In September, Inline will release a Double Hung with a U Value of 0.16 using just triple glass. It also boasts a U value of 0.17 and SHGC of .46. The air infiltration is a .07 which is better than virtually all fiberglass casements. I am not aware of a better fiberglass double hung window.

Make sure it's clear what measurment they're using for U-value (center of glass or whole unit).

To be clear, it's the whole unit. Please visit the NFRC website under 9000 double hung for all test results.

UnitedHomeExperts, we only use whole unit to encourage consumers to compare products, apples to apples.

Thank you for bringing this up.

http://search.nfrc.org/search/cpd/cpd_search_detail.aspx?cpdnum=IFL-A-19