The fenestration structures include external frame segments facing the interior and exterior surfaces of a building wall, and an internal frame segment 'sandwiched' between the external frame segments. The external frame segments are made from a relatively hard, dense and/or resilient material, such as hard wood, while the internal frame segment is made from paulownia which is a moisture-resistant, dimensionally stable thermal barrier material. The external frame segments therefore provide durable and attractive exterior and interior surfaces of the fenestration frame, while the internal paulownia segment provides superior resistance to thermal transfer across the frame, while also remaining dimensionally stable across a variety of moisture and temperature levels. Internal paulownia segments may be positioned around the entire periphery of the fenestration frame to create a comprehensive thermal barrier.

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