Preservative treatments suitable for hardwood window joinery

Abstract

An eight-year model window exposure trial was used to determine the best treatments available for hardwood windows. Severe decay was found in painted untreated Eucalyptus regnans, Eucalyptus delegatensis, and Shorea spp. window frames. Light or light-moderate decay was detected in Eucalyptus sieberi and Eucalyptus obliqua frames. There was generally only light decay in Thuja plicata windows, although two windows had deeper patches of decay. Only light decay was found in eucalypt windows treated by vacuum pressure impregnation (VPI) with azole LOSP (light organic solvent preservative) when they were painted. There was no difference in performance of azole LOSP-treated E. regnans windows when painted cream or brown. However, similar unpainted windows had moderate to heavy decay. E. regnans windows dipped in azole LOSP performed much better than untreated windows. TBTN-treated E. regnans windows performed worse than azole LOSP-treated windows. The VPI treatment of unseasoned E. obliqua, E. regnans, and E. delegatensis with boron to meet H1 requirements for lyctine borer control also mostly protected the windows from significant decay. The retention of boron achieved in E. sieberi did not improve decay resistance. When the ends cut after VPI boron treatments were resealed by dipping in boron solution, there were nine examples of end-grain decay. In comparison, all of the azole LOSP-treated eucalypts where cut ends were resealed by a 3-min dip in azole LOSP were sound. The least decay for windows was found in E. regnans that contained borate/fluoride diffusible preservative rods.