Cross laminated timber (CLT) is a relatively new construction method that allows multi-storey construction using solid wood instead of concrete or steel. It is a method favoured by progressive architects and recognized for its much lower environmental footprint. Wood is an excellent construction material and is a long-term carbon sink that also performs well in the event of earthquakes and fire. Obviously being made of a natural organic material, durability performance against mold and wood destroying organisms is a concern. Even if it were possible to permanently keep the wood dry to prevent wood rotting basidiomycete fungi, some pests are able to attack wood that is wetted temporarily (e.g. mold fungi) or even relatively dry wood (e.g. subterranean termites, wood destroying beetles, dry wood termites and carpenter ants. In most parts of the world there is an active remedial treatment market dealing with decay in structures that were also supposed to be kept dry. In the USA damage caused by decay fungi is thought to be even greater that that caused by subterranean termites (Smith and Whitman 2007). The relatively modern use of indoor plumbing and air conditioning has increased interior moisture issues due to water leaks and condensation. In single family residential construction, when problems occur it is relatively easy to carry out remedial treatment of infested structural wood and wood replacement. Even whole structural subfloors sometimes need replacement due to decay caused by high humidity in conjunction with air conditioning. However, it is NOT easy in multi-storey mass timber construction. For this reason, a global representation of wood scientists has discussed this issue and even signed an open letter on the need to address durability of mass timber construction (Taylor et al. 2016). Countries with subterranean termite problems typically have good standards that address them. Soil poisons, primary barrier wood treatments (to a certain height of the structure) and physical barriers, are well entrenched as control methods (Lloyd et al., 2015). However, none of these methods control the other wood destroying organisms. Wood treatment applied to the bottom ~1 meter of a structure was developed originally in Japan and, with glycol borates, is now the leading commercially applied termite preventive treatment in the USA (to meet the US Department of Housing and Urban Development, State and building code requirements). This can be extended to whole house wood destroying organism control (Lloyd et al., 2015). Possible approaches to address durability include the use of naturally durable wood or pressure treatment of wood with appropriate preservatives. A topical treatment (to avoid significantly re-wetting otherwise dry wood) with a diffusible preservative (to move in the event of enough moisture to support decay) and a moldicide may also be adequate. Such treatments could be carried out by spray, dip or double vacuum application and do not typically cause dimension changes (Jermer & Lloyd, 2000). Borates (especially glycol borates for dry wood e.g. Turner, (2008)) have the advantage of being effective against both decay fungi and insects at low retentions (Lloyd, 1997) and are even bait toxicants against subterranean termites at low retention (e.g. Jones, (1991). Such approaches have been successfully used in residential construction such as under the old German Standard DIN 68-800 where lumber was dipped in a 10% disodium octaborate tetrahydrate solution. The use of an inorganic salt as the active ingredient is also good in topical applications where organic systems can be destroyed by UV light during construction (Lloyd et al., 1998). Even non wood destroying insects such as psocids and cockroaches can be controlled via the use of borate treated wood (Lloyd et al., 2003). Borates have low acute mammalian toxicity are globally available from a number of suppliers. To enhance moldicidal performance there are a number of good active ingredients including quaternary ammonium compounds, carbamates, chlorothalonil, azoles and isothiazolinones. An example of such a treatment includes the Arch/Lonza Frame Guard system commercialized in the USA (Lloyd, 2015). Using topical treatments also has the added advantage of being able to change formulations relatively easily (for example you could use different borate concentrations for geographies with and without subterranean termites) and to ensure that all of the construction components are treated (the mass timber itself but also framing and plywood and many other SKUs that typically go into a building. A 40% disodium octaborate tetrahydrate in glycol formulation (commercially available as Boracare from Nisus Corporation) has performed very well in 10- and 15-year field tests in Mississippi (Williams & Amburgey 2003) and Florida (Barnes et al., 2014) against subterranean termites, in preventive and remedial drywood termite tests (Austin & Gold, 2006; Lewis 2012), and in decay tests such as AWPA E 10 (Amburgey et al., 2004). Formal testing was also carried out in Japan as part of the JWPA approval procedure. In addition, a field test was carried out specifically for the Shimojishima Airport project. A separate series of diffusional penetration and rain exposure leaching tests that we carried out in Japan were also useful. Mitsubishi Estate Co., Ltd. organized the construction which was designed by Nikken Sekkei Ltd. architectural office and built by local contractors for Mitsubishi Estate Co. Ltd. The commercial operations of the Airport are carried out by Shimojishima Airport Management Co. Ltd. Topical treatment was carried out using a 40 % DOT glycol borate diluted at a 1 to 1 volume dilution in water (Boracare to equal a 23 % DOT solution concentration). 0.8 % diecyl dimethyl ammonium chloiride (1% as Moldcare available commercially from Nisus Corporation) was added as a surface moldicide. The diluted material was applied with roller coating at Yamasa Mokuzai Co. Ltd., the CLT manufacturer. An application of 300ml m2 was used in compliance with JWPA requirements to treat 6,000 m3 of CLT. This is a higher concentration and higher application rate than required by US EPA registered label against all non-subterranean termite organisms but was chosen to help drive diffusion over time, to give protection against subterranean termites, to enhance spread of flame performance, and to supply some reservoir to compensate for dilution due to diffusion into the wood. All testing and treatment results went well, and the construction is now complete. The inaugural direct flight from Narita to Shimojishima was March 30th 2019 and many more flights will be able to land in an airport that is effectively storing carbon, and will remain free of drywood termites and other wood destroying organisms for the decades to come. If mass timber and especially CLT is to gain the position it deserves in future construction, it is essential that long term durability is addressed. This project has shown that is relatively easy and cost effective to do so.

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