Hydrocarbon mixts. used to prepare low density foams. The hydrocarbons include alkanes, alkenes, alkynes both in linear, branched or cyclic form as well as having multiple conjugated or non conjugated unsaturation as well as aromatic cpds. Hetero atoms introducing other functional gps. such as ketones, ethers, esters, amides can be included. The low density foams include polymeric inorganic and organic materials which have densities below 300 kg/m3. The cpds. can be in the liq. or gaseous phase at room temp. A liq. blowing agent at room temp. is combined together with a gaseous blowing agent at room temp. The compsn. of this mixt. is such that the gaseous blowing agent can at least partially be dissolved into the liq. blowing agent with or without the use of pressure. More specifically these mixts. are used to prepare polyurethane foams with densities below 100 kg/m3 which can be open or closed celled. Particular rigid closed celled foams are of interest since these are know n for their thermal insulation properties which require gasses with a low thermal conductivity to be captured inside the foam structure (the thermal conductivity of the gas being below 25 mW/mK). In the case of polyurethane foaming CO2 gas may additionally be present which is formed in situ during foaming by for instance the reaction of isocyanate gps. with water. A particular example of a hydrocarbon mixt. is the combination of iso-pentane (the liq.) with iso-butane (the gas). Typically the ratio between iso-pentane/iso-butane is 99/1 - 50/50. When sufficient pressure is used this expands up to 20/80 ratios. This mixt. can be either on the polyisocyanate or the polyol side or divided over both sides or added separately via yet another stream be used to make the polyurethane foams. Another example of mixts. include the use of iso-pentane with HFC134a (1,1,1,2-tetrafluoro ethane, a gaseous cpd.) with an opt. addition of acetone. Another possibility is the use of C3-C5 HFC, e.g. HFC245fa, in combination with C3 to C7 hydrocarbons.