A carpet knife can work well for cutting away old caulking from house siding. Slice down alongside it from both directions with the hook-like blade, then use the knife to lift out the old caulk bead intact.
Most greenhouses have frames of wood or aluminum. Wood will lose no significant heat through the frame. Over time the wood fades unless you keep it painted or finished, and it may rot or warp. Aluminum frames retain shape well, won’t corrode, and are often color-coated. Some heat is lost through the aluminum.