Wall Assembly Question
Emel
| Posted in General Questions on
We originally planned to use rockwool on the exterior and interior of our home. It is officially out of the price range. We live in northern WI. My question is, if we used plywood sheathing, dense packed cellulose as the insulation, a smart vapor barrier like Intello, and plaster walls, would we need the exterior insulation? We’re trying to avoid condensation in the walls that would lead to mold or moisture issues. There’s so many ways to build a wall, it’s left us a little confused on best practice but also one we can afford.
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What type of WRB would you have on the outside of the plywood sheathing? And would you paint the plaster, if so, would it just be water-based/latex paint?
The wrap was going to be Benjamin obdyke and Benjamin Obdyke Slicker Classic Rainscreen over this and then corrogated metal siding hung vertically with perforated drip caps above and below to allow air flow. I cannot remember the name of the paint but it was a paint meant to be used with plaster that doesn’t seal like latex, I think it’s meant to be more breathable. I want to say it’s ECOS brand.
Moisture will still accumulate in the sheathing if you don't have enough exterior insulation to control the dew point, but the vast majority of homes in the US are in that situation and perform just fine.
One key is to limit how much moisture gets into the wall; in cold climates, a variable permeance interior membrane does a good job of that.
Another is to allow the wall to dry readily to the exterior; an impervious cladding such as metal and a minimal rain screen such as Slicker Classic isn't really enough. I recommend a deeper rain screen vented at top and bottom to ensure that any moisture gets evacuated quickly.
Finally, cellulose is a good option because it can safely store a bit of moisture and its borate content protects anything it touches, including the sheathing and framing. Other types of insulation leave the framing and sheathing as the most-absorbent part of the assembly and don't protect them from microbial action.
In other words, I would add a variable permeance membrane and a deeper rain screen.
Thank you Michael! We were told because we chose vertical corrugated steel siding that this would technically count as a rain screen and would not need furring strips. We still added the other two products because I didn’t like the idea of part of the metal directly touching the sheathing. With this knowledge of vertical corrugated siding, the other two wraps and the vented top and bottom, do you still think the extra rainscreen strips would be necessary? We do have it in the plans to use intello smart membrane as the interior vapor barrier.
It's not situation with one clear answer. The corrugations in metal some amount of air movement, just like Slicker Classic provides some amount of air movement. My job is to design buildings that will remain safe and mold-free for decades, so I'm on the conservative side and prefer to have a generous amount of venting. If you skimp on the rain screen, there is a good chance that you will have no problems. But there is a greater chance of no problems with a deeper rain screen.
This makes sense. They are using solid pine board and batten instead of sheathing with interior horizontal strapping for added structural support. Would this also help with the drying as I think it would breath better that plywood sheathing?
Yes, board sheathing is typically more permeable than sheet goods, largely due to the gaps between boards. But the sheathing only helps the wall dry if the moisture has someplace to go, preferably dry air, which is why the rain screen gap is important.
Even a small gap will drain liquid water, whether from stormwater infiltration or condensation, but to get air flow you need at least 1/2" or so.
With vertical cladding, the best rain screen is cross-strapping with 1x3s or 1x4s. That provides a nailing surface for the cladding and a vertical drainage gap. If you don't want the cladding to be 1 1/2" from your sheathing, there are a few workarounds, including various plastic materials, but the simplest is probably installing the 1x3 strapping diagonally.
Thank you for your thoughts on this. We are requesting the thicker rainscreen to help drying. What are you thoughts if we did the same wall assembly… plaster, Intello membrane, 4x6 walls insulated with dense pack cellulose…. But what would be your thoughts if instead of doing the plywood sheathing, we used a zip sheathing product or one of their insulated products? My one concern about this would be the inability for air movement and drying. Am I wrong in thinking that? I know many builders here in central WI are now solely using zip.
“Would we need the exterior insulation?”
The first question is what is the local code min insulation requirement you must meet and do you have a goal in mind above the legal minimum?
Can you meet or exceed the min without external insulation in your walls?
Moisture accumulation is not an issue if have no external insulation. If you have any it must exceed 30% to avoid the problem.
If the walls will be 5.5 inches thick the cellulose will come in at R12 that seems a little low for your area.
Would you consider making the wall thinker to make room for more cheap insulation or are you going to buy more expensive insulation.
You might find this article interesting.
https://www.greenbuildingadvisor.com/article/how-to-design-a-wall
Walta
Thank you for the article link. For exterior insulation, the main hang up is that while rigid insulation around the exterior may be possible, it puts a vapor barrier on the exterior and in our region a vapor barrier on the interior is what’s required. Exterior rockwool is completely out of our budget so I’m not really sure how to remedy this. Possible options I can think of… 1. a double wall structure with no exterior insulation (although some say the cold sheathing would still cause moisture issues as well as the deeper walls), 2. a regular 2x6 wall structure using 1” zip r sheathing and using a vapor smart barrier on the interior side. But this may not be enough insulation and would still put the vapor barrier on the exterior.
Emil,
Properly designed double stud walls simply don't experience moisture problems in their sheathing.
You can safely design walls with small mounts of exterior insulation and interior vapour-barriers. This article talks about how: https://www.greenbuildingadvisor.com/article/rethinking-the-rules-on-minimum-foam-thickness