i am about to start framing a superinsulated house. the specs call for
1/2" structural fiberboard sheathing (brace-rite by knight celotex).
no osb on the corners. this is not something i chose, but it is on
contract so i'm stuck with it. no additional bracing is required.
what do you engineers out there think of this system? i've heard of
houses which creak in the wind, but that was not the structural
fiberboard. there is no additional bracing specced.
anyone used this much? i'd really really like to sheath my walls
before standing them up, but am wondering if this is even possible with
this shit. for example, can you even walk on a wall, or will it break
between studs? can i cut it with a rotozip?
It's listed as a permissible braced wall product in the IRC (method #4, 1/2"
or 25/32" structural fiberboard applied vertically or horizontally, on studs
spaced 16" o.c. maximum, and installed per table 602.3(1).
The kicker is it's only certified for 16" OC. [ R602.10.3 (4) ]
For a single story home in SDC (Seismic Design Category) A or B, 100 mph
wind or less, it's installed at each in and 25 feet oc (min. of 16% of the
wall). For SDC C or wind 110 mpg or less, it's 25% of the wall, for SCD D-1
it's 30% and for SCD D-2 it's 40% of the wall. ((Same as OSB)).
Fastening is 1-1/2" gal roofing nail, 6d common nail (8d if structurally
rated) or staple 16 ga. 1-1/2". All fastening is 6" oc field and 3" oc.
edge. (man that's a lot of nails.)
Advantages appear to be high permeability rating of 20 and an R-value of 1.3
(yawn). It appears to be ok for up to a 3-story home, <110 mph wind, and
seismic category D-2 or less.
Restrictions stated are; "Knight-Celotex Fiberboard products must not be
used in close proximity to chimneys, heater units, fireplaces, steam pipes
or other surfaces which could provide long term exposure to excessive heat
(maximum 212*F) without adequate thermal protection. Consult the appropriate
heating appliance manufacturer's instructions before installation."
The Modulus of Rupture, is 400 psi min.
Water Absorption for 24 hrs. is 15% max.
Tensile Strength (perpendicular to surface) : 800 psi
Warnings are: "Provide proper ventilation and use of respiratory protection
as required by the manufacturer. Avoid dust inhalation. Use caution when
creating large amounts of dust because of potential
explosion hazard. Refer to manufacturer's MSDS data for these and other
precautions." "Panels must be installed vertically."
Installation instructions include: "It is recommended that the exterior
finish be applied within 30 days. If applied sheathing should get wet, wait
until completely dry before application of exterior finish."
In looking at the ICC Evaluation report ESR-1978, conditions for use state
that: (5.1) it is not to be used as a nailing base for finish siding. (5.2)
it's limited to type V-B construction and IRC structures. (5.4) if installed
on exterior side of exterior wall, must be protected a water-resistive
barrier. (Product information requires this to be done within 30 days.)
Keep in mind that the stuff will not qualify as fire blocking (if that
My **guess** in looking at the spec's is, you can walk on the sheathing
Personally I would have specified 24" oc studs to gain more R-value, and
OSB. (But that's opinion only.)
That's a big expense to cover a building twice - who's supposed to pay
for it? If you're talking about just adding the OSB over the
fiberboard on the corners, that's going to cause an issue with the
siding surface. Please clarify, Bob.
one of the reasons they are calling for fiberboard is because of its
high perm rating. so adding OSB would defeat that purpose.
my main concern is that it provides enough rigidity. after all, this
is a half million dollar project, and here we are using a product that
is regarded by many as inferior to OSB. well, I guess i'll find out.
thanks for the replies.
If the building is designed for wind load only then the fiberboard product
should work fine. The literature I've read allows for up to 150 PLF
loading for this product, but only for wind load. I would like to spec
this product, but most of my designs are governed by seismic loads and the
product has not been tested for that type of loading.
Perhaps I was a little too flip about my earlier answer. For that I
apologize -- it has been a tough couple of weeks. I am now thinking you
could possibly use either let-in 1x4 wood bracing or perhaps Simpson Wall
bracing straps. These would stiffen the house with very little change in
Bob Morrison, PE, SE
R L Morrison Engineering Co
thanks, bob. we don't have seismic loads here (northern mn). here's a
question for you--what gives a stiffer frame--sheathing the corners
with osb or let in bracing?
at this point after talking to the owner and the architect, i think i
am going to try the fiberboard. if it seems to shaky, I'll sheath the
I'm guessing the OSB will give you a stiffer wall, but I think you will be
surprised at how stiff the fiberboard wall is when you are done.
Remember, it has a rated capacity of 150 PLF. That's 3000 pounds for a
20-foot long wall. I expect that the most noticeable deflection will
occur around openings. There is a way to add horizontal straps to
"reinforce" the openings in order to get the wall to be stiffer. I can
send you a detail if you contact me off list. See e-mail address below.
Bob Morrison, PE, SE
R L Morrison Engineering Co
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