The reinforced steel stabilisors along all 4 sides of the blank mean that this product is stable like no other! In independent tests the blank achieved the best possible value, "Tolerance class 4". This blank is therefore virtually guaranteed not to bow, and indeed, we offer a 10 year guarantee against it doing so. Claims against this product for bowing are a thing of the past. We are also able to supply this product with an aluminium moisture barrier as well as with a special PU Foam, which gives the blank even better insulation properties. The Plus in the FERRO ClassicPlus: The high level of insulation! This product is a piece of quality engineering: the 68mm blank attains for example an U-value(W/m2K) of 0.96 according to EnEV! The only blank better than this is our own "Climate champion" - the FERRO Passiv.