The fact is that most homeowners’ understanding of how their home operates as a structure in its own unique environment is about 50 years out of date.
Once upon a time all houses were built without any insulation, and all heating was what we call transitory – here for just a second, then out through those un-insulated walls, where it would evaporate any incidental water that got behind the siding on its way out. You have to constantly feed your heat source, so you send the kids out to chop more wood.
Move forward in time to oil heating. You don’t want to run out of oil in the middle winter and it costs more, so you decide to insulate your walls- and there you have the very beginning of modern construction – keep the heat in, keep the cold out.
But, when you have a differential in heat from outside in, you have a differential in pressure. So that heated air inside draws in the cold and wet from the outside just like sucking on a straw. Happens in nature every day, we’ve all seen cold fronts sweeping into areas of low pressure on our weather maps.
Since everyone knows that’s going to happen, you then get building paper… more recently like Tyvek. Any water or airborne moisture will run into the paper, condense there if it is airborne, and then run down the wall and out…and that’s how your interior wall structures stay dry. The paper goes over the nail fin flashing the window was installed with, so the two work together.
Install a new window in that hole without pre-flashing the opening and securing it to the window and integrating it with the building paper in an airtight situation, and you will have 360 degree leaks – those are the facts. Long term, invisible…rot and mold inside your walls.