Sheathing-less Walls
I have been enjoying the questions and subsequent discussion in this form for quite some time. So far I have found the broad range of topics information very helpful.
One topic I haven’t been able to find a conversation on is sheathing-less construction. I realize that there are plenty of buildings that don’t use OSB or plywood sheathing and simply use Polyiso or XPS as sheathing/continuous insulation /air barrier/WRB over wood framed and wind braced walls. The builders I have seen using this method seem to be selecting this assembly primarily for the cost savings by not installing OSB or plywood and not for any sound building science reasons.
I’m located in mid-western Ontario and have been building for almost 30 years. During my career, I have always used OSB or plywood sheathing. However over the last couple years, my company has been moving in the direction of high performance and passive building which has led us down the path of super insulated walls and all of the good stuff that comes with this method of construction. So my question for this forum is……
With the advent of products like Solitex Mento Plus from Pro Clima which can be used in directly over the exterior of a stud wall to create a sheathing-less enclosure and is robust enough to accommodate dense pack cellulose or fibreglass, is “cold sheathing” still a concern for thick (double stud) walls? To me the idea of installing a skin like this on the exterior with a vented rain screen and using a smart vapour retarder on the interior of a dense packed double stud wall would create a very “dry-able” wall. I realize that OSB or plywood might be necessary at times as a structural requirement but in situations when let in wind bracing is suitable a sheathing-less wall seems to make sense. Also, nice and light to stand.
Ok enough blabbering from me, have at it! I welcome your feedback.
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Replies
I've always wondered a few things about houses sheathed without structural engineered wood.
1. In the event a corner is damaged, does the house want to twist under the weight of the roof?
2. The cellulose may help here, but in the event of a fire, does an XPS/eps sheathed house burn in minutes or seconds? Are the glass mat faced gypsum panels a good use case here, behind the foam sheathing?
3. What is the actual costs saved all things being equal, when you (hopefully) have to make sure you're always hitting a stud, for anything besides brick? Insulation costs more than osb right now, it actually seems more expensive to sheath in foam, depending on the wall assembly.
4. If you send a nail though the foam with a nail gun, does it hit a guy working in the house?
Half joking, half serious. I've always felt that anything less than fully sheathed is a fools errand. If you can't afford a house with osb, you can't afford a house at all.
Kyle,
4. if I don't have any hand nails in my pouch, I hold back the nose safety on my gun and shoot a couple into a piece of wood. If it's about a foot away they stick and are easily pulled out. Much further than that and the nails start to cartwheel and can bounce back unpredictably Shot at a tree say ten feet away they will bounce off harmlessly.
Alright alright, i know guys, that was the joking part. Tough crowd :)
Messing with nail guns is all good fun.
"1. In the event a corner is damaged, does the house want to twist under the weight of the roof?"
Highly unlikely, unless we're taking about a wrecking ball or a car driving through the corner. The interior gypsum board provides supplemental shear strength as would certain claddings or polyurethane foam in the cavities, not that I'm advocating for the latter.
I too suspect we'll start to see sheathing value-engineered out of more and more structures. Look at how much less steel is built into a vehicle today compared with decades ago. Certainly doesn't make cars more robust or safer but the trade is made for greater fuel economy and first cost.
Opinion warning -
Value-engineering away sheathing is only possible is you devalue human life to $0.00. It's easy to look at homes in historically non-seismic and non-tornadic(?) zones, and with a flick of a pen throw OSB on the corners and rigid foam everywhere else. It makes sense, right? More insulation, faster build, lower costs, all things that benefit the customer. Right up until you have a seismic event, or the first tornado through the area.
The most striking example is the tornado through KY (my home state). We don't have tornadoes, at least, we didn't, until that one killed more people than any other tornado in history. Those homes without structural sheathing are setting ducks. Zero racking resistance, practically transparent to wind blown debris, and no safe zones within the house to speak of.
That's the hard sell though. It might not happen until 25 years, 50 years, 75 years after the home is constructed. But that's long enough time for one of those 100 year storms to come through, on average. It's not just about the first owners initial payback period either. Hopefully, the home will be occupied by a family for its entire existence. We have people commenting on this forum that are in houses built in the '40's, and I expect in 100 years, we'll have people commenting on the houses we are building today.
Engineering away safety IS a fools errand, and it's cheap enough that even if a destructive event never happens, structural sheathing is easy insurance against sagging and other time-related creep that happens to all structures at some point.
Kyle,
I agree. I oversee a 1938 Community Hall built without sheathing. It's a designated muster point in an emergency - but as the most likely emergency here is an earthquake, it probably won't be of much help. The task of removing the still serviceable cedar siding and adding sheathing is an almost insurmountable job for both financial and practical reasons. I wish it had been done when it was first built.
Two other more prosaic reasons I think we should include sheathing:
- A sheathed wood frame house distributes loads in ways which add structural redundancy. The walls become large box beams and the disparate structural members are tied together where they meet. This means the building can experience quite widespread failure of it's parts through rot of other damage without losing it's overall structural integrity. Anyone who has tried to demolish a wood frame building can attest to that. It's hard to pick one thing that holds it up.
- To me the sheathed wood framing is the core of a house to which things get added or removed without compromising its (for lack of better word) essence.
A house becomes a house a lock-up, when for the first time there is a clear distinction between inside and outside.
The only time I would omit the exterior sheathing would be if it were being moved at a different location on the exterior walls.
"Value-engineering away sheathing is only possible is you devalue human life to $0.00."
One could make the equally valid argument that continuing to consume resources at the pace we have been devalues all life on the face of the earth to about $0.00. Reducto ad absurdum.
I've never worked in regions of seismic concern. Sheath the houses there if you must. Code likely mandates it anyway.
I for one do not imagine a 7/16" layer of OSB on all fronts will save a blasted thing in the path of even the smallest tornado. Build a basement if you must. Or an engineered shelter anchored to the slab. Save lives; saving the house is the fool's errand. IMO.
Jason,
I wasn't disagreeing with you, per se, just commenting on the fact that at the extreme end of value engineering, we've lost site of what it is we are trying to accomplish, in my eyes.
Houses that last a very long time save resources, compared to building new again in the same spot. Building made of lumber are essentially renewable, and houses that stand a very long time hold within them a great amount of sequestered carbon.
On the 7/16" sheathing front, you don't have to imagine. It has been measured and documented, and it works, at least up to a reasonable wind speed. Nothing is going to stop a direct hit from an EF5, and that is where an appropriate shelter comes into play. That out to be a part of the initial design, so that it can be implemented as cost efficiently as possible. Check out some of the reports here:
https://www.apawood.org/wind-weather-seismic
The Forrest products lab even has published plans for integrating wood based shelters into rooms that a resident couldn't point out otherwise.
https://www.fpl.fs.fed.us/research/research_emphasis_areas/howdoesitapply.php?rea_id=2&view_id=3
With a highly permeable exterior surface there is nothing that moisture can build up on. A sheathingless wall with rain screen gap is a very robust assembly.
Technology has definitely improved since but we have been down the road with sheathingless walls before (old houses with clapboard over tarpaper directly on studs) and they had a lot of issues that were not fixable without complete exterior rework. I'm confident Solitex folks have the durability figured out, call me olde school, I'm still hesitant to trust a membrane with some pressure behind for the long term.
My bigger beef with this type of assembly is how to keep critters out. You have to rely now on your siding as the main critter barrier which takes a lot of care to detail it that tight.
For the money, fiberboard+housewrap is still a better moisture open assembly to me. The 1/2" stuff would have problems with dense packing but it would provide a lot more strength to the walls and have some extra R value.
Gypsum sheathing also a great vapor open alternative. Nice part about it is you can score and snap like drywall, except for the itch factor, pretty easy install.
> selecting this assembly ... not for any sound building science reasons.
A little moisture accumulation on foam is less harmful than on OSB. I consider that a sound building science reason.
> Polyiso or XPS
Being more vapor permeable, EPS or GPS should be the first choice.
The primary purpose of the sheathing on walls is structural, both to provide racking resistance (the wall falling over with the studs all tilting over like dominos), and to provide something to hang siding or cladding on. The sheathing is often used as an air barrier, but that's not it's main function.
You CAN use things like let-in bracing (usually a diagonal 1x4 cut into ("let in") the the edges of the studs so that it's flush with the stud edge) across the wall, a diagonal steel brace, or "shear panels", which is structural sheathing that is only in key locations specified by an engineer and not along the entire surface of a wall. The downside with both is it's a bit tricker to hang siding, since you have to hit the studs.
Responding to Kyle's post #1:
1- This is why you need the racking resistance of structural sheathing or similar, but as I mentioned above, there are multiple ways to do this.
2- I haven't seen any real data on this, but I don't think the insulation is going to make a lot of difference in a wood framed structure. Polyiso will be better than EPS or XPS here, and rigid mineral wool is basically fireproof. Either way, the wooden structure itself is flammable, and I'd be more concerned about vertical air pathways than the type of continous insulation used in terms of fire spread concerns.
3- You'd have to carefully consider the savings in time and materials if you use no structural sheathing, and offset that with the cost to engineer shear panels and the extra labor that may be needed to hang siding when the fasteners will have to hit studs and can't just "go anywhere".
4- I highly doubt this would be an issue. I know the claw on my nail gun will just squish into rigid foam, so it won't ever engage enough to fire (the claw has to retract to let the gun fire, so you have to press it against something with a fair bit of force). Even if you disable that feature or get it to work on foam, the foam will slow the fastener and it's not going to travel far. Nail guns aren't like guns that shoot bullets -- nail guns actually drive the nail in with a piston, so once the piston stops moving, the nail rapily decelerates.
BTW, I prefer screws to hang foam anyway :-)
I personally prefer a wall with continous stuctural sheathing as it's pretty solid, easy to build, and isn't very complex in terms of what needs to be done to get it right.
Bill
Jeremiah,
I built two work shops this summer without exterior sheathing - mainly because of the high price of sheet goods. A few things I hadn't anticipated were difficult, but nothing insurmountable.
- Penetrations, both large ones for windows and doors and small ones for ducts and wires, need thinking through.
- Installing the Tyvek WRB in windy conditions was a nightmare. Until you get the whole building closed in, the pressure differential wants to blow it off from the inside.
- You have nowhere to rest a ladder. Everything requires scaffolding.
- It's hard to anticipate everywhere you need backing for siding and trim.
Those ladder braces with the arm that clamps across the top can help here. The arm will span two or three studs and give you places to rest the ladder. I have found that the "two or three studs" always puts the ladder about 6" off from the ideal spot, so it's still a pain in the butt, just a slightly less painful amount of pain....
BTW, wrap the tyvek all the way around the last stud in the wall, staple so that the staples go through at least two layers, then unroll with tension on the roll as you go. I've done this before with poly sheet, it does help when the wind is out to get you. The downside is as the you unroll more of the material, the sail you create wants to pull you over. Fun times.
Bill
Isn't' sheathing-less construction common in the SW US? I seem to remember this site having a lot of phots showing extremely poor install of the WRB on homes from this region.
John,
Which raises the question as to whether there is a real distinction between doing it to save costs or for performance reasons? Regardless of motivation and meticulousness of the construction, is the result in both cases an inferior building to one with sheathing?
Why not use wood fiber board on the exterior? Not really sheathing, but should be robust enough to reinforce the membrane containing the fill insulation
Andy,
That's an awful lot of work and expense to end up with something which lacks many of the important attributes of exterior sheathing.
As expected, lots of great insight and advice!
I am certainly aware of the challenges that eliminating structural sheathing pose and there are several. I have only been involved in one project where sheathing was omitted and 1x4 let in bracing was used. The exterior was skinned with Solitex Mento 1000 and Polyiso was used as continuous insulation over a standard 2x6 wall. Took some getting used to but ultimately the process was smooth.
The main reason I was presenting this question was in regards to the “cold sheathing” issue that can sometimes be problematic with double stud walls, as I wasn’t sure the issue was caused by using wood sheathing or simply the fact that continues insulation wasn’t installed on the exterior of the WRB. From what I gather the challenges of keeping critters out and installing siding are more worthy of extra focus then the cold sheathing concern.
Take a look at what the insurance companies have invested in to make homes more resilient (= less insurance claims) and to show what fails thru lab testing full size structures. https://ibhs.org/about-ibhs/ibhs-research-center/
Watch how wind damages a building without wall sheathing.
Not every one is in a high seismic area or high wind area but the better your building is the less likely you will be in a motel, come that normal extreme event.
The ICC codes have an importance factor in the loads your house is designed to resist, your typical house is not in the essential facility category. I feel it is essential.
I'll add my two cents having built two "sheathing-less" homes for myself, and having two friends who have done the same. All the construction had diagonal let-in or steel bracing, and continuous exterior foam board insulation, and full 1" pine or hemlock siding.
Speaking for myself, I live in an upstate NY heavily forested area with plenty of critters - mice being by far the most dertermined to get inside - and have not had any issues to report. Never a mouse in our home, and we've never heard any in the walls or smelled the tell-tale odor of a decomposing rodent in the walls. Cumulatively we saved a small fortune on both projects by not using plywood sheathing, and if there was any additional labor to install the bracing over that required by plywood sheathing, it was likely minimal.
Daniel
The cost and availability of a sheathing panel will eventually make it a rare choice. The question I come back to is how my home should respond to the world I live in, and what my real needs are. I recognize that there is a general tendency to see houses as monolithic edifices, castles for the common man, pictured as standing for a century or more. There is a strong aversion to the idea of a house as a temporal creation, built to be rebuilt. The house as a skill, one that can be repeated many times with what materials come to hand.
I have seen photos of huge windrows of debris, stretching for miles following one of the many catastrophic events that we have invited into the world. Most of it is building material, little of of it is reused. Too costly,it seems, to recycle. Do we up the ante with nature and try to win with more fortifications? Or do we rethink how and where we build?
I now think of a house as a boat. Small, built for a turbulent life, readily repaired, even relocated. That's the future.
I like the idea of a flexible skin as both climate control and support. The main issue is more or less tensile integrity as far as structure. We took the route of solid walls, but many cultures didn't They provide examples of achieving shelter with a different emphasis ,most often due to limited types of materials, but also as a matter of lifestyle.
I have built greenhouses from scratch, using wood combined with a 6 mil poly covering. I have been amazed at how well this combination works. I have only used them open as a greenhouse. but builders, especially those who work on boats, often use such structures for protected working space. I have seen mine weather some pretty stressful events. (The first I built was not adequately anchored and took off like a kite across the road. It mostly just needed to be reassembled with a few new parts)
I believe that a continuous skin , especially well attached to structural frames, gives a very respectable rigidity to any structure. Stronger skin material could easily give as much racking resistance as a solid panel, if it is well fastened and as continuous as possible. By well fastened, I mean reinforced wherever fasteners are used. On the greenhouses I build I mostly use fasteners just on the end frames. If applying material over that, such as insulation board, I would use strapping over the foam. That would give you plenty to aim at for siding, and of course be a rain screen. I would also consider metal panels as a siding material, or maybe another skin.
Brian,
You may enjoy How Buildings Learn by Stewart Brand
I'm from that generation when building seemed like a radical thing. Loved the vibe, and it's been amazing to me to see the evolution.
Brian Carter above has an interesting take on the general approach to building in our society. One thing that has kept us in this cycle of bigger and more expensive fortifications as he describes it is artificially low cost of funds (mortgage rates) and subsidized risk insurance. At some point normalized rates may return (through hyper inflation of material costs, deflation of the currency, government pullback from risk subsidies, and/or rising interest rates to reflect the true credit or interest rate risk). At that point we may be forced to confront these considerations more carefully. We are starting to see a little of that when 1/2 inch sheets of plywood were fetching $50 and a 2x4 hit $6 in some places.
Sitting in my Thermoply sheathed house from 1989, I can't help but notice how products like Thermoply sit sort of in the middle as a sheathing product. It's structural, has a WRB pre-applied and taped seams (probably very low perm?), and is super cheap. One of our local yards mentioned they had actually sold some quantity over the last year or two, because it was pretty attractive at sub-$10 a sheet during the pandemic. Usually they just sell the odd sheet or two for repairs to existing houses, from a old-stock skid covered with dust in the back storage barn.
Of course, it's installation sensitive like everything else, so mis-taped/untaped seams and unsealed penetrations can ruin the best intentions. My house has some 6 inch holes cut with a utility knife for a box penetration, not sealed at all and hiding behind lap siding.
It also doesn't act like a nailbase at all, so you're still required to hit the studs for a siding install, and still have to install blocking for pretty much all exterior stuff.
I hate the stuff, after dealing with it for repairs on this house.
It actually has its uses for kneewalls and such, but I can't see wanting to sheath a house with it because of the lack of nailbase. Even if it had some benefits like a higher perm, it's still only a partial solution to sheathing.