We are replacing the original double hung windows in our 1920's house with vinyl replacements. The weight pockets are not accessable from the front wall (behind the moulding) so we are planning to use expanding polyurethane foam (think "Great Stuff") to fill the voids. We like the high R-value and the closed cell sealant properties. Does anyone have experience or opinions on this method of insulation? Advice on where to purchase in bulk or which products work best (we have 30 windows). Thanks.