Insert Backer Rod into Gap

this piece of backer rod fits that space very well so I'll just cut this piece of back Rod you know put it there this is a piece of hybrid backer rod from sashco what you want is a sealant to to um or caulking to stick to the the two sides the two edges that way any expansion contraction it can stretch and flex so I'm going to end up using um sash Go's Big Stretch right here Big Stretch bonds extremely well has a lot of flex has a lot of stretch so so I'm going to stuff this back a rod down in there far enough now that I can get just a nice bead of sealant over the top of that