Inorganic polymer cement paste was used as cleaner binder for the design of lightweight matrices as insulating envelopes and panels in building and construction industries. Sponge-like structure with a homogeneously distributed pore network, low density and low thermal conductivity permitted to classify the geopolymer-wood fiber composites promising clean insulating materials. Matrices with the density similar to 0.79 g/cm(3),bi-axial four-point flexural strength of similar to 4 MPa presented thermal conductivity between 0.2 and 0.3 W/(mK). The possibility of substituting the sodium silicate with rice ash-NaOH system and the efficiency of the matrices to constitute an effective tortuous road for the thermal gradient improved the sustainability and quality of this new class of products. The pores network and the microstructure approximated by a spatial periodic geometry suggested a "macro transport" mechanism to explain the movement of heat across the matrix of light geopolymer composite. (C) 2015 Elsevier BM. All rights reserved.