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I can’t understand why I see post after post assuming sheathing is even needed. If its racking and shear strength you want,
use cross bracing that is attached to each stud; or, use some occasional posts and girts in the wall (hidden or visible).
Roof and wall sheathing is overkill, overbuilding and redundant.
If its an exterior bulk water barrier you need; your siding, roofing and water wrapping can do that.
Sheathing frosts up, condenses and reduces the R value of your insulation unless you use mechanical ventilation (and who
mechanically vents and dehumidifies a wall cavity — see below — you should vent the cavity in a way you can control its
humidity level).
If you don’t like housewrap stapled to the ouside of the cross-strapped studs and rafters (surfactants may breakdown the
wrap) use polyethelyne and strip-caulk the studs/rafters to bulk-water seal your wall and roof cavities.
Since the cavities are sealed on the interior, sealing the exterior will resolve those endless debates about
convection-across-insulation. Insulation debates are incomplete without putting moisture factors and mechanical
venting/dehumidifying into the formula. No insulation will work to spec under live conditions.
Vent the walls and roof cavities at the tops and bottoms, into the interior environment and let your HVAC system look
after condensation. HVAV will manage humidity/frost-condensation levels more efficiently than any passive ventilation of
the cavity to the outside could.
Raise your siding and roofing off the poly/housewrap covered studs/rafters with firring strips or purlins enough to prevent a
capillary sandwich of siding against wrap/poly and keep the drain plane unrestricted. On the roof, purlins will serve as
cross bracing and separate roofing material from the water-wrap.
If you use Type 2 EPS or XPS foam board on the outside, combining it with sheathing will turn your wall/roof into a
sponge and rot the framing structure eventually. Whether its exterior or interior ply or OSB, it doesn’t matter.
Replies
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Tedd,
My idea is to use foil faced foam w/ taped seams instead of poly or tyvak...and then fir strips between sidng and or roofing....Also in framed bays use dp and no poly...then drywall...and no venting of closed areas..just control moisture and penetration air leaks and allow dp to control inner assembly moisture...what do you think?
Fred...How about you too?
Jack : )
*tedd. For starters let-in wood or metal corner bracing does not work. Furthermore Simpson Strong Tie warns against using their metal strapping or let-in bracing for anything other than temporary bracing until structural sheathing is installed. No building inpector in seismic zones 3 and 4 would allow you to do as you propose, even if he acknowledges the constribution made by horizonatally applied gypsum board. GeneL.
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Gene ...
I don't want to side track this possible discussion by narrowing in on one element of what I think we need is a thinking man's, not a conventional man's (always did it that way) approach to design/materials' selection and building performance. That's why I question sheathing in the first place -- everybody does it but I don't think they have a good reason in this day and age.
But .. in specific reply, light guage steel cross bracing with 1'sq corner gussets is actually 3 to 4 times stronger than plywood sheer walls with a lateral resistence of about 5000 lbs.
The use of steel cross bracing has been developed/applied to Zone 4 residential buildings in Canada and California.
For arguments sake, if the bracing is salable to one's local/colloquial building inspector, what about the rest of the idea ?
... and ....
Jack ...
I understand your approach but I am skeptical about our ability to actually make a wall cavity, air and moisture tight. Since insulation is a sponge, I would like to have it vented to the internal environment so I can manage humidity levels and dry out any frost/condensation (keep up the R value during hot humid summers and damp and fluctuating northern climates - raining one day and -35F the next).
I realize that your use of foam will probably eliminate much of the frost and condensation problem but I don't believe we can seal the cavity from absorbing both internal and external moisture. Which approach will get us closer to a perfect exterior weather, temp and moisture seal and a seal that lasts over life of building ?
Since we have mechanical ventilation, dehumidifying and energy recovery systems in virtually every new house, why not use it where humidity is a threat to building performance -- the wall and roof insulation cavities ? This, by the way, is why I think that external attic vents (passive or active)is a dated concept and a performance reducing/cost incurring waste.
tedd
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Tedd,
I'm basically agreeing with you if you look at my post again....the foam helps with condensation and is easy to seal very tightly if used on the outside of studs...the inside wall is much less sealed because I propose to still put holes in it for electric boxes in walls and not gasketting top and bottoms of drywall and no poly or kraft under drywall...So my wall "breathes to the warm side and may not have any water worth talking about embeded in it. What does diffuse in will stay relatively uncondensed and diffuse out...the ceiling is another matter...no holes in it...drywall continuos above all interior partitions...insulation above it should be covered with open cell foam to reduce surface area and micro-eddy high delta T losses...I think a better ceiling would actually be cathedral in nature and follow my wall principles exactly...
What do you think...By the way I have always thought standard construction thinking to be rediculously backwardly copying "lowest common denominator" thinking and am glad to have another "out of bounds" thinker out posting about.
Willing to go beyond the edge,
Jack : )
*GeneI don't know where your stated seismic zones are, but the Ontario Building Code dropped i anyrequirement for corner bracing about 10 years ago, be it sheathing, let in, or otherwise as long as the interior is sheathed with horizontal d/wall. Shocked the hell outa me!!
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Thanks Jack ... I will chew on your design for a few days. I see you have applied the same principles and reached for same goals but its better thought through the means than I.
It seems that convention is largely based on myths that don't fit with today's materials. The potential for finding economies and efficiencies at the same time as making building earier and designs more logical (simpler and symetrcal) is available if we can challenge the conventions openly.
It seems too, that keeping up with the research, the new materials hitting the market, and following the amendments in building codes are important. The codes are not my "expert reference point" (for sure) and are no more scientific or tested than the conventional hogwash we are trying to clean up in the trade.
Some people should take a few building apart once and a while to see what really happens inside. Others should do dynamic stress tests on materials and designs to see how things really operate. It is not in the manufacturers interests to know the answers -- like how their product performs when installed in concert with another product or building feature. To many debates are taken out of the context of the living and dynamic building as if perfection can be arrived at in isolation.
Tedd
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Patrick. Tedd posted his drop the seathing under tow headings. Your quite right about Canada, but let me repart here waht I said in other post.In Canada for part 9 buildings, national and provincial building codes do not require bracing where exterior walls will have one of the following interior finishes: gypsum board with taped joints, plywood, hardboard,, insulating fibreboard, particleboard, waferboard or strandboard.
The American codes, UBC anyway, do recognoze the contribution made by interior gypsumboard.GeneL.
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Gene ... I can't find that other posting location .. I thought I asked the web master to remove the other one. If you posted something there would you mind reposting it here.
In regards to Canadian code, those optional interior wall coverings (noted in your post) represent a huge array of performance characteristics re: shear strength, racking and deflection resistance. Just what is enough ?
By sheathing outside of studs with ply or OSB then covering with another layer of siding plus the internal wall covering, we have a two compression flanges and a tension flange making the wall over built. Its my understanding that even the dead plus live load requirements for roofs don't demand a compression and tension flange for engineered panels, stressed skins; etc.
Since I last posted I discovered light steel wall cross braces spec'ed for 6000 lbs of lateral force.
I still think sheathing is tradition without engineered validity, a throwback to days when plywood was the greatest thing since sliced bread to combat shrinking clapboards and disintegrating mortar in houses that leaked like sieves and barley hull insulation turned into dust.
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Tedd, Patrick et al. It is now noon and I'm sitting here waiting fot the possible 1 foot of snow.
The Canadian building sciientist A.T Hansen, in his paper, "Shear Resistance of Wood Frame Walls, "in building Practice Note 61, 1985,says, "Although most wood framed buildings continue to be sheathed, this is primarily to permit rapid protection from the weather and, in the case of insulating sheathings,to reduce heat loss. TRADITION, however, is also considered to be a significant factor in retaining sheathing,"(my emphaisi).
The 1977 Leger house used no structural exterior wall sheathing, choosing instead, DOW Styrofoam. It still stands(the double cross laminated interior gypsum board helped considerably)has not rotted out or had window or door problems because of a lack of struuctural exterior sheathing.
However, my reply was based on the research done at Forest Products Laboaraotry (FPL) by Tuomi and Gromala and reported in their paper, "Racking Strength of Walls: Let-In Corner Bracing, Sheet Materials, And Effects of Loading Rate." A second research report is by Wolfe,(also of FPL) "Contribution of Gypsum Wallboard to Racking Resistance of Light-Framed Walls." All three engineers agree on the uselessness of wood corner let-in bracing: it will not provide the Federal Housing Administration (FHA) required racking strength of 5200 pounds.
Wolfe found that gypsum wallboard installed horizonataly contributed 250 pounds of resistance per lineal foot of wall lenght. Gypsum board manuafcturers' sponsored testing yielded values as high as 660lb/ft. The 1997 Uniform Building Code(UBC)recognizes between 100 lb/ft to 250 lb/ft depending on whether the wallboard is 1/2-inch or 5/8-inch, , two ply, or blocked or not blocked and nail spacing.
The codes are strangely quiet on the combination of non-structural(RFBI) sheathing and let-in bracing. The Cabo One and Two Family code allows the use of "...approved metal strap devices installed in accordance with the manufacturer's specifications;" BOCA in its Commentary Volume 2 says, "The code official may approve [the use of metal stapping] as an alternative if the proprietary brace has been tested in compliance with ASTM E72 and evaluated in accordance with the specified code requirements." What manufacturer's metal strapping meets the FHA requirement of 5200 lb/ft?
Wolfe also looked at the let-in corner bracing issue. Both he and Simpson Strong Tie found that metal bracing does not approach the FHA minimum standard of 5200 pounds.
The purpose of this 2nd post is to provide the source of my saying that let-in wood or metal bracing does not work. It is not to defend or argue for sheathing.
Please provide name of research report on steel cross bracing that support ypour claims about metal bracing. The 1997 edition of UBC has no such provision, so how convince a UBC building official? GeneL.
I have more quwations for you but will do so in another post.
*GeneDon't pine away waiting for snow, I'll send you a foot or two and still be drowning in it.i ". . . double cross laminated interior gypsum board" as in one layer vertical and one layer horizontal??? Pour quoi?? As in Why???i Up to my ears in the white stuff with no end in sight!!!-Patrick
*Let it snow, let it snow, let it snoooooooow. . . .Joseph FuscoView Image
*Patrick. We got about 10-inches. It is a bright suny, but cold Monday afternoon.The first layer of gypsum board was installed horizontally, and the 2nd layer was placed vertically. The 2nd layer had fewer screws and was also glued. This increased the house's mass, and gave a wall approaching a plastered one at considerably less cost. By t he bye. Only the exterior walls were double-layered. This was SOP in many of the houses I did.
*Hi GeneWe got almost 3' of snow between Thurs & Sat, now it's cccccold and wwwwindy. i I'll have a little Roquefort with that whine.Was the added 'mass' and 'almost like plaster' finish the primary reasoning, and if the 'plaster-like' quality was a i reason(as opposed to merely a result) why not double up all of the walls??? In most rooms only 2 out of 4 walls would have this i better finish.I thought you were derisive of i massas in: better insulation instead of "mass and glass", hence the deveploment of your "MESH" and "Leger House" designs???Inquisitive as always!-Patrick
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I just came across the bracing issue on a design build project. Straight from a plan checker in the
Sierra Nevada foothills in California, all of Ca. is seismic zone #3 or #4, the UBC does not allow diagonal bracing at all. Now we need a 4' shear panel at each end of a 25' wall ( or 4' of shear panel for every 21' of wall ). What is commonly done on a long wall is to split up the required shear wall at each end of the wall.
Jerry
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FREDL. I concur with your technique. Our first layer was 1/2-inch and the second layer was 3/8-inch. The OSB has the advantage of being as strong in one direction as in the other, whereas the gypsum board as the inuitial layer is strongest in the horizontal position because of the paper.
PATRICK. I'm dismissive of the hype about mass. The active and passive solar comunities argued for solar systems and mass and glass; we argued for tightness and insulation. If any of you are into electronics you know of the RC filter which is made up of a resistor (R) and a capacitor (C). A capacitor is able to store an electric charge.It takes time to charge or discharge the capactor.The RC circuit has a time constant or TC. The TC of an RC circuit is equal to RC: TC=RC.
A house may be seen as an RC circuit where the mass of the house is its capacitance,(C) and the insulations are its resistance (R). It takes time to charge-heat- a house and time to discharge-cool down, and vice versa. .
For the solar crowd the house's TC= rC; tons of mass C. R is the R found in the FHA MPS house. (I assume that by now we all know these acronyms). For the MESH house practitoners TC=Rc: tons of insulation (R)and very low leakage,and the normal mass(c) found in the avergage American house.Many years of real world experience proves that insulation(R) is more effective and less costly than mass (C).
Therefore,a house with a long time constant TC, once heated--charged- takes a long time to cool-down.Once the same house is cooled it takes a long time to heat-up. For too many years the solar communities refused to acknowledge this fact. Solar Age almost to the very end of its publishing existence ignored so-called super insulated houses. The 2x6 R-19 wall with 1-inch of RFBI on the exterior is the equal thermal performance wise of a mass wall: 8-inch concrete masonry block plastered on the interior side, and insulated on the exterior with 2-inches of RFBI.
The reason for the cross laminated layers of gypsum board was to acxhieve the qualities of a plastered wall minus the cost. For those who think mass matters I mentioned that we had increased the mass of the house.
JERRY. You are correct. The 1997 UBC does not permit let-in bracing( and it says nothing about metal bracing)in Seismic Zones 2,3 and 4. There are a number of alternatives allowed by the code one of which is 2 full 4'x 8'sheets o*f plywood, OSB etc., installed vertically on each side of the corner.However, additional bracing is required every 25 feet.
Furthermore, there is published evidence--which I do not have at hand--that the interior gypsum board will not hold under certain conditions. GeneL.
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Gene, I think you have missed the point of supplemental Solar heat. It is not suitable for all locations. Obviously in our Canadian climate, the heat loss through the glass (where the indoor/outdoor temperature differential may be 100*F) would far outstrip any solar gain from our weak winter sun. If you consider Arizona, Texas, Florida, its a completely different situation.
The general idea behind the Solar house is: once the mass is heated the flywheel effect will keep it at a reasonably constant comfortable temperature. As much as possible, the flywheel is kept going by energy supplied by the sun, rather than expensive fossil fuels.
A well-insulated house is thermally efficient no matter where it is located. Now if we replace some of that insulation with "solar glass" and look at the result, we will find that the heating effectiveness of the solar glass is determined primarily by climate and thus geography.
There comes a point (geographical) where the energy needed to overcome solar glass heat-loss exceeds the solar energy gain. This is where the solar glass becomes ineffective. If you think about it, it doesnt matter how much insulation is in the rest of the house; it is the loss/gain ratio through the solar glass that determines whether or not that glass is worthwhile at any particular location.
I believe in well-insulated houses but I just wanted to explain one reason why Solar people can ignore the insulation: whether or not solar glass should be employed in a house at any geographical location is determined solely by the heat gain/loss ratio through the glass. If there is a net Solar heat gain then it is worthwhile...independant of the insulation in the rest of the house. The free Solar gain substitutes for the expensive fossil fuel. (Ignoring for the moment the installation cost of the Solar glass and mass vs fossil fuel cost savings).
My use of the term "solar glass" means extra glass installed solely for the purpose of capturing the sun's energy, apart from the normal windows one would expect to find in a house.
By the way, I think that no matter what the Codes say, anyone who relies on gypsum board instead of OSB or plywood sheathing to provide structural integrity is an idiot. Just one example: You know what happens to gypsum board when it gets wet. I'd hate to come home from vacation to find my house flat on the ground simply because of a water leak on the upper floor.
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I can't understand why I see post after post assuming sheathing is even needed. If its racking and shear strength you want,
use cross bracing that is attached to each stud; or, use some occasional posts and girts in the wall (hidden or visible).
Roof and wall sheathing is overkill, overbuilding and redundant.
If its an exterior bulk water barrier you need; your siding, roofing and water wrapping can do that.
Sheathing frosts up, condenses and reduces the R value of your insulation unless you use mechanical ventilation (and who
mechanically vents and dehumidifies a wall cavity -- see below -- you should vent the cavity in a way you can control its
humidity level).
If you don't like housewrap stapled to the ouside of the cross-strapped studs and rafters (surfactants may breakdown the
wrap) use polyethelyne and strip-caulk the studs/rafters to bulk-water seal your wall and roof cavities.
Since the cavities are sealed on the interior, sealing the exterior will resolve those endless debates about
convection-across-insulation. Insulation debates are incomplete without putting moisture factors and mechanical
venting/dehumidifying into the formula. No insulation will work to spec under live conditions.
Vent the walls and roof cavities at the tops and bottoms, into the interior environment and let your HVAC system look
after condensation. HVAV will manage humidity/frost-condensation levels more efficiently than any passive ventilation of
the cavity to the outside could.
Raise your siding and roofing off the poly/housewrap covered studs/rafters with firring strips or purlins enough to prevent a
capillary sandwich of siding against wrap/poly and keep the drain plane unrestricted. On the roof, purlins will serve as
cross bracing and separate roofing material from the water-wrap.
If you use Type 2 EPS or XPS foam board on the outside, combining it with sheathing will turn your wall/roof into a
sponge and rot the framing structure eventually. Whether its exterior or interior ply or OSB, it doesn't matter.
*
Rod,
Your water leak idea doesn't make sense unless your home has absolutely no outside siding and there also was a 60 mph wind while all of the drywall on all walls was soaked totally through...Conditions of a huricane knock down houses anyway!
Near the stream and having a hard time knockin down any home even with 3/4 of the walls chainsawed out.
J