Handwoven fabrics, which made with cotton and silk, have unique characteristics and feel and are being produced in many parts of Thailand for making handicrafts such as pieces of clothing, embellishments, and home furnishings. It should be one of the largest income sources for many Thailand people, especially in rural areas (Fujioka, 2002) . In general, handwoven fabrics are not woven as tightly and strongly as machine-produced woven fabrics, resulting in fraying and shifting fabric shape

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... during the construction process. To solve this weakness, fusible polyester interfacing liner tends to be ironed onto the back of handwoven cotton and silk before cutting and sewing since it makes the handwoven fabric stiffer and keeps its shape. It results producers can cut handwoven cotton and silk easily and sew them with fabric less-distorted. However, it also affects the thickening of the fabric, resulting negatively to breathability and ability to transfer heat and humid between wearer body and an inner layer of clothing. Thailand locates in the tropics, which is hot and humid across the nation. This research began with the question if adding layers, like fusible interfacing liner, is appropriate in a hot and humid climate and why this technique has used for a while in Thailand. Limited studies have examined the thermal comfort of multi-layer fabrics, especially with fusible interfacing liner (Shabaridharan, 2012). Therefore, the purpose of this study to investigate if the use of fusible interfacing liner is appropriate and how it affects the thermal comfort of the wearer by measuring air permeability in handwoven cotton and silk. The air permeability, the degree of air in milliliters that is passed in one second through the material, is a very important consideration in the performance of fabrics. Air permeability helps with transporting heat and wetness from the skin to the surface setting (Ogulata, 2006) . Previous research has presented that air permeability mainly dependents upon the fabric's weight and thickness. Air permeability decreases when fabric thickness increase. Also, lightweight fabrics create better air permeability. Three different 100% handwoven cotton fabrics (CO) and six different 100% handwoven silk fabrics (SL), which are used popularly in Thailand, were selected as the surface fabrics and tested in two different conditions; 1) fabric only (FO) and 2) fabric with 100% polyester fusible interfacing liner (FFIL). The fusible interfacing liner was ironed onto the back of each of handwoven fabric specimen. To measure fabric thickness, AMES Digital Thickness Tester (model: BG1110-1-04) were used with the test standard ASTM-D1777. For fabric weight measurement, all fabric specimens were cut 5 inches in length by 5 inches in width. According to ASTM D3776/D3776M standards, a specimen should have a minimum of 20 square inch area. Fabric specimens were weighed by the Mettler Toledo weighing scale. Finally, to measure the air permeability, The TEXTEST Air Permeability Tester (model: FX 3300 LabAir IV) was used with the test standard ASTM D737-04. The specimen was placed under the test head and it was confirmed that there was no wrinkle. All measurements were repeated five times for each specimen and the average value of the variables was presented in Table 1 . As shown in Table 1 with a descriptive analysis, though adding fusible interfacing liner, fabric thicknesses of handwoven cotton and silk have increased by about 11% to