Tag Archives: air infiltration

Porous vs. non-porous hurricane protection

17 Sep

As a homeowner in a hurricane prone area, it is important to know the performance differences between porous and non-porous hurricane protection. What? You never heard of such a thing? Don’t feel like the Lone Ranger – neither have most homeowners. As a matter of fact, the person selling you your current hurricane protection probably hasn’t either. Here’s the way it has been explained to me:

Porous type protections have open areas around their edges or on their surface that equal more than 5% to 10% of the area covered by the shutter. This allows air and/or water to pass through or around them. Some have as much as 50% open area (like some screens or mesh). They provide only impact protection and rely on the door or window they are protecting to resist the water penetration and air infiltration from both negative pressures (those directed away from the house) and positive pressures (those directed towards the house). So, porous systems block wind-borne debris and are usually built out far enough so that they will not deflect into the glass when impacted by the standard 9-pound 2×4 lumber missile traveling at 34 mph. However, they don’t always do the best job of reducing wind pressures or water leaks. The result is that the windows and doors behind them sometimes experience the full effect of the wind pressures and may leak about as much as an unprotected window or door.

Non-porous hurricane protection systems should be¬† water and air tight or very close to it. In many cases, even protection labelled as “non-porous” will still allow outside pressures to affect the window or door behind it. This is due to the fact even though the non-porous protection has less than 10% open area, it’s still more than 0% (0% being considered water resistant) If the windows behind them are not strong enough they can still be blown in, which allows wind pressures and driven rain to enter the structure.

Here are 2 examples:

This..

You have a corrugated metal or plastic panel-and-channel system over your 48″ tall x 96″ wide living room picture window. To deploy the system, the panels are tipped towards the wall, slid up into the upper channel and then are pushed back against the wall at the bottom (or window sill), and then either dropped down into the floor of the bottom channel or bolted to an angle below the window sill. This leaves a gap at the top and bottom where the corrugations come away from the building. These gaps at the top and bottom allow wind and rain to get in behind the shutter and attack the window. There are also gaps at the vertical edges of both end panels where they meet the outside wall of your living room.

When the wind blows, some of the pressures and rain are allowed attack your windows. If you were to open the inside window during the storm, you would definitely feel wind blowing on you. This is technically known as air infiltration. Any rain or water that gets past the shutters is called water penetration and you will most likely feel this, too. This is considered a porous system – it allows air pressure and wind driven rain to get at the protected window. If you have newer wind rated windows, this system will be fine for you. It will keep direct hurricane force winds and large missile wind borne debris away from your windows.

Vs. this…

You have purchased and cut a flat cellular polycarbonate twin-wall sheet to the over-sized dimensions, per the instructions, to fit over the same picture window. You have the proper support bars in place to prevent deflection during impact and your anchor holes are all lined up and you’re ready to deploy the sheet. You affix some standard foam weatherstrip (with adhesive backing) to the backside perimeter of the sheet in the oversize area. You hold the sheet into the proper position and drill and insert the anchoring hardware and tighten per instructions.

In this case, when the wind blows, there is no attack of the protected window. Theoretically, if the weatherstripping is working properly, you can open the inside window during the storm and light a candle and it won’t blow out. The weatherstrip should keep the wind and water from getting in. This is a non-porous system. If you have older, single or double strength aluminum or pvc framed windows from the ’60s, ’70’s or ’80’s, this might be a good system for you. And it lets light in so it won’t make you feel like you’re in a dungeon and will save you a bunch of battery power and candles when the power goes out. It isn’t going to be cheap, but it will be lightweight and easier to deploy come storm season. If you have large windows to cover, you could even invest in hurricane rated mullions (support bars) that are the same color as your window trim and leave them in place permanently. That will decrease your deployment time and as long as they are strategically located, won’t obstruct your view too badly and won’t be too noticeable when you look outside.

Finally, in some cases the porosity of the system is determined by the installation itself. The farther away from the face of the structure the system is mounted, the larger the gap for wind and rain to enter. Make sure that you have it in writing as to which system is being installed on your building.

Hurricane impact windows are non-porous hurricane protection, as well. Evolution Hurricane Shutters can be installed as either porous or non-porous protection. We recommend  the non-porous method because it provides the opening with the greatest protection and the greatest energy conservation and sound insulation.
The lesson here? Although non-porous protection is superior to porous protection, you may only need to have non-porous to satisfy your insurance carrier or local building code. Take a common sense approach to your hurricane protection needs. Is seasonal storm protection all that you want? Worried about vandalism? Does your yard serve as the local ball field for the rest of the neighborhood? Do you want energy savings, too? Will you always be around to deploy the protection? What conditions does your insurance company dictate? If you are only a seasonal resident, you may be required to have an “installation contract” in place with a local company to prove to your insurer that your hurricane protection will be in place in time to protect your structure from the storm. Just a reminder – read the news and realize that the insurance companies are getting tighter and tighter about what they are going to pay out. They are ticked off that hurricane protection costs them revenue but glad about reducing their risk. They don’t want to pay out any more money than they have to and their inspectors will start nit-picking your protection at the rime of the policy start and they will also be more adamant about post storm investigations to lower their settlement figure. Taking short-cuts or getting lazy about putting protection in place by the homeowner will only draw the ire of the underwriter and get you a step closer to a fraud charge.

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