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Q: Building Material ( Answered 4 out of 5 stars,   0 Comments )
Subject: Building Material
Category: Science > Instruments and Methods
Asked by: leeferg2-ga
List Price: $5.00
Posted: 03 Nov 2005 08:34 PST
Expires: 03 Dec 2005 08:34 PST
Question ID: 588401
How long (years)is the adhesives and molding process used in OSB (wood) rated?
Subject: Re: Building Material
Answered By: redhoss-ga on 03 Nov 2005 14:40 PST
Rated:4 out of 5 stars
Hello leeferg2, I have wondered this same thing. I knew that OSB is
water resistant, but also that it had limitations. Here is a very good
discussion of OSB and moisture:


Oriented Strand Board (OSB) is a wood structural panel product. We are
often asked to comment on popular opinions regarding any special
sensitivity of OSB to water.

What Happens When OSB Gets Wet?
OSB is rated Exposure 1 for durability. Panels with this designation
are intended for protected construction uses not permanently exposed
to the weather, intended to resist moisture due to construction
delays, or other conditions of similar severity. In order to be marked
Exposure 1 OSB panels must meet specific (i.e. moisture cycling) bond
Technological advances in resin and wax formulation over the past few
years have resulted in panels that are much more resistant to the
effects of moisture and to swelling. For most applications, OSB panels
with Exposure 1 durability will offer excellent performance, even when
subjected to normal moisture conditions during construction. Today's
OSB has a degree of inherent moisture resistance, imparted by the
resin and wax formulation contained in the product. Panels are also
treated with a sealant on the edges to retard moisture penetration. In
addition, OSB is bonded with waterproof adhesives. In summary, OSB is
not extremely sensitive to small amounts of water, and it absorbs
water slowly.

In the event that you need a panel with superior moisture resistance
and durability, specialty OSB panels are now manufactured and
available. Careful decisions in design, construction and maintenance
will ensure you continue to get the performance you expect from OSB.

Preservative Treatment for OSB
When marginal moisture conditions are anticipated, the addition of low
levels of low-toxicity fungicides can be used to upgrade the
durability of OSB. For equivalent fungal resistance to
Douglas-fir-faced plywood, Forintek recommends a combination of 0.2%
zinc borate added during manufacture and 10 mg/cm2 Oxine copper
sprayed on the surface.
For applications where the moisture content remains above 28% for
extended periods, such as siding, higher levels of preservative (0.75%
zinc borate) are added to OSB during the manufacturing process. Zinc
borate additionally provides termite resistance. A higher level of
dimensional stability can also be designed into the product if these
conditions are anticipated. Preservative-treated OSB and wood
composite siding are now available in the marketplace.

Proper Handling of OSB on the Job Site
Panels should be protected from excessive wetting during storage and
construction. The panel edges are especially vulnerable and therefore
manufacturers protect the edges with paint or a special sealer. This
sealer is usually coloured. If a field cut of a panel slices away a
sealed edge, take extra care to protect this now vulnerable side
during construction. Manufacturers also recommend a 2mm (1/8") gap be
left between panels during installation to accommodate expansion. Do
not allow panels to sit in pools of water. Store panels indoors or
under cover, with enough support to keep panels flat. Schedule
delivery as close as possible to time of use and close in the
structure from the weather as quickly as practical.

Appropriate OSB Applications
Conventional OSB is suitable for all dry sheathing applications and
treated, stabilised OSB should be used where moisture tolerance may be
required. There are some applications where OSB is not advised by
manufacturers. OSB is not recommended for use where it would be in
sustained contact with a moisture source, such as soil or concrete
within 150mm of soil level.
Like most other wood products, OSB is designed for use under dry
service conditions. OSB is manufactured dry and should be transported,
stored and, ideally, installed dry. Installing dry is difficult in
rainy climates, however, brief periods of wetting should not cause
decay problems, provided OSB dries before the air barrier and cladding
are both added. Interestingly, in above-ground field tests of small
boards, OSB fully exposed to drying on both sides remains free from
decay for many years. Once the building is closed in, the ability of
the wall to dry is dramatically reduced. This means the OSB - and all
other components of the framing - should be kept dry through the life
of the wall. This is true no matter what the choice of construction

From reading this I understand that OSB will survive exposure to
moisture during construction and is not harmed "provided OSB dries
before the air barrier and cladding are both added". Assuming this is
done, this next reference gives the answer to your question:

The adhesives used in SIPs (a designation of panels of which OSB is a
member)  have a track record of more than 25 years, and SIPs have been
used in the USA for more than 40 years. Where the SIP system has been
tested and is protected from moisture, a life of a least 60 years is
expected. A recent European Technical Approval Guideline ETAG 19
provides performance criteria and tests for SIPs.

I would say that your answer is that OSB adhesives/panels would be
expected to have a life of at least 60 years. The key phrase is
"protected from moisture". If not protected from moisture, it appears
that an OSB panel would have a very short life.

This should answer your question. However, should you require further
info please ask for a clarification.


Request for Answer Clarification by leeferg2-ga on 04 Nov 2005 13:16 PST
This is more of a second question versus clarification, but if the
adhesives of OSB have expected life of 60 years, how does that compare
to traditional lumber (non-OSB) construction expectancy?

Clarification of Answer by redhoss-ga on 04 Nov 2005 13:49 PST
Another very good question. Some wood is more resistant to decay than
others and here are some good examples:

Exotic wood over 50,000 years old now available in the US. 

Now you can actually have and use the world's oldest wood
- Every tree grew for at least 1,200 years
- Every tree has been preserved underground for up to 50,000 years
- Every board is certified and guaranteed

The house first appears on St. Augustine's tax rolls in 1716, but it
was constructed before then. By 1788, the building was only "in fair
condition," according to a Spanish map of that time.

What I am saying is that I don't believe there is an exact answer to
your question. A friend of mine lives in a cabin that is 100 years old
and it is in very good condition.
leeferg2-ga rated this answer:4 out of 5 stars
Exactly what I was looking for.

There are no comments at this time.

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