One-component systems (I) in pressurised containers, for prodn. of rigid polyurethane (PU) foam, comprise mixts. of (a) NCO gp.-contg. prepolymers obtd. by reaction of polyisocyanates with cpds. of mol.wt. 62-10,000 contg. at least 2 NCO-reactive H atoms, (b) known additives etc., (c) volatile liq. propellants with b.pt. below 0 deg.C/1-13 hPa, and (d) 0.25-2.5 wt.% (w.r.t. a) liq., satd. aliphatic hydrocarbons with dynamic viscosity 110-230 mPa.s at 20 deg.C and density 0.845-0.890 g/m3. Also claimed is rigid PU foam obtd. by hardening system (I) by the action of moisture after it leaves the pressure can. USE/ADVANTAGE - Used (esp. in aerosol cans) for filling applications, e.g. between window frames and walls, holes in walls etc., and fixing, e.g. door frames. Incorporation of (d) prevents the usual change in vol. or thickness due to diffusion processes, changes in temp. and/or humidity sec., and gives 1-component PU foam with improved dimensional stability.