8.1 In common with all timber products OSB is subject to moisture movement. As a guide, an increase in
moisture content of 1% increases the length and width of a board by 0.3 mm per metre run.
8.2 Under similar environmental conditions, OSB will take longer to equilibrate and will attain an equilibrium moisture
content approximately 2% to 3% lower than solid timber.
8.3 To avoid distortion and damage to finishes, movement gaps, in accordance with the recommendations of
BS 8103-3 : 2009, should be provided when installing the board.
8.4 To minimise subsequent movement, before installation all wet site operations should be completed and the boards
conditioned as close as is practicable to the environmental conditions likely to occur in service. To achieve this, the
moisture content of the board prior to installation, determined with a properly-calibrated moisture meter, should be close
to the service class equilibrium moisture content (emc) values given in DD CEN/TS 12872 : 2007, Table 1:
• service class 1 4% ≤ emc ≤ 11%
• service class 2 11% ≤ emc ≤ 17%
• service class 3 emc > 17%.
8.5 Damp-proof membranes and vapour control layers should be incorporated as necessary in accordance with the
requirements of BS 8103-3 : 2009 and BS 5250 : 2011.
8.6 The water vapour resistance factor (μ) of OSB, as given in BS EN 13986 : 2004, should be either taken as the
design value given in BS EN 12524 : 2000 [30 (wet cup), 50 (dry cup)] or determined in accordance with
BS EN ISO 12572 : 2001. Such values may be used in any interstitial condensation calculations to BS 5250 :
- The experimental value of water resistance factor determined in accordance with BS EN ISO 12572 : 2001
(wet cup) for the 9 mm thick Sterling OSB/3 board is 219.
8.7 In accordance with normal good practice for wood-based sheathing materials used in cold frame construction,
external walls in which the product is incorporated must include an effective vapour control layer on the room side,
suitable weather protection on the outside surface, a ventilated cavity and damp-proof courses. The product should be
treated as conventional plywood sheathing with regard to detailing at openings, eaves and sole plate, the fixing of
wall ties and breather paper, and the effect of openings on racking strength.
8.8 The moisture content of sheathing material is affected by the humidity conditions existing in the cavity of which
it forms one face. The cavity should be of conventional construction for timber framed buildings, freely drained and
ventilated. The outer masonry leaf should have adequate resistance to wind-driven rain, particularly in regions classified
as severe exposure. Raked mortar joints or high-porosity masonry should be avoided, particularly in these latter areas.