Materials Wood-based panels
Service class
Temperature
Approx RH*
EMC**
Examples from the UK National Annex to Eurocode 5 [7]
‘humid’) that has a coating system and an enhanced MDF.H (non-structural ‘humid’) that uses chemically modified fibres, allowing it to be used uncoated in very harsh environments such as in contact with the ground.
1
20°C
65%
12%
Warm roofs, intermediate floors, timber-frame walls – internal and party walls
Biological durability
2
20°C
85%
3
Conditions leading to higher moisture contents than service class 2
20%
Cold roofs, ground floors, timber-frame walls – exterior walls, exterior uses where member is protected from direct wetting
>20%
Exterior uses – fully exposed
*RH Relative humidity: level not to be exceeded for more than a few weeks each year **EMC Maximum equilibrium moisture content for most softwoods. In similar conditions the EMC of panel products will typically be lower
Figure 1: Service classes and examples from Eurocode 5
referred to as ‘moisture resistant’. This doesn’t mean they are waterproof but just that they have an enhanced resistance to breakdown in the presence of moisture. Their structural integrity will break down given enough time at elevated moisture contents (i.e. exposure to direct wetting as opposed to the occasional high humidity for which they are designed). Panels of this description are particularly intended for internal situations, where there is a risk of accidental wetting or high humidity, for example in bathrooms and kitchens. Painting this type of product without any manufacturer guarantee will enable its use in limited exterior situations but not as a construction product – only where it is for temporary use and where its failure does not pose a risk, for example a site hoarding. Indeed there are proprietary OSB products available for use as site hoardings that come with factory primed surfaces for painting on site. There are a few innovative products available that can be used in exterior situations. These fall outside European classification because they are new and have not been in circulation long enough for them to be officially standardised. These will have been tested and certificated to demonstrate their effectiveness in specific end use exposures, and have to be installed and maintained according to manufacturer requirements. The type of products that fall into this category would be particleboard and MDF, because there are no official ‘exterior’ grades for these products. For example, there is an enhanced P5 (structural ‘humid’) particleboard that has a weather protective coating to protect it during construction. Then there is exterior MDF (non-structural www.trada.co.uk
Once a suitable grade (dry, humid or exterior) in terms of the resin binder’s capability to resist moisture has been selected, the durability (biological durability) of the wood used to make wood-based panels should be considered. The predominant wood type used in the manufacture of particleboard, OSB and MDF is softwood, which has a low natural biological durability. Hardwood plywood will have durability relevant to its wood species heartwood. This would be reduced where there is a proportion of sapwood present as all sapwood is non-durable. Wood-based panels by their nature will generally use all of a log, which means they will contain both sapwood and heartwood. There are exceptions – such as marine plywood made to BS 1088, where heartwood is used in the build-up of the product – but apart from that all wood-based panels will contain sapwood and can therefore be considered non-durable. This is where preservative treatment, the use of modified wood or simply painting can come into play to enhance durability. >>
Cabershield Plus is a waterproof P5 flooring grade particleboard system. Photo: Norbord
Timber 2018 Industry Yearbook
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