Vinyl over existing plywood

Old 01-16-10, 09:12 AM
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Vinyl over existing plywood

I plan on residing my 35 yr old house with vinyl. The existing siding is plywood over ~1" sheathing under the foam. I don't want to apply the vinyl directly on the existing plywood because: 1. It is in poor condition. 2. It is overlapped at the seams and thus uneven. I plan on replacing all windows and would like to attach nailing strips on the exterior surface so that vinyl trim can be properly installed......the existing windows nailing strips appear to be nailed to the frame and the foam-plywood leaves them barely standing proud of the plywood. It appears that my best bet is to remove all plywood siding and install new OSB on the existing foam, apply thin leveling foam (green board) and apply new vinyl. Or, should I strip down to the frame and resheet with OSB, apply green board and then vinyl.

I like the idea of keeping the existing 1" foam but it doesn't seem structurally correct to not have sheathing directly applied to the framing. Thanks for any help/advise.
Old 01-17-10, 05:47 AM
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I really hesitate to reply on this one, because I have no strong opinion one way or the other, and I don't know that one way is right or wrong. So let me just say a few things that may help, whichever route you go.

There are plenty of houses out there that ONLY have foam sheathing. The reasons this is allowable, is that they likely have additional bracing that has been let into the framing, or maybe steel bracing that helps to square the walls. Some houses will have plywood on the corners of the home, but foam on the rest. When vinyl siding is installed on these types of houses, you just have to locate and nail to the studs.

On whether or not you need to remove the plywood siding, let me say that it would have to be in pretty bad shape for me to consider that one.

Whichever router you go, your new windows should have a nailing fin, and that nailing fin should be applied on the outer layer of sheathing. So if you put OSB over your foam, you would put housewrap (your weather resistive barrier or WRB) on that, and flash your windows to the WRB. If your foam is your outer layer of sheathing, you would need to install the foam in a manner that makes it a WRB by taping all the seams with a compatable seam tape. Personally, I don't like it when the foam is the outer layer.

Your fanfold foam insulation is usually only needed on houses with an irregular surface such as lap siding. It would be redundant to use it on a surface that is already flat, especially if you have sheets of foam sheathing as your outer layer. The only benefit of it on the exterior of your osb would be as a cushion, and its pretty questionable whether that is necessary either.

If you remove the 1" foam you could potentially run into some problems if the foam runs behind any walls or flashings. You might be opening a can of worms by removing the foam. So I'd tend to steer you toward leaving it on. And I would rather have a solid surface on the outside to nail to, but as I mentioned earlier, it's not absolutely necessary.

A WRB behind the vinyl is the most important thing, since vinyl siding lets water behind it in various locations, especially around window openings.

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