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I am about to have my windows replaced at my home. The openings are all block frame, which from what I understand generally means that the exterior trim of the window doesn't need to be disturbed.

A previous owner had aluminum siding installed on the house. Unfortunately, the installer did a rather poor job where the siding meets the trim. In areas, there's a rather large gap between them and, as a remedy, a large amount of caulk has been used to fill the space.

Since I'm having the windows replaced, now seems to be the appropriate time to also clean this up. One of the window salesmen that visited my house recommended it just being ripped out and recaulked more carefully with new trim. I've also heard of something called j-channel being used but I'm not sure whether this is just for vinyl siding. What's the most pragmatic way of dealing with this?

Below is an example. This particular example doesn't have too bad of a gap (from what I can tell) but it gives you an idea of the existing state.

caulking

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Both vinyl and aluminum have similar issues regarding thermal expansion. The siding must be installed with a gap to fixed objects in order for there to be room for expansion. If this is not done, the siding will expand against the obstruction. Not being able to move in that direction, it will buckle along it's length, causing unsightly bulges.

Therefor, J-channel is a common element in installation of both materials and is readily available in both aluminum and vinyl. I would consider J-channel the best solution, but it does involve more work than simply re-caulking.

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I spoke with two different contractors and they both independently suggested caulking or gluing the J-channel to the trim board as a retrofit solution. Thoughts? –  YWCA Hello Nov 20 '13 at 18:00
    
That's about all you can do, you cannot properly nail J-channel when retrofit like this. The process involves removing the trim, inserting the J-channel with its long leg behind the siding. Caulk (my preference) is applied to the channel where it meets the trim. The trim is pushed into the caulk and then the trim is nailed in place. Be sure the J-channel is not pushed tight to the siding when installing the trim, that sizable gap to siding needs to be there. –  bcworkz Nov 20 '13 at 23:18

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