Building sciences are both fun and fraught with peril. I'm somewhat excited and cautiously optimistic about Boxabl from a housing manufacturing perspective.
OSB is a fine air barrier, and most water vapour volume happens through air leaks. Vapour diffusion tends to be a smaller percentage:
* https://www.youtube.com/watch?v=FXXgjvOJcYI
The main place that vapour concentration really becomes a problem is at the highest point of the house (e.g., ridge):
* https://www.buildingscience.com/documents/building-science-i...
The main point is to not have your condensing surface on the inside of your structure:
* https://www.buildingscience.com/documents/insights/bsi-001-t...
Which is why so many jurisdictions are encouraging / mandating external insulation. When the sheathing is the coldest surface, of course there's going to be condensation, but if it's the same temperature as the inside air how would moisture accumulate.
> If structures don't breathe they rot.
If structures don't dry they rot. There are plenty of of <1.0 ACH@50 structures that do not "breathe" that have no moisture/rot issues because they take care of water mechanically, e.g., ERV/HRV and (whole house) dehumidifiers.
In theory vapor accumulation is only really problematic in the overhead plenum spaces. In actual practice (setting aside acute water ingress) 100% of the time moisture problems start in the crawl space and then spread to the plenum.
Yes absolutely it is possible to design complex interlocking systems that at least in theory both provide high efficiency sealed construction and control moisture. In practice the complexity of these systems is sufficient that even top-end contractors frequently run into issues that lead to full blown abatement projects. There is also the minor issue of what happens if all of these systems aren't subjected to aggressive inspection regimes and/or a structure goes a significant amount of time unoccupied. The one thing all of these super high-efficiency sealed systems have in common is they quite literally tear themselves apart if neglected or the power gets turned off for any meaningful amount of time.
The statement "If structures don't dry they rot" is absolutely true, but only in the context of traditional lumber products that are comparatively resistant to rot in the first place and are capable of weathering swell/shrink cycles without falling apart. Highly engineered products degrade aggressively just by getting damp in the first place as swelling wood fibers break down bonds with the adhesive that's holding the material together.
I absolutely stand by my original statement that OSB shouldn't be code-compliant and with the exception of shitty flat-pack furniture has no business anywhere in a home.
> In theory vapor accumulation is only really problematic in the overhead plenum spaces. In actual practice (setting aside acute water ingress) 100% of the time moisture problems start in the crawl space and then spread to the plenum.
Which is a good argument for conditioned crawl spaces.
* https://www.buildingscience.com/documents/bareports/ba-0401-...
Shit is wack yo. Perfectly usable houses are remodeled all the time.
I hate OSB so much. And it no longer is that much cheaper than plywood, which still can die in water but at least doesn't die immediately.
* https://www.greenhomeguide.com/askapro/question/i-m-concerne...
* https://greenhomeguide.com/askapro/question/we-have-osb-that...
Unless you're using planks for siding and your flooring, you're going to have to use some kind of glued product product. Further your OSB is generally behind your drywall (sheet rock, gypsum) which can act as an air barrier.
Ideally you should have an HRV/ERV that exchanges the air in your dwelling constantly.
The fact that you have to cut that many backflips just to get the stuff to work should tell you something. Once it finally does get wet all hell breaks loose because OSB cannot tolerate wetting cycles in the same way plywood can.
If you're ever curious about just how big the gap in durability is here's a free demonstration: next time you see a new house going up stop by after the framing crew has the place sheathed and ask for a small scrap of OSB sheathing and plywood they have sitting in the scrap pile. Take both home and let them sit out in the yard for six months. The plywood will warp a bit, and it might discolor. The OSB will disintegrate unless you live in a desert.
1850s-1960s: Termite damage followed by rot. This time frame is notable for ready availability of dense, tightly grained building materials. Lumber from this time period shrugs off all but the most egregious wetting cycles. So what happens is high humidity attracts termites which break down the structure. This in turn gives rot a plate to establish a foothold and spread (slowly).
1970s-1990s: This period is notable for a steady decline in quality of building materials and introduction of first and second generation engineered products. First-to-market siding products, condensation issues due to the aluminum craze in the 80s, and material adhesives edging out toward the end of their life expectancy all contribute to problems with mold/rot. Looser grained building materials also mean that when a problem is present it will quickly spread to larger areas of the structure than older materials would permit under similar conditions.
2000-2010: Easily the absolute nadir of home building in the US. The industry saw a massive influx of "budget" engineered materials, with no substantive changes to code to address the deficiencies of these materials. My personal favorite from this era include OSB siding that turned into a kitchen sponge whenever the paint layer was breached.
2010-present: same as it ever was. The market is still flooded with engineered materials that have a fraction of the life expectancy of more traditional materials. Building codes have largely caught up with the obvious limitations of these materials, however now the biggest issue is as a nation we are short two full generations of trained craftspeople in the construction industry and as such installation errors are rampant. This leads to more and bigger issues, bigger abatement projects, and in significantly newer homes. Case in point: a pinhole leak in a caulk seam on a window surround that resulted in all of the structural members surrounding that window, the wall cladding, the sill beam, a section of the floor, and several joists rotting out in short order. Root cause: didn't use plywood. Engineered sheathing acted like an enormous sponge both retaining and broadcasting moisture to all of the surrounding materials.
So yeah, you're not wrong inasmuch as according to theory and per code it is within the realm of possibility to use these construction methods and materials successfully. In practice, however, the least competent subcontractor on any given jobsite presents a hard ceiling to what one can get away with. You design a fault-intolerant system that has any flavor of complexity to it's installation and odds are good someone's going to screw something up. The Achilles Heel of modern vapor tight building systems is the fact that houses leak. Either through incompetence during the initial build or breakdown of materials over time all houses leak. Whereas older construction methods would tolerate this to varying degrees, newer systems do not.
- Hardwood timber framing
- Stone or masonry curtain wall from the foundation to the bottom of the window sills.
- Fully sealed crawlspace with inline registers broadcasting conditioned air into the space
- Insulate the curtain wall instead of the interstitial space between floor joists
- Standard soffit-to-peak venting in the attic space
- Two layers of plywood subfloor separated by a layer of tar paper
- Double layered sheetrock on all interior walls
- Wall-to-wall sheet vinyl floor treatments in all of the rooms where water is a thing.
- 3/4" hardwood flooring everywhere water isn't a thing.
- Passive/active solar combo meal on the roof to offset any efficiency losses incurred by "loose" construction methods
- Temperature & humidity sensors in the crawl space & plenum
- Wood window frames and sills. Modern plastic window frames and sills are excellent at hiding a problem until it's turned into a $50k project (see also: aluminum siding). By comparsion wood trim acts as a bellwether. I'd much rather have to scrape, recaulk, and paint a window than be looking at deconstructing an exterior wall that's rotted to the foundation.
Doesn't this risk any water that does end up getting in, to accumulate and cause more damage over time since there's no route for it to dry out?
I was always taught in my building science courses to put the water/air/vapour control layer inside the wall, before the insulation, so vapour can dry out from both sides.
If you really want to rot a floor though the absolute best A+ gold star method for doing so is adding a gas log fireplace with surround. I had a house a couple years ago that had moisture from the flue pooling inside the surround and under the unit. Rotted through 3/4" red oak hardwood flooring, 2 layers of 3/4" plywood, and bit off the top 4" of the 2x8 joists under the unit. Interior space was a solid sheet of black mold. That was one of those bag-the-room negative air pressure break-out-the-space-suits jobs. Six months later I was on a job where condensation off another gas log flue pooled inside a surround and got into the OSB cladding, then wicked 9' straight up. Only time I've ever seen cedar siding actually rot. We ended up having to cut the entire fireplace surround and a portion of the roof off the side of the house and rebuild it studs and all. Never a dull moment I tell ya.
> ... and the floor treatment rejects both vapor and spills.
That's a good point I didn't consider. Drying out the inner surface works well for interior vapour that gets into the enclosure, but won't handle spills, or standing water well.
My biggest concern with this system is given this system's "tightness" to water vapor (similar to SIPs), all of the same issues with mold and related air quality are inherited. If structures don't breathe they rot. i