Method for increasing dimensional stability of a paper or a board product

A method for producing of a paper or a board product comprising:

• providing never-dried fibres;
• providing a strength composition comprising a permanent wet strength resin component and a sizing agent;
• adding the permanent wet strength resin component to the never-dried fibres, thus providing treated never-dried fibres;
• combining the treated never-dried fibres with a fibre material selected from recycled fibres, fibres originating from broke, dried fibres and/or fibres produced by mechanical pulping, thus providing a fibre slurry comprising 15-70 weight-% of the treated never-dried fibres, based on a total dry weight of the fibre slurry;
• forming a treated fibre slurry by treating the never-dried fibres with the sizing agent of the strength composition before the combining step or treating the fibre slurry with the sizing agent of the strength composition;
• introducing the treated fibre slurry to a forming section to produce a web;
• and
• introducing the web into a press section for producing a paper or a board product.