Cross-laminated timber (CLT) has become popular in Europe for the prefabricated construction of wall, roof and flooring elements. The use of CLT in North America is gaining interest in both the construction and wood industries. Several North American manufacturers are in the process of product and manufacturing assessment or have already started pilot production.
For general principles of durability by design, the Best Practice Guide for Wood-Frame Envelopes (CMHC, 1999) and the Building Enclosure Design Guide – Wood-Frame Multi-Unit Residential Buildings (HPO, 2010) should be referred to for the design and construction of CLT buildings. The use of prefabricated CLT panels does not change the basic heat, air and moisture control design criteria for an exterior wall or roof assembly. However, different from conventional stick-built wood-frame buildings, the design of CLT building enclosures requires additional attention due to the unique characteristics of the product. CLT panels are massive solid wood elements and therefore have low vapour permeability and may provide a considerable level of insulation. They have a certain level of inherent air tightness but usually require an additional air barrier. The panels may absorb a large amount of moisture when exposed to excessive wetting and the consequent drying may be slow due to the mass of wood in such panels.
This chapter focuses on best practice heat, air and moisture control strategies for wall assemblies that utilize CLT panels in North American climate zones. The overlying strategies are to place insulation in such a way that the panels are kept warm and dry, to prevent moisture from being trapped or accumulating within the panel, and to control airflow through the panels, and at the joints and interfaces between them.
It is intended that these guidelines should assist practitioners in adapting CLT construction to North American conditions and ensuring a long life for their buildings. However, these guidelines are not intended to substitute for the input of a professional building scientist. This may be required in some jurisdictions, such as Vancouver BC, and is recommended in all areas at least until such time as CLT construction becomes common practice.