WATER/MOISTURE RESISTANCE OF A CLT STRUCTURE
Freshly sawn wood is usually saturated with water however, we understand that a high humidity concentration in an object lowers its moisture resistance rate. Moisture resistance is the ability of an object to resist absorbing moisture from its surrounding. Cross Laminated Timber also when in use is usually exposed to changes in relative humidity and temperature of the surrounding air, which induce changes in wood moisture content. Built-in moisture has a far greater effect on its durability since it increases the risk of mould growth within the timber. If the timber gets wet, the building needs to be managed so that the damp dries out quickly.
Moisture control consists of:
- Preventing water intrusion and condensation in areas of a building that must remain dry.
- Limiting the areas of a building that are routinely wet because of their use (e.g. bathrooms, kitchens) and drying them out when they do get wet
In response to this, during the manufacture of CLT, the built-in moisture in the timber is drained by seasoning.
Seasoning of timber is basically a long-term process of reducing its moisture content to a required level. Due to the porous permeable and hygroscopic nature in timber, vapor continuously escapes out of the timber through the pores causing them to reduce in size. If done naturally, this could take approximately 6months if subjected to good temperatures. For consistence in market supply, factories season huge chunks of timber in kilns so as to save time and increase production of timber with moisture content values.
CLT if not correctly constructed in a controlled environment with permeable insulation and a membrane layer without penetrations, accumulated moisture may lead to mould and rot. Therefore, emphasis is be put on the surface layer (vapour layer) to regulate the moisture content of the CLT by preventing water intrusion into the inner timber panels.
It is best practice to limit overheating of interior temperatures therefore insulation must be swapped to the interior to keep the CLT warmer and thus less at risk to reach high relative humidity levels. Also, one must take care of moisture during transportation, storage, installation and sealing up with protection from all moisture sources
Best tips in regulating temperature include:
- Reduce glazing from triple to double glazing and reduce solar gain.
- Continuous mechanical ventilation
- Airtight fabric to reduce humid air infiltrating into the assembly
- External shading to reduce solar gain
- External finishes to increase fabric emissivity
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