The glass gets stronger as the temperature lowers. This is true of any amorphous material. The actual breakage point of any glass object is strongly dependent on the surface condition. breakage initiates in small scratches on the surface. Also the Chemical composition of any surface coat has a large effect. For example, this is why when you are cutting glass, it is a good idea to wet the scribe mark. The hydration forms a larger molecule which stresses the glass in microcracks. Also why your grandma's prized vase broke while you were washing it. microcracks plus hydration = stress. The point is that such considerations will overwhelm the changes in intrinsic strength over the short -20C to 50C temperature span. I may be able to help more if I knew what your problem is.