A process for producing a polyurethane or polyisocyanurate construction board, the process comprising of providing an A-side reactant stream that includes an isocyanate-containing compound; providing a B-side reactant stream that includes a polyol, where the B-side reactant steam includes a blowing agent mixture including isopentane and a blowing agent additive that has a Hansen Solubility Parameter (δt) that is greater than 15 MPa0.5; and mixing the A-side reactant stream with the B-side reactant stream to produce a reaction mixture.