We present WinSyn, a dataset consisting of high-resolution photographs and renderings of 3D models as a testbed for synthetic-to-real research. The dataset consists of 75,739 high-resolution photographs of building windows, including traditional and modern designs, captured globally. These include 89,318 cropped subimages of windows, of which 9,002 are semantically labeled. Further, we present our domain-matched photorealistic procedural model which enables experimentation over a variety of parameter distributions and engineering approaches. Our procedural model provides a second corresponding dataset of 21,290 synthetic images. This jointly developed dataset is designed to facilitate research in the field of synthetic-to-real learning and synthetic data generation. WinSyn allows experimentation into the factors that make it challenging for synthetic data to compete with real-world data. We perform ablations using our synthetic model to identify the salient rendering, materials, and geometric factors pertinent to accuracy within the labeling task. We chose windows as a benchmark because they exhibit a large variability of geometry and materials in their design, making them ideal to study synthetic data generation in a constrained setting. We argue that the dataset is a crucial step to enable future research in synthetic data generation for deep learning.