replacing the siding – if I’m putting a layer of 1/4″ fanfold insulation over the sheathing, taping the edges and overlaps, does that take the place of housewrap or do I need a layer of Tyvek over the insulation?
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Fanfold has very little r-value in that thickness. The main purpose of using it in a residing application is to provide a flat surface to apply the new siding to. Taping the seams will be of little good for either air or water holdout and could posibly cause more damage than good by creating a double vapor barrier. A layer of Tyvek would be a good addition to your project and help in two major ways. First, it is an air barrier. This will stop the wind washing effect and allow your insulation, including the fanfold, to be more effective. Thus help keeping your heating and cooling costs down. Drafty homes are expensive on utilities. Second, it is a barrier to bulk water(rain). If you are using vinyl siding this is a big bonus. Vinyl siding is loose fitting by design and can allow a fair amount of water through seams and joints. Look at the weep holes on the bottom of your siding.