When installing Fullback siding insulation with vinyl siding, you have to accommodate for the additional thickness of the foam. If you do not have access to the wider, recommended j-channel, you can use a standard j-channel with a foam shim underneath to accommodate the thickness of the insulation.
To install the foam shim, simply nail it up around your window, then place your j-channel overtop. When installing the siding, push the Fullback against the foam shim, and slide the siding into the pocket of the j-channel.
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