All our external folding doors have been re-tested for their U value ratings. And good news! -They are now more thermal efficient achieving better ratings than ever! Every door range has achieved a high rating making them more thermal efficient than ever before. The rating are now as follows: V44 Supreme has a new U value rating of 1.6 W/m2k V54 Elite has a new U value rating of 1.5 W/m2k V58 Solid Oak has a new U value rating of 1.4 W/m2k Make sure you choose the right folding sliding doors for you with a good U value for maximum thermal efficiency.