A formulation system comprising: (a) a pre-polymer that is a reaction product of a polymeric isocyanate and a polyether polyol having an average functionality of three or more and eight or less, where: (i) the polymeric isocyanate contains five weight-percent or more methylene diphenyl diisocyanate based on total polymeric isocyanate weight; (ii) the polymeric isocyanate has a functionality of 3.0 or less; (iii) the polyether polyol is present in the pre-polymer at a concentration of one weight-percent or more and 25 weight-percent or less based on total pre-polymer weight; (iv) the polyol has an equivalent weight of 50 or more and 500 or less grams per equivalent; (v) the -NCO concentration of the pre-polymer is 15 weight-percent or more and 31 weight-percent or less based on total pre-polymer weight as determined according to ASTM D2572; (vi) The pre-polymer is free of isocyanate trimers as determined by nuclear magnetic resonance spectroscopy; and (b) a polyester polyol component comprising 10 weight-percent or more and 25 weight-percent or less of free glycol based on total polyester polyol component weight and the balance being polyester polyol; and (c) a blowing agent that contains less than five weight-percent water based on total blowing agent weight; where the ratio of pre-polymer and polyester polyol are such that, when mixed together they result in foam having a trimer content of 12 weight-percent or more and 22 weight-percent or less based on total polymer weight in the foam, and an -NCO index of more than 300 and less than 700. (a) a pre-polymer that is a reaction product of a polymeric isocyanate and a polyether polyol having an average functionality of three or more and eight or less, where: (i) the polymeric isocyanate contains five weight-percent or more methylene diphenyl diisocyanate based on total polymeric isocyanate weight; (ii) the polymeric isocyanate has a functionality of 3.0 or less; (iii) the polyether polyol is present in the pre-polymer at a concentration of one weight-percent or more and 25 weight-percent or less based on total pre-polymer weight; (iv) the polyol has an equivalent weight of 50 or more and 500 or less grams per equivalent; (v) the -NCO concentration of the pre-polymer is 15 weight-percent or more and 31 weight-percent or less based on total pre-polymer weight as determined according to ASTM D2572; (vi) The pre-polymer is free of isocyanate trimers as determined by nuclear magnetic resonance spectroscopy; and (i) the polymeric isocyanate contains five weight-percent or more methylene diphenyl diisocyanate based on total polymeric isocyanate weight; (ii) the polymeric isocyanate has a functionality of 3.0 or less; (iii) the polyether polyol is present in the pre-polymer at a concentration of one weight-percent or more and 25 weight-percent or less based on total pre-polymer weight; (iv) the polyol has an equivalent weight of 50 or more and 500 or less grams per equivalent; (v) the -NCO concentration of the pre-polymer is 15 weight-percent or more and 31 weight-percent or less based on total pre-polymer weight as determined according to ASTM D2572; (vi) The pre-polymer is free of isocyanate trimers as determined by nuclear magnetic resonance spectroscopy; and (b) a polyester polyol component comprising 10 weight-percent or more and 25 weight-percent or less of free glycol based on total polyester polyol component weight and the balance being polyester polyol; and (c) a blowing agent that contains less than five weight-percent water based on total blowing agent weight;