The effective heat conductivity (lambda) of reacting melter feed affects the heat transfer and conversion process in the cold cap, a layer of reacting feed floating on molten glass. A heat conductivity meter was used to measure k of samples of a cold cap retrieved from a laboratory-scale melter, loose dry powder feed samples, and samples cut from fast-dried slurry blocks. These blocks were formed to simulate the feed conditions in the cold-cap by rapidly evaporating water from feed slurry poured onto a 200 degrees C surface. Our study indicates that the effective heat conductivity of the feed in the cold cap is significantly higher than that of loose dry powder feed, which is a result of the feed solidification during the water evaporation from the feed slurry. To assess the heat transfer at higher temperatures when feed turns into foam, we developed a theoretical model that predicts the foam heat conductivity based on morphology data from in-situ X-ray computed tomography. The implications for the mathematical modeling of the cold cap are discussed.