A process for producing a polyurethane or polyisocyanurate construction board, the process comprising (i) providing an A-side reactant stream that includes an isocyanate-containing compound; (ii) providing a B-side reactant stream that includes a polyol, where the B-side reactant stream includes a blowing agent that includes a pentane and a blowing agent additive that has a Hansen Solubility Parameter (δt) that is greater than 17 MPa−0.5; and (iii) mixing the A-side reactant stream with the B-side reactant stream to produce a reaction mixture.