Straightening bowed PIR board

[garyo]

Hi chaps,

Is there a knack to straightening a bowed PIR board? I have a few 1.2/2.4's 70mm non foil that have a 15mm bow and I really need them to lay flat against plasterboard. There's no simple way to apply fixings in the middle because I'm sliding them in to a stud cavity. The bow is across the short side.

I've done the obvious for now: they're on the floor with some weight on them, but I'm not confident.

Thanks

 

[cjard]

Had this exact situation after I picked up some bowed insulation from secondsandco for cheap. I think it had been made to fit an arched roof of some sort as the bow was perfect and identical in every board..

Might have to or someing at either end so your weight is bowing the other way. If it won't lie flat, then flip it over so it's like a U shape and weight the ends, then make several cuts with a Stanley knife across the board (so as to relieve the internal stress causing it to bow). Get a can of gun expanding foam, poke the gun tip right into the cut and gun as you remove it. Trigger off, move 3 inches down and repeat. The cuts that enabled the board to lie flat will now be filled with foam, and after it sets, the board will be flat

 

[garyo]

Thanks. I have around six like this and they're all bowed slightly differently, so I think mine are down to bad storage (an ex builder have them to me as a yard clearance when I was buying something else from him)

I'll see how my weights go and then take to it with a knife as you suggest.

 

[cjard]

My boards were 120 and bowed such that one end was at least 200mm off the floor when you stood on the other end. At 15mm yours are near perfect so I'm sure that the weighting will help though it may need some counter bending.