A shell system for a building, facilitating internal bottom-up flow of air inside the shell system, the shell system includes a plurality of modular thermal panels, and connecting means for interconnecting the plurality of modular thermal panels or a portion thereof Each of the modular thermal panel includes an enclosed frame having two side faces, a top face and a bottom face, wherein two openings are formed in the frame's faces: a sealingly enclosed internal face and a sealingly enclosed external fac wherein an inner gap, filled with air, is formed between the internal face and the external face. At least one frame-opening is formed in each of the faces of the frame, allowing air to flow between adjacent modular thermal panels, that are sealingly interconnected, while allowing the inner air flow.