We present a class of porous polymer monoliths with high visible light transmittance and low haze synthesized using nitroxide-mediated stable free radical polymerization (SFRP), a form of controlled radical polymerization, to suppress large pore structures. Such materials can find applications in thermally insulated windows. Aerogels with adjustable optical and pore properties were achieved by modulating the nitroxide radical equilibrium. Controlled radical polymerization resulted in materials with average pore widths <10 nm, porosities of 43%, and high visible transmittance >60% (for 2.8 mm thickness). These results provide general design rules for radical polymerization of clear, colorless porous polymers.