Replies

1. Expert Member
   Michael Maines | Dec 19, 2022 02:54pm | #1

   The only thing I'm aware of along those lines is that you can get impact-rated glazing for areas that are likely to experience wind-blown debris that could break the glass and allow in a gust powerful enough to blow the roof off. Alternatively, in those areas you can beef up the structure.

   When you're going from something like 3 or 7 ACH50 to 1 or less, the difference is a few square inches to a few dozen square inches of air leaks, usually spread all over the house, so I don't think airtightness would make a measurable difference for storm resistance.

2. Expert Member
   BILL WICHERS | Dec 19, 2022 04:37pm | #2

   In short, no, doing a good job of air sealing does not change the wind survival rating of a home. The amount of wind that leaks through little cracks and pinholes is so small as to be inconsequential in terms of force exerted (or, in this case, not exerted) on the structure. To visualize this, take eight sheets of plywood put together in a line to form a wall 32 feet long. Assume the builder is an avid reader of GBA, and taped all the seams between those panels so that the wall is perfectly airtight -- 0.0ACH50! Yeah!
   That's 256 square feet of surface area. With 50 mile per hour winds blowing straight into that wall, the wind is exerting about 1,638 pounds of force against the wall.

   Now let's assume you didn't tape the seams between sheets of plywood, and left 1/8" gaps between the sheets. Even worse, you didn't land the edges of the panels on studs (horrors!), so the wind can blow right through those gaps. There will be a total of seven of those gaps between the 8 sheets, and they'll total about 0.6 square feet of total area. That leaves 255.4 square feet of plywood surface for that same 50 MPH wind to blow against, but now only about 1,634 pounds is exerted on the wall.

   All those totally open seams only saved 4 pounds of wind force, which was about a 0.24% reduction in this example -- not enough to worry about. In an actual structure, there will probably be something behind the plywood (drywall, etc.), so the wind will still "hit" something and exert force, even if it ends up leaking through the wall eventually. Even with completely open seams, there are still some aerodynamic forces that are going to act on the airflow so that the "leaking air" still exerts a little force and doesn't quite completely sneak through that wall.

   I would say from an engineering perspective that wind forces on a structure assume a "perfect" air seal, since the calculation is just force over the area of a wall -- no allowance is made for holes in that wall. If your window blows out or the roof flies off, then that's a different story :-)

   Bill

3. 
   millionsofpeaches | Dec 20, 2022 07:20am | #3

   Awesome! Thank you both!