Do foil radiant barriers degrade signal reception?
bluesolar
| Posted in Green Products and Materials on
Hi all — Have you installed any foil radiant barriers or foil-faced insulation? Have you noticed a reduction in cell or broadcast TV reception?
If I understand correctly, a metal roof or arbitrary metal sheet in between oneself and an antenna will significantly degrade the signal. I’m not sure if the thickness of the metal matters, and thus whether thin foil facing will have a significant effect…
Thanks.
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Only anecdotal evidence here, but my RB doesn't seem to effect my cell service or my WAN network functioning. I'm sure if my walls also had an RB that would be a different situation.
It can potentially degrade the reception of your satellite phone, but TV & cell phone not so much, since the latter are coming at a low enough angle that it's coming through walls & windows, not through the roof, not downward out of the sky. Even with Aluma Flash™ as the WRB on wall, windows are big enough apertures for the wavelengths/frequencies you're talking about.
The "skin effect" of the thickness of the conductive/reflective material isn't at all important at the frequencies of interest is. The sheet-to-sheet connection/conductivity and the size of any slots/holes is the key thing.
Thanks. I wasn't thinking of the skin effect at all, since that's about AC current through a conductor. I only mentioned the thinness of the foil in reference to its RF blocking ability, wondering whether thickness matters.
>"I wasn't thinking of the skin effect at all, since that's about AC current through a conductor"
RF is also AC, just much higher frequency than the power grid or AM radio. Skin affect is relevant across all frequencies, but at the frequencies used for TV & telecom even the thinnest aluminum in RB are very reflective.
I have foil faced OSB roof sheathing and foil faced polyiso on the walls. Reception is substantially reduced, but decent near a window. I have also been a cord cutter for a dozen years. I left the foil faced OSB root sheathing off a section of my attic where the OTA antenna is located and it works well.
Even after a year and a half reading GBA posts I still have trouble deciphering some of the more arcane acronyms. Please translate.
OSB = Oriented Strand Board. It's the panel product that looks like a bunch of pressed wood shavings glued together.
OTA = Over The Air. Basically any signal broadcast for general coverage, so an "OTA TV antenna" would just be a regular antenna intended to receive broadcast television signals.
Bill
Hello etekberg! Were you able to just peel the foil off your radiant barrier. I need to do the same. Thanks! 😀
If your primary concern is TV reception, it will be easier for you to use an outdoor TV antenna and not worry about the foil radiant barrier. An outdoor antenna will likely work better regardless. If your primary concern is cellular service for your phone, that will depend more on the walls than the roof in terms of any shielding effects from the foil.
Bill
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It can. Various materials interfere with Radio Frequency (RF) signaling. Metal, concrete, moist paper, rain and human bodies being some of them. A “Faraday” box or cage built from conductive metal completely blocks RF.
Yes, if you completely cover your home in foil-faced insulation, you WILL have reduced cell signal strengths inside the home. A metal roof will have less effect on terrestrial signals like cell phone service.
Dana is basically correct, skin depth doesn't matter at cell phone frequencies (some as low as 700MHz, but mostly up around 1.9+GHz these days). "holes" that will have an effect will be things like windows and doors. Nail holes will have nearly zero effect on the RF signals coming in. The amount of signal that can "leak" through a hole is dependent on the wavelength, and 1.9GHz is around maybe 16cm or so, so holes big enough to matter from an RF shielding perspective are going to have to be bigger than about 2cm.
If you're in a low-signal cell coverage area, foil-faced insulation WILL make things worse for you. If you're in an urban area with "four bars" all the time, you'll probably still get enough signal that you'll be fine, but with a bar or two less signal strength inside your home. An advantage is you'll hear your neighbor's WiFi signals less, and they won't hear yours as much either.
Note that you DO NOT need a conductive connection between shielding panels for them to "work". At high frequencies like this, capacitive proximity effects are enough to "connect" things together. If you use foil tape between panels, you've actually made a VERY effective shield for your home.
If you are actually trying to block signals, you need to do more stuff. I get to design these types of facilities occasionally, and they're fun to demonstrate -- when the door closes, your cell phone goes to "NO SERVICE". They are also a bit on the pricey side to build. We usually use layers of copper screening and drywall on the interior to do it. For an extreme example of an RF shielded room, look up "anechoic chamber".
Bill
Thanks. Do holes like windows help if you're not standing next to them?
Seems like nail holes wouldn't really be "holes", if they're filled with (metal) nails.
I don't know where the whole thing about the skin effect and skin depth came from – it's not something I was thinking, since that applies to AC current in a conductor, not metal foil perpendicular to an RF signal.
Interesting about the work you do. I'm familiar with anechoic chambers as used in device development, like smartphones.
RF is an AC signal, and as all alternating magnetic fields do, they generate a current in conductors within the field. This is how antennas work. Skin depth comes into play with shields, it’s just that above a few megahertz or so the depth of the conductive surface doesn’t matter much since the “skin” where the current is concentrated becomes so small anyway.
Window holes help because the RF signal will both enter and bounce around inside the structure as well as refract around the edges of the window. You may find more localized good/bad spots as a result due to destructive interference patterns, but it won’t be a “works in front of the window but not a few feet away” kind of problem unless you already have a VERY weak signal anyway.
Ferrous metals aren’t as good at microwave frequencies like your typical cellphone signal, but they do act more as a “plug” than an open hole would. The interesting thing is that even with no plug, a nail-size hole makes little difference since the hole is small relative to the wavelength of the RF signal. Holes smaller than about 1/8 wavelength or so don’t actually leak much signal, this is why screen works as a shield, and why you can have perforated reflectors on satellite dishes to reduce wind load without much affect on their ability to reflect the RF signal from the satellite. As the frequency increases, the wavelength shortens, and you need finer and finer mesh screen to maintain the same shielding effectiveness.
My contracts for shielded spaces had multiple layers of air gapped metal screen since I had specific attenuation targets to hit, usually 90+ dB. The goal was usually either to protect what was inside from interference or to make sure no one outside could detect the signal inside. A few sites wanted to be EMP hardened, which requires all kinds of fancy stuff on EVERYTHING entering or leaving the shielded space. Cables had to be in specific diameter pipes of specific lengths to act as waveguides below cutoff and things like that. Fun projects with big budgets, my very favorite kind :-)
Bill
Bill, Can you comment on the effect of water/ moisture instead of metal as a barrier to RF? It has been my experience, for example, that Bookshelves are surprisingly effective at blocking WiFi between classrooms in a school building. While the focus of this discussion is all about conducting barriers aren’t other materials also barriers?
Moisture has little effect on RF signals below about 8-10 GHz or so, which includes all WiFi and cellular frequencies currently in use. The 5GHz WiFi band is actually pretty close the the 4GHz “C band” which is some of the quietest (lowest natural background noise) spectrum there is. Water absorption is frequency dependent, and if you’re thinking of the thin film of water on your house during a rain storm, any effect will be so small as to be Immeasurable.
I’ve designed a lot of WiFi networks for schools. Usually it’s block walls that are the biggest issue. My rule is the signal can usually penetrate up to two walls with sufficient signal strength, so we like to put access points every fourth class room or so, in the hallways. This ensures pretty good coverage of a typical wing of a school building.
Every material that isn’t a vacuum has some amount of effect on signal strengths. This is due to what is known as “dielectric losses”, basically. Some materials will be worse than others. An empty wooden bookshelf probably won’t have much effect, but fill it with books and you have a wall of nearly solid wood pulp about 5-8 inches thick. That will likely have a measurable effect. I’ve never tried measuring it, but I suppose I could (I have the equipment to do it accurately).
What you typically see is a combination of losses, which attenuate (reduce) signal strengths, and reflection/refraction, which cause reflected signals to arrive at the receiver either out of phase or otherwise delayed in time with respect to the direct signal. All of this causes a reduction in the receivers ability to accurately recover the data from the signal. You can see what the receiver sees by looking up “eye diagram” pictures that show how the signal can degrade by out of phase signals (it’s a little different, but similar in concept).
The usual way to deal with all of these issues is to ensure high enough signal levels to avoid problems. This means more closely spaced access points in commercial buildings than you might expect.
Bill
Interesting, I don't think I knew that RF is AC.
I've been reading good things about cell signal boosters for homes. I'm surprised that they seem to work well, and Wi-Fi calling is an option too.
We're looking at a steel-framed home, probably a moment frame like Blue Sky Building Systems' (https://blueskybuildingsystems.com/offerings). Possibly coupled with ThermaSteel or SIC Systems or similar walls, which are steel frame sandwiched EPS panels. That means a steel cage basically. With maybe 18 inches between studs on center, that's plenty for the relevant RF signals to pass. But I'm not convinced that the frame will have *no effect* on signal. The ceiling will have lots of beams or joists, and then there will be a roof frame above it, all steel, maybe 18 or 24 inches OC. I was wondering what foil-faced insulation would do on top of all that, mostly in the roof. I can't predict yet what angle and orientation the house will have relative to towers, since we don't have a site yet, but I wouldn't assume that the signal is only needing to go through the walls horizontally, and not the roof at a more oblique angle.
The steel frame itself won’t have much effect on the higher frequency signals used for cellular and WiFi services. The signals will reflect and refract through the stud cavities so you’ll get good signal levels inside the building.
If you cover the building with foil faced polyiso, you WILL see reduced signal levels inside the structure on cellular devices. WiFi is generally coming from inside the structure anyway, so the exterior building covering doesn’t really matter.
Cell boosters DO work, but they can be finicky. I’ve used them before. You absolutely MUST maintain spacing between the indoor and outdoor antennas or the devices will “see” themselves (like feedback in an audio system), and shut themselves down. I’d try to avoid cell boosters unless I had no other option. You can also use picocells, which are like mini cell sites that you can put in your house to get good cellular coverage, and they run your cellular signal through the internet back to your carrier.
For access points, avoid using any “boosters”. Those “booster” devices work by retransmitting your WiFi data, so reduce your bandwidth every time you add one. With a good access point that is properly located, you can usually get coverage over an entire house with only one access point.
Bill
FYI, I'm not worried about Wi-Fi. That's inside the house, as you noted. I'm only talking about cell and OTA TV.
The downside with pico- or femtocells is that they're specific to one carrier. Don't they also drop the call when you leave the home? Signal boosters work with all carriers, which is a big plus for me, and calls are seamlessly transferred to towers when you leave. I've been looking mostly at the Wilson and weboost models.
>" Do holes like windows help if you're not standing next to them?"
Yes- windows are quite LARGE holes compared to the wavelengths of telecom RF or even FM radio (though in the FM band they take on slot-antenna type characteristics. The higher frequency stuff that gets through the windows gets reflected around by the back side of the RB foils.
>Seems like nail holes wouldn't really be "holes", if they're filled with (metal) nails.
A nail hole is way too tiny to matter. For the frequencies in question a bug screen is fairly equivalent to a continuous foil.
Thanks Dana, that helps.
I installed foil faced poly iso on all of my exterior walls. Anecdotal evidence says there was some cell service degradation, but it definitely did not go to zero signal. I don't have any metal on the roof. It was a bit of a pain for a couple of years, but then cell service over Wi-Fi became a thing and I no longer care about cell service inside my house, as Wi-Fi does the job much better and without any effort from me (aside from having properly functioning Wi-Fi, which I would have anyway). I can't speak to TV reception, as I have an external antenna.
Thanks, interesting about Wi-Fi calling. I didn't think of that. Do calls drop if you leave your house?
Calls don't drop. It switches back to the cell network pretty seamlessly (hard to say if a hiccup in audio is the switch or just cell coverage issues). The Wi-Fi works outside the house pretty well, despite the foil, so the handoff has a bit of time to happen. Generally cell phones work better in my home now than before, everything that has happened in the last 10 years considered (foil faced poly iso, Wi-Fi cell coverage). I can't say that this is true for a visitor that hasn't enabled cell over Wi-Fi -- I don't have any data points for this.
Windows don't help much, hard coat loE is pretty attenuating
I have an rf remote to shut off solar powered lights in the back yard, and I have to open a window or slider to get it to work. it is right at the edge of its range, and I think that is the key
If you are next to a cell tower, your metalized insulation won't do much, it still drops a vew dB, but at 100 percent signal, it doesn't matter