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Windows and Doors/window replacement over T1-11


Hi Russell,
My son-in-law is asking for my help in installing new windows in his house.  I have some DIY experience with windows in my own house, but am unsure how to proceed with his setup.  He is installing new vinyl windows with integral J-channel and brick mold.  He currently has T1-11 siding and has hired a contractor to install vinyl siding over the existing T1-11 after the windows are complete.  He plans to use 1/4" fanfold on top of the T1-11 and install the windows over the fanfold.   

Is nailing the window through the fanfold appropriate?
Is he better off using a house wrap vs. the fanfold.
Does installing vinyl over T1-11 require fanfold for leveling?
What if a layer of house wrap already exists under the T1-11? ( I will find out soon ).

I'm concerned about fanfold under the window nailing fins and also  fear that fanfold on top will not allow for the siding to slip into the J-channel and fear the windows will be recessed in respect to the siding.  

I plan to use sticky back bituthene  or equivalent as well as Z drip cap on header.  Does anything you propose change the way these should be installed?  

Thanks in advance.


I do not see a need for fan fold if you are installing vinyl siding over T1-11 wood siding. So I am clear on what is on the house have 4x8 or 4x9 sheets of wood siding on the house now? In my opinion the only thing fanfold is good for is when a home owner would install new vinyl siding over old horizontal or lap siding. Or if it is old house and the exterior walls are not straight. It does little to insulate since it has an R Value of only 1. Yes you can nail the window fins through the fan fold. You just use the same clear tape over the roofing nails that is used to cover the seems in the fanfold. if you want to install the fanfold, then butt it up against the edge of the window nailing fins. If you already have house wrap under the T1-11 siding then I would not add another layer. If I was doing the job, I would not recommend putting house wrap over existing T1-11 siding because it can hold moisture between the wrap and old siding and it will rot and or get mold. As far as your concern about the J Channel..the supply house in my area has two different sizes of J Channels for the very reason you have concerns about. Check with where you are buiying the siding to see if the siding manufacture offers the normal size J channel and one that is a 1/4 of an inch larger.

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Russell Spataro


Construction, home repair, tile, cabinets, flooring, framing, windows, siding, concrete, new construction, remodeling


14 years as a custom home builder

25 years as a carpenter

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