Straightening a twisted frame

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If you are leaving a huge gap in the middle, surely the thickness of insulation is pretty secondary? I did a set of double doors recently that were very similar (4x2 100m celotex) and nailgunned them together as the strength was coming from the skin. Made an angled join in the middle so they shut flush though
I have two sets of bifold doors on my workshop, spanning 12 feet. Each door roughly 3 feet wide by 7 foot high. I used 4x2, but with the 2 inch being the thickness. Glued Bridle joints on all corners and dowelled cross pieces. Each frame glued and clamped between two sheets of 1/2 WBP ply, and clamped to carefully levelled trestles to make sure they were straight. 20 years on and still straight. Personally I think your problem is the storage time. If you get them from the yard straight then you want to use them straight away. Quite difficult to store them without them taking on a twist, and really asking for trouble if they have twisted and you still use them anyway.