It turns out, 5/8 plywood is about the best window insurance you can buy
I ran into the This Old House crew on a bus at the International Builder's Show, and shamed Tom Silva into letting me sit next to him. Tom and I talked about hurricane and flood resistant construction, which happens to be one of the things I am interested in for 2018 coverage. Naturally, I have my eyes open for air cannons.
I learned later that Tom, Norm, Kevin, and Richard are working with SGC Horizon on the Skilled Labor Fund, which was launched last year as a channel to help feed the construction and remodeling industry with a talented, knowledgeable labor pool.
The serendipity of this video coming through my YouTube feed right when I arrived home was too much to overcome. "How can I not promote a video of an air cannon shooting 2x4s at a window?" is pretty much went through my mind, and against most of my headline-writing instinct, I wrote that headline.
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In this episode of Ask This Old House, Tom Silva walks through fundamental hurricane resistance
With a hurricane, you have to worry about three things.
- You have to worry about a storm surge, which can be exacerbated by high tides raising the water level higher.
- The wind
- The rain.
The second two work together to mess you up.
"During a hurricane, any object on the ground can be picked up by the wind and thrown at your house as flying projectiles. And if they hit the house at the weakest link, like your windows or your doors or even a skylight, they'll break it and the wind and rain can get inside your house, destroying the content and blowing the structure apart from the inside out."
—Tom Silva, This Old House
There is considerable pressure during a hurricane. The difference between the pressure inside your house and outside is significant, and if a hole is broken through the home's exterior skin, that pressure equalizes quickly; it can literally blow the roof off from the inside. Windows are one weak spot. Garage doors are another.
Using that conversation as an excuse, Tom takes his guest to a testing facility where engineers shoot 2x4s at windows with an air cannon.They test the options for covering windows and doors during a hurricane. The gun shoots 2x4s at 34 miles per hour because
"...that's the speed that an object is picked up and thrown through the air like a trash barrel or a tree limb or in this case a 2x4."
"In most cases, you see people boarding up their windows and doors using plywood. Now, it's important that the plywood is cut to fit the opening, and the building code says that that plywood has to be a minimum of five-eighths of an inch thick/.."
Why 5/8 Plywood instead of 1/2 in OSB? Two words: Cannon tests.
2x4s fly right through the OSB and through the window, but it bounces off the 5/8 inch plywood, leaving the window in good shape.
The plywood is stronger because the long-stranded laminates run opposite directions. OSB or oriented strand board are many small pieces of wood, glued together. The sheets have racking strength, but not projectile resistance.
Next, they look at some fabric choices. Ballistic nylon, to be exact. Held in place with brackets, the super fabric can be installed before a hurricane and removed afterward. After shooting a 2x4 at it, they discover that the nylon, indeed kept the 2x4 out of the building, but it did break the window.
"And in many jurisdictions, that's acceptable by the building code."
Another product is a bi-folding metal shutter which is permanently installed over windows. Some shutters allow you to close them from the inside; some require a ladder, others can be closed remotely. When the test engineers fire the cannon, they are required to aim for the center of the covering, which was a weak spot for the shutter in this video clip. The shutters open, and the window breaks.
The final product that they look at is what's called a passive system:
"That means you don't have to do anything."
The insulated window has impact glass; the outside layer of glass is doubled, with a membrane in between them.
"So if a flying object should hit your window, it will shatter like a car window but you won't get a hole in it."
After a final test, the results are in:
"Oh-ho! Well, it broke the glass but no hole and your house is still protected against wind and rain."