Saw this posted on another forum I frequent, and thought it would be relevant to every one here as well.
- A Contractor Talk forum had this comparison:
Here are the highlights to support my utilization of CDX instead of OSB.
OSB swell is generally greater than in plywood due to the release of compaction stress in OSB created during the pressing of wood chips into an OSB panel. Plywood that has swollen will return to its nominal thickness as the wood dries. OSB will remain swollen to some degree after it dries because the panel will still have the higher "compaction ratio" that was present as of the date of manufacture.
The comparison below, undertook by the APA, lists the thickness swell (in percent) using a water soak test.
Plywood Average swell= 6% to 8% depending on thickness
OSB Average swell= 10% to 15% depending on thickness
Over a period of time, when subjected to high humidity or a series of dramatic wetting events, OSB is more prone to panel swell than plywood especially at the edges. Panel swell is most noticeable along the edges where it is critical for flooring and roof sheathing to match-up as not to show through the flooring or roofing materials.
Research done by the USDA Forest Products Laboratory (Research Paper FLP-RP-574) shows that over an extended period of time, under low constant loads and temperature, and in high-humid conditions, OSB will sag or "creep" more than plywood.
50% RH Fractional creep values
85% RH Fractional creep values
Cyclic RH 50%-85% creep values
Long-term, plywood sags less than OSB in humid conditions. In humid areas with vented attics and crawl spaces, using plywood for roof and floor sheathing can reduce risk of sagging roofs and floors.
APA Technical Note N375B states that plywood panel bending stiffness is 10% greater than OSB at equal joist spacing. Panel bending stiffness is the capacity to resist deflection.
Density of plywood is 34-36 pounds per cubic foot compared to OSB at 38-42 pounds per cubic foot.
One 23/32" 4'x 8' plywood piece would weigh approximately 67 lbs.
One 23/32" 4'x 8' OSB piece would weigh approximately 78 lbs.
Plywood is approximately 15% to 19% lighter than OSB. While the additional weight of OSB does not mean increased strength, it just means that it is heavier to handle on the job. In addition, OSB's higher weight means higher thermal conductivity (thus slightly less R value) than plywood.
Plywood and OSB: Screw withdrawal or holding ability.
Group 1 plywood, made from the strongest species of wood such as Southern Yellow Pine, holds screws better than OSB.
Plywood and OSB: Nail withdrawal or holding ability.
As shown in APA report T2001-3A, plywood generally has higher nail withdrawal values using plain-shank, ring-shank, and screw-shank nails, even though OSB has a higher density than plywood. The numbers below reflect dry test conditions. The numbers vary some when tested under dry to wet-redry conditions.
Withdrawal strength (lbs/in. penetration)
Plain Ring Screw
Mean: 79.7 316.3 83.7
Mean: 67.6 281.5 63.9
Nail withdrawal strength measures the force to pull the embedded nail from the nailed parts.
After roof sheathing or sub floor panels are nailed to the framing, nails will remain in place better in plywood than OSB.
Under severe weather testing, plywood is shown to be more impact resistant than OSB. Plywood outperforms OSB in the South Florida Building Code (Dade and Broward Counties) mandated use of the "large missile" impact test for materials used in walls and roofs.
Plywood is more impact resistant than OSB. This provides resistance to flying objects in high wind situations. More impact resistance provides added durability against accidents caused by impact on floors, roofs, and walls on the inside and outside of the home.
Check out this link for more information.