bales as the plaster key, as in traditional straw bale construction. (I do professional plaster work so cost does not factor here.)

– Furthermore, class I/II vapor retarders are prohibited by the strawbale appendix of the IRC. The logic of those authors posits that a completely vapor open assembly is best for straw walls. I agree with this logic, even in zone 6.

Two questions for this forum: first regarding the building science principles at play here, then the code.

1. My thinking was that I would pursue as vapor open an assembly as I could. With a carefully detailed continuous membrane exterior air and moisture barrier wrapping the whole house, a likewise carefully detailed interior plaster layer with connections to an air barrier / vapor retarder of some kind at the ceiling and any non straw sections of wall, and a mechanical ventilation system, my intuition is that I don’t need an interior vapor retarder over the straw. According to straw bale traditionalists, I shouldn’t have to worry about interstitial cavity condensation, as strawbales have tremendous hydroscopic and wicking properties compared to fluffy insulation products. I just need to avoid condensation on the sheathing. In my assembly, the 1x rough sawn pine boards are the most vapor closed material within the wall, around 10 perms I’m estimating. I aimed to mitigate that condensation point by adding the timberboards (also getting me to a reasonable R value in zone 6). I am thinking an R value of 12 (3.5″ thickness), that being a 70/30 ratio and a total r value of R40 with no thermal bridging.

However, I am not completely confident in this logic and don’t have the tools or experience to remedy my doubt with data. What does this forum think? Will this assembly have moisture issues? Should I / do I need to add a vapor retarder behind the plaster in this assembly? Do I have enough exterior insulation to prevent condensation? I have read in other GBA and BSC articles that 65/35 is recommended, but that is assuming less permeable sheathing.

2. Meanwhile, there is the question of code compliance. I imagine my local code official will not know what to make of the contradiction between the vapor retarder requirement in conventionally insulated walls and the prohibition of vapor retarders in straw walls. Any tips how to approach this issue if I decide to proceed with the assembly described above? 

Thank you for reading.

– Tyler