You have selected our website for Lithuania from United States. You would like to stay on this website?
The main differences between OSB 2 and OSB 3 and 4 are:
a. domain of use (service class):
b. Swelling limits → ≤ 20% (OSB 2) / ≤ 15% (OSB 3) / 10%-12% (OSB 4)
c. Strength (MOR) and stiffness (MOE):
Higher durability and moisture resistance are the most important advantages of OSB 3 compared to OSB 2.
As from the mechanical properties point of view (MOR, MOE, internal bond) there is no difference between OSB 2 and OSB 3, it is the swelling rate that makes the difference between the panels and limits the domain of use for each type of board:
According to the harmonized European design standard for timber structures EN 1995-1-1 (Eurocode 5) and to EN 335 (wood protection), the outside sheathing of external building components like roof and external walls is classified as usage class 2, requiring materials classified as service class 2, that is OSB 3 or OSB 4.
From this point of view, OSB 2 can only be used for internal structural floors and partition walls or for non-ventilated roofs, where the insulation is installed above the roof sheathing.
Due to its reduced swelling rate (≤15%), OSB 3 swells, shrinks and expands less than OSB 2 (≤20%), which means reduced stress-strain of the fasteners due to change in panel thickness, which further implies a reduced risk of squeaking.
Other advantages: better sound insulation and better reaction to fire of OSB 3 and OSB 4 TOP (class D-s2, do) compared to OSB 2 (reaction to fire class E) for thickness range 9-12mm, due to slightly higher density (≥ 600 kg/m3 versus ≥ 580 kg/m3).
The dimensional changes in thickness, length and width are normally calculated as % change (shrinkage/expansion/swelling) in length/width/thickness per each 1% difference in moisture content of the boards.
The difference in moisture content of the boards represents the difference between the equilibrium moisture content (EMC) that the boards are expected to reach in a given environment conditions, and the initial moisture content of the panels when delivered from the manufacturer’s warehouse.
Table 1 from CEN TS 12872 gives a general indication of the range of moisture contents of wood-based panels in various conditions, whereas Table 2 gives an orientation on the dimensional changes in length/width/thickness of wood-based panels expected at a 1% increase in the board’s moisture content:
Having these values, we can compare the dimensional changes due to moisture uptake of OSB 2 and OSB 3 panels (board size 2,440x1,220x11mm) installed as roof-sheathing (say SC2 conditions: RH ~75%, EMC ~ 15%), assuming that panels moisture content in dry state (when leaving the factory) is 6%:
First, there is the board composition:
Second, there is the structural behavior:
However, OSB is 40-60% more efficient than particle boards in terms of static design values for bending strength (MOR) and bending stiffness (MOE) → this means that generally we may choose smaller thicknesses of OSB in structural applications than for particle boards (ex: 12mm OSB instead of 15mm P5).
The use of OSB is limited to service class SC 1 (interior structural use in dry conditions, RH . 65%) and SC 2 (interior or external protected structural use in humid conditions, 65% . RH . 85%), whereas plywood can be used structurally both inside (in dry SC1 or humid SC2 conditions) and outside (SC3 highly humid conditions, RH > 85%), depending on its technical class.
The strength of plywood varies widely, depending on the wood species, number of layers and resin type. As an example, the modulus of elasticity on the major axis ranges 4,000-8,000N/mm2.
The bending strength and bending stiffness (modulus of elasticity) of solid wood is much higher than OSB’s, since solid wood is used for the structural elements of the timber frame (beams, studs, rafters), that should resist to all the static and dynamic loads that are transferred to them via the building’s floors (made of OSB-sheathing).
Just as an example, the design value of the MOE for solid softwood is ~ 10,000 N/mm2, whereas the design value for OSB 3 on major axis is 4,930 N/mm2 (or 6,780 N/mm2 for OSB 4 TOP).
In case of using softwood-glulam, this value goes up to 11,000 N/mm2, or up to 12,500 N/mm2 for hardwood-glulam (beech, oak). On the other hand, the MOE on minor axis is much lower for solid wood compared to OSB: