The physical properties of Galactic cirrus emission are not well characterized. BOOMERANG is a balloonborne experiment designed to study the Cosmic Microwave Background at high angular resolution in the millimetre range. The BOOMERANG 245 and 345 GHz channels are sensitive to interstellar signals, in a spectral range intermediate between FIR and microwave frequencies. We look for physical characteristics of cirrus structures in a region at high Galactic latitudes where BOOMERANG performed its deepest integration, combining the BOOMERANG data with other available datasets at different wavelengths. We have detected 7 emission patches in the 345 GHz map, consistent with cirrus dust in the IRAS maps. The analysis technique we have developed allows to identify the location and the shape of cirrus clouds, and to extract the flux from observations with different instruments at different wavelengths and angular resolutions. We study the integrated flux emitted from these cirrus clouds using data from IRAS, DIRBE, BOOMERANG and WMAP in the frequency range 23–5000 GHz (13 mm to 60 μm wavelength). We fit the measured spectra with a combination of thermal and non-thermal spectra considering two models for the thermal emission. The first model assumes the emission to be isothermal with a variable spectral index. The second model considers two temperatures in the cloud, both the components spectral indices being set to 2. The two models are statistically equivalent and the estimated temperatures are consistent. A 10 K component has been detected at high latitudes. In our sample, assuming the isothermal model, the temperature of the thermal component varies in the 20 – 25 K range and its emissivity spectral index is in the 0.5 – 1.5 range. The spectral index of the non-thermal emission at lower frequencies covers the -1.6 – -2 range in antenna temperature. We could not identify a clear physical relation between temperature and spectral index as had been proposed in previous works. This technique can be proficiently used for the forthcoming Planck and Herschel missions data.