5 ms·

Mold abatement & home restoration contractor here. OSB should not be considered code-compliant material, full-stop. Imagine your home has been constructed with

Mold abatement & home restoration contractor here. OSB should not be considered code-compliant material, full-stop. Imagine your home has been constructed with slabs of sponge that have are purpose-engineered to provide optimal growing conditions for fungi when moisture is introduced. Now consider that over the lifetime of a structure some combination of exterior cladding failure, roof leaks, ground moisture issues, and plumbing fails are not only likely, but basically guaranteed. 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

hedgehog 4y ago
Do you see that in modern construction? As long as there is adequate insulation and the vapor retarding layer is in the right place (per code) the OSB shouldn't ever be more than damp for any extended amount of time.
- forgetfreeman 4y ago
  I've seen catastrophic moisture damage in every form of home construction utilized in North America from the 1850s to present day. Anecdotally the most common failure modes break down roughly thus: 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.
  - wpietri 4y ago
    This is fascinating stuff. So what would you do if you were building a house today?
    - forgetfreeman 4y ago
      Assuming I was building in the Southeast where I live, that someone handed me a blank check, and without drilling down to specific products? - 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.
      - hedgehog 4y ago
        Fascinating, I understand most of it but why vented attic vs conditioned?
        forgetfreeman 4y ago
        Simplicity. Sealed crawlspace prevents the plenum from ingesting ground moisture, so as long the roof is maintained that space shouldn't see any significant issues at any point in the next century.
      - saeranv 4y ago
toomuchtodo 4y ago
All excellent points, and why moisture vapour transmission direction/rate must be taken into account during a build. If you live somewhere where AC is the primary mode of interior conditioning, you will build to where the AC will pull moisture in from the structure for removal via the condensate line. If you live somewhere not so tropics, you're going to build where the moisture is pulled out of the home by the natural environment. Building sciences are both fun and fraught with peril. I'm somewhat excited and cautiously optimistic about Boxabl from a housing manufacturing perspective.
signaturefish 4y ago
It is indeed a concern, and I suspect that's why they're recommending any wikihous project include a full-house MVHR system (see About=>Product, the bent arrow near the middle of the image map). MVHR is the solution I've settled on for my house retrofit project - it should allow the house to breathe in a controlled manner, without leaking warm air in winter.
throw0101c 4y ago
> OSB should not be considered code-compliant material, full-stop. 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 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-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer 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-the-perfect-wall 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. * https://www.youtube.com/watch?v=CIcrXut_EFA https://www.youtube.com/watch?v=CIcrXut_EFA
- forgetfreeman 4y ago
  OSB is absolutely a fine air barrier assuming <60% humidity at all times. That does not make it a sane choice for sheathing/flooring material given the likelihood of moisture ingress. Every form of seam sealant known to man bites it at some point due to thermal expansion cycles, uv damage, etc. and that's assuming perfect installation of all layers of the moisture control stack. It isn't even a good choice for cabinetry due to it's propensity to devolve into Nature Valley Granola when put in proximity to a sink, dishwasher, or refrigerator (if a cold water line is part of the install). 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.
turtlebits 4y ago
OSB is fine with a proper WRB, vapor control and venting. Things like rainscreen/strapping under siding and roofing go a long way of letting your house dry out when water gets in.
- forgetfreeman 4y ago
  Sure it is, for the first year. Or the first five years. Or however long it takes for a caulk seam around a window or door to get sprung or a plumbing fitting somewhere to develop a drip. How long do you expect your home to last without major restoration work? 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.

hedgehog 4y ago

Do you see that in modern construction? As long as there is adequate insulation and the vapor retarding layer is in the right place (per code) the OSB shouldn't ever be more than damp for any extended amount of time.

forgetfreeman 4y ago
I've seen catastrophic moisture damage in every form of home construction utilized in North America from the 1850s to present day. Anecdotally the most common failure modes break down roughly thus: 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.
- wpietri 4y ago
  This is fascinating stuff. So what would you do if you were building a house today?
  - forgetfreeman 4y ago
    Assuming I was building in the Southeast where I live, that someone handed me a blank check, and without drilling down to specific products? - 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.
    - hedgehog 4y ago
      Fascinating, I understand most of it but why vented attic vs conditioned?
      - forgetfreeman 4y ago
        Simplicity. Sealed crawlspace prevents the plenum from ingesting ground moisture, so as long the roof is maintained that space shouldn't see any significant issues at any point in the next century.
    - saeranv 4y ago

toomuchtodo 4y ago

All excellent points, and why moisture vapour transmission direction/rate must be taken into account during a build. If you live somewhere where AC is the primary mode of interior conditioning, you will build to where the AC will pull moisture in from the structure for removal via the condensate line. If you live somewhere not so tropics, you're going to build where the moisture is pulled out of the home by the natural environment. Building sciences are both fun and fraught with peril. I'm somewhat excited and cautiously optimistic about Boxabl from a housing manufacturing perspective.

signaturefish 4y ago

It is indeed a concern, and I suspect that's why they're recommending any wikihous project include a full-house MVHR system (see About=>Product, the bent arrow near the middle of the image map). MVHR is the solution I've settled on for my house retrofit project - it should allow the house to breathe in a controlled manner, without leaking warm air in winter.

throw0101c 4y ago

> OSB should not be considered code-compliant material, full-stop. 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 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-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer 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-the-perfect-wall 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. * https://www.youtube.com/watch?v=CIcrXut_EFA https://www.youtube.com/watch?v=CIcrXut_EFA

forgetfreeman 4y ago
OSB is absolutely a fine air barrier assuming <60% humidity at all times. That does not make it a sane choice for sheathing/flooring material given the likelihood of moisture ingress. Every form of seam sealant known to man bites it at some point due to thermal expansion cycles, uv damage, etc. and that's assuming perfect installation of all layers of the moisture control stack. It isn't even a good choice for cabinetry due to it's propensity to devolve into Nature Valley Granola when put in proximity to a sink, dishwasher, or refrigerator (if a cold water line is part of the install). 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.

turtlebits 4y ago

OSB is fine with a proper WRB, vapor control and venting. Things like rainscreen/strapping under siding and roofing go a long way of letting your house dry out when water gets in.

forgetfreeman 4y ago
Sure it is, for the first year. Or the first five years. Or however long it takes for a caulk seam around a window or door to get sprung or a plumbing fitting somewhere to develop a drip. How long do you expect your home to last without major restoration work? 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.