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Replacing aluminum windows with vinyl in aluminum siding do I caulk j channel?

Replacing aluminum windows with vinyl in aluminum siding do I caulk j channel?

Old 08-01-11, 12:25 AM
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Replacing aluminum windows with vinyl in aluminum siding do I caulk j channel?

Hello I am getting ready to replace 4 aluminum windows with vinyl windows. I will be removing the aluminum siding and using new construction windows. With the new construction windows I would use flashing tape on pan first then the top and side flanges after installing the window.

My question is does the aluminum siding go in the window's built in J-channel? Home depot guy told to make j channel fillers with aztek plastic trim and caulk them? This makes no since to me. Do I caulk the J-channel tops and sides to prevent water from getting behing the siding. My house has that Thermoply foil cardboard stuff under the aluminum siding. I thought if water gets behind the siding the thermoply is the water barrier? The current aluminum windows have no j channel and the aluminum siding is caulked as it buts up to the windows(looks like hell).

Residing in vinyl is not an option at this time. I cant afford 18 windows and siding right now. My thought is to replace windows first(bad seals, cant see through some of them, icy, sweats) then after all are replaced in few years replace the siding with vinyl.

Thank you
Old 08-01-11, 06:07 AM
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Assuming that your aluminum window and your vinyl window are exactly the same size, then yes, your aluminum siding "should" fit inside the integral nailing flange. The guy at the store might have been assuming that you have trim around your aluminum window, which would mean that your aluminum siding won't be long enough to reach the j-channel of the vinyl window.

Yes, if you have an integral j-channel on your new window, you can expect water to get behind the siding. I don't like/trust the cardboard/foil sheathing, but if it is in good shape, I'd recommend that you cut a strip of tyvek (or better yet, aluminum trim coil) that will extend from the bottom of the rough opening over the TOP of the last piece of aluminum siding that you have not taken off- the one that is fully below the window. It just needs to extend over the nailing fin and interlock of that siding. What this does is it directs any water that gets behind the siding back out to the weep holes in the siding as soon as possible. You would then install the window as you suggested, using flashing tape as directed.

As an alternative to putting the window on top of the cardboard/foil sheathing, you could cut it back about 6" or so, and install a 12" wide strip of Tyvek (or similar) housewrap around the perimeter of the window instead. That way, sometime down the road when you tear off the old siding (and probably tear off that foil stuff too) you will be able to integrate the new housewrap for the new siding into the housewrap you installed around the window.

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