More important than the thickness of the polycarbonate panel is the method used to mount the panel in your opening.
Also not sure if you want 4mm or 6mm from your post, I would not expect 4mm panels to technically pass the requirements for impact, but 6mm panels might....of course 1/2 plywood wouldn't pass the test and even 5/8 and 3/4 plywood don't always stop the 2x4 either. I would definitely consider the 6mm over 4mm.

Speaking to your engineering background, laminated glass is used for the vast majority of impact windows. Although there are different options, laminated glass typically consists of two glass layers bonded to a plastic interlayer, usually PVB or polyvinyl butyral, but often SentryGlas which is brand ID for duPont (now Kuraray) Surlyn ionomer film that is stiffer and more tear resistant than PVB. SG is similar to acrylic or polycarbonate in stiffness and tear resistance (although somewhat less tear resistant than either of those materials), but it bonds to most metals and glass under the heat and pressure of the laminating process; something neither polycarbonate nor acrylic can do without a bonding layer of a different material. Although I am aware of companies using polycarbonate in a laminated glass application, the requirement for the bonding layer (usually a polyurethane) makes it more difficult to produce and there are issues with field failures due to substantial differences in coefficient of expansion between glass and plastic.

The thinnest laminated glass that I have seen pass impact is about 7mm (2.3mm glass / 2.28mm interlayer / 2.3mm glass), 6mm polycarbonate should have substantially more impact resistance than a 7mm laminated glass in that configuration. The thickest I have seen was 1 1/4" overall (exact make-up proprietary last I know of) for tornado impact testing. Hurricane impact is 8', 9#, 2x4 at 50fps. Tornado impact testing is 12', 15#, 2x4 at 150fps. The tornado testing that I witnessed resulted in a shattered 2x4 on almost every impact and less than half of the glass samples even broke.

All of which comes back to using thin sheet polycarbonate as a shield over your window.

Once again the major consideration is how you mount it, and when mounted correctly, (I am NOT recommending, but), I personally would be comfortable using a 6mm polycarbonate shutter over my windows in a hurricane to protect my home. But keep in mind the primary purpose of shutters, or impact glass, or whatever else is used is to protect the home, not the occupants. The best place to be in a major storm is elsewhere.

As an aside, I have watched youtube videos of people building their own shutter systems out of wood, one out of reclaimed pallet wood, and while they look pretty nice in some cases, I would not give them 10 minute survival time in the case of a major storm. Using polycarbonate is a MUCH better idea.


Background: I have performed, supervised, observed and/or evaluated the impact test (MDC and ASTM) over 3000 times over 20 years and also spent a few years on the ASTM committee that specifies the test requirements.

 

.... would the 4mm panels work?

Better than nothing … but not by much. Do you need the panels to meet building codes? Do you need them for homeowners insurance?

Theoretically, the 4 mm panels will have 1/4 the strength of the 16 mm panels. But 4 mm is only … less than 3/16”.

 

Thank you both for your kind replies. In 2016 Hurricane Michael hit the panhandle of Florida. The storm track litterally came down the street where I live at 162 mph (confirmed 1 mile away). I had aluminum hurricane rated panels on the front & sides of the 1st floor, reinforced plywood panels on the inside of the upstairs windows (all windows ...heavily reinforced with glued & screwed 2x2's), but no protection at all on the backside of the house on the 1st floor (5 windows).
We had 2 windows broken on the top floor but only the outside pane of the double glazing. Fortunately none of the 5 unprotected windows broke.
We got by with relatively minor damage. The two broken panes, lost shingles, and numerous lost soffit material. Both houses to the North of us were destroyed and subsequently razed, the home to the south of us was 50% destroyed but since rebuilt and the next home south was destroyed & razed. In all 7 of the homes in our small community of 20 homes were destroyed or mostly destroyed.
In my mind I am convinced that with out my hurricane shutters our house would have been destroyed also. I figure we got very lucky the back windows didn't go.
Sorry for the long story, but you see my reason for wanting those back windows protected in the future. My intent is to use 6 mm polycarbonate panels with properly spaced ss hanger bolts embedded in the 2x4 window framing with large washers and wingnuts.
Again thanks for your replies.

 

I forgot to mention that the 6mm panels are double wall.

 

Curious why you prefer ALL polycarbonate panels. I use polycarbonate storm panels but I mix them with aluminum panels. The hurricane panel distributors I get them from sells the same "h" headers, and L tracks, then for the panels you can get them in steel, aluminum and polycarbonate. They are the same size and corrugated the same pattern.

To me steel is heavier, aluminum much lighter, and poly the lightest. But poly is only a little bit lighter than the aluminum, but aluminum is much lighter than steel ones. So I opt to get most of the panels in aluminum.

Like you said, polycarbonate is more expensive, but it has one disadvantage, it is much thicker than the metal panels, so much so that when you are installing the poly panel, it is actually very snug when inserted into the upper header, you may think snug is good, but the problem is when you install these panels, and they are all poly, panel 1 overlaps panel 2 by one raised pattern, and in that case two poly on top of each other, many people struggle to fit two successive polys into the head without deforming the aluminum header. The solution? Don't get all poly. I calculate my order so I will end up with 1 or 2 polys per window because you want some light into and visibility outside during a storm. So a window with say 8 panels I do 7 aluminum and 1 poly, this is cheaper, lighter and much easier to install, if you go shop for poly, try to take two polys overlap them and see how difficult it is to jam that into the header, because these headers were machined for metal panels and poly panels came later.

In addition, if you use roofing panels or non standard protection methods, you will not get credits for your windstorm insurance, and if you did because the wind mitigation inspector missed it, you most likely will not get paid in the event you have damages and they can point to the roofing panels.

 

I will keep my aluminum panels on the front & sides as well as the plywood interior panels on the second floor. The poly panels will only be on the east side of the house. The poly panel are not the corrogated type you mention. See the post below. As for insurance ,,, not a problem .. few in florida can afford it Well over $3000/year.

 

Do you mean this?

 

No. I was thinking of modular storm panels like this.

The one in your linked image works too, the only thing is these are not as common because typically you need to cut a sheet the size of the entire window, which means you need to store dozens of large odd size sheets when not in use, and also they are more difficult to put up when a storm is approaching because of the panel size. Imagine a 72"W X 48"H window, you need a panel size 80"X56" with 4" overlap on all sides, and you need to get on a ladder to install the wing nuts on top even for a window at the ground level, and if you have a two story house, it's not fun dragging an 80X56 sheet to cover a second floor window, typically it needs two people with a ladder on each side. The regular corrugated panels with top h header and bottom L tracks are much easier to store, put up and take down.