Adding Aluminum Sheets to Wall Assembly
marleyandbowie
| Posted in General Questions on
To help mitigate electromagnetic fields from a nearby source, we’re considering adhering some sheets of thick aluminum foil to the drywall, and then covering that with pine shiplap. Would this cause condensation issues and/or interfere with the wall’s vapor permeability in a way that could cause mold to form inside the walls?
This is for a mid-century home with wall assemblies that have never been updated.
Thanks!
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Replies
marleyandbowie,
As long as you do not use air-conditioning extensively, and the wall assemblies are designed to dry to the outside, you should be fine with a vapour-barrier on the interior of the walls.
You would need to tape the seams of the foil. But have you considered installing rigid foam with a foil facing. You would then get the benefit of greater wall insulation.
The aluminum will act as a vapor BARRIER, so all the same precautions needed with vapor barriers would apply here. From a vapor/moisture control standpoint, aluminum sheets are no different than a sheet of polyethylene and all the same issues apply.
In terms of shielding against electromagnetic fields, what is the source you're trying to protect against? Aluminum might not be the best option here, and you might not need sheets. If you want to protect against radio signals, high frequency things like cell phones, wifi, etc., then you can use aluminum screen (copper screen is slightly better), and then you don't have vapor issues. If you're trying to protect against lower frequency things, like AC power waveforms, you need something ferrous, so you'd need steel sheet, not aluminum. Either way, for maximum protection you need to wrap the ENTIRE room, to create a faraday cage. If you don't wrap the entire room, including the floor and ceiling, you'll distort the field, and have diffraction patterns so that the energy can still get into your room.
Bill
Maybe they want the siding material to match their hat?
Can't you at least reduce the signal significantly with a partial solution? After all, just being on one side of a building or the other makes a huge difference in cell reception. I get ZERO cell reception inside my steel skinned pole barn that has no other factors. Even with the door wide open, zilch.
The advice about moisture is spot on. If you are living someplace that is very humid and need lots of cooling you will need to be very careful.
As for magnetic fields aluminum foil How sure are you that you want to prevent your cell phone from getting reception in your home? Will you have WIFI in your home?
If the concern is high tension power lines the foil is likely to thin to be affective at that long a wave length and will have little effect on magnetic fields but will stop radio waves.
Walta
Steel roof and siding how small a mesh would you need in the slab the close the Faraday cage.
4-inch sound a little to big but 1 inch could do.
Steel doors yes but the lack of windows may hurt the resale value.
This is starting to sound like the building the Bell system built every few miles or so for the exchange equipment.
Walta
I've built shielded rooms before. We would use a layer of copper screening, a layer of drywall, and another layer of copper screening. The two layers of screening were isolated from each other, but otherwise continuous around the room. The size of the mesh depends on the frequency, since the opening in the mesh is related to wavelength. I've used 1/8" and 1/16" mesh before for the low microwave region, but it should work up to maybe 20-30+ GHz easily before you start to see any real leakage. You can have windows, but the shield has to cover them.
The tricky part is doors and services (power, wiring, plumbing). Anywhere anything penetrates the shield, you need a special fixture. Most of the time for air ducts and pipe this is a honeycomb-like device that looks like a punch of small tubes fused together. It is a type of filter that operates on a principal known as a "waveguide below cutoff". Such a device blocks any frequencies with wavelengths too large to fit down the little tubes, basically. These fixtures are very expensive.
Power was harder to filter. You need devices known as "feedthrough capacitors", but really big ones -- most are pretty small. Typical values are 0.01 or 0.001 microfarads or so. These are NOT the same as regular capacitors since the conductor passes through the middle of the capacitor, so you have a sort of capacitor built all the way around the wire. These are also expensive.
Doors need RFI gaskets, which look like weather stripping with a copper mesh tube over it. These are expensive. The doors themselves can usually be typical commercial steel skin doors, but it's common to use two that swing in opposite directions on a common frame.
The fun part is when the customer wants to test the final product, you walk into the room, and then watch your cellphone lose service when you close the door to the room. Prices to build rooms like this is pretty high and I've only done it a few times for special facilities.
The bell system wasn't built this way. The old Autovon switches were just built underground, and they had blast doors and special ventilation systems. The soil above the vaults did most of the filtering work. Regular telephone exchanges don't really have any special shielded in the walls, although they are usually built with pretty solid structures. Basic gas discharge tube surge protection is on every copper wire entering the facility, but that's about it for filtering there. One of the nice things about fiber optic cabling is that it is immune to EMI and RFI, so you don't have to worry about signals coming in on that -- you would only need to shield the electronics on the ends, and the regeneration sites along the route about every 40-60km or so.
Bill
Hi,
I had good results with Y-Shield HSF54 shielding paint. It uses graphite as the conductive element in the paint. The paint is black, it takes a few coats of primer to change the black color to white.
I have screen type fabric covering the windows. I got it from LessEMF.com
I have serious doubts about the effectiveness of that paint. Part of my doubts arise from goofy use of terminology in their data sheet for the product, but also from the claim that it has to be grounded. Shields do NOT have to be connected to earth ground to work. The same company also sells "earthing" products, even a chair cushion to supposedly protect against low frequency fields. If someone wants to send me a sample of this paint to test, I could check -- I have the equipment necasary to check it up to around 26GHz or so. I doubt very much it would live up to the claims.
Resistive coatings don't make good shields. Resistive coatings can help to absorb fields, but they won't block them. A typical application for such an absorptive coating would be a coating using to reduce radar reflections from stealth aircraft. Such coatings usually use various metal oxides as a resistive material. For shielding, to actually block a field, you want a cood conductor. It's not necassary for good electrical connections in all places either (solder, etc.), because capacitance is enough in many cases to bridge a gap. There is never any need to connect to earth, either. Shields work by essentially short circuiting a field and conducting the energy around the object being protected. There is no need to divery energy to ground -- that's not how it works. A shielded enclosure will work just as well sitting on your lawn as it will flying around in an aircraft.
Another issue is any shield that has gaps larger than around 1/20 of a wavelength isn't really doing anything. For 5GHz wifi signals, that means gaps can be no larger than about 3 millimeters -- about 1/8" or so. This is why you can get into your all-metal car and still get good cellular service -- the windows and other "holes" in the metal screen are plenty big enough for the signal to fit through.
If you really want to block RF energy, it takes a lot of materials and diligent installation -- way more than is usually required for a good air sealing. If you want to block very low frequencies, you usually need to use ferrous enclosures with more thickness to be effective. Running a few wires in your sock isn't going to do anything, for example.
Bill
I've used the shielding paint in audio applications. It's highly conductive, not resistive. It's used to shield areas that are irregular and don't lend themselves to metal shielding, like a cavity carved in the body of a musical instrument. In this application you're trying to block signals in the audio range, below 20kHz, and mostly 60 Hz hum and its harmonics.
The paint is expensive, I recall that it's sold in pint containers.
Thanks everyone! In case anyone is interested, the reason I am doing this is to provide some protection from a smart meter on the outside of a bedroom wall. I have a really good EMF/RF filter, and I have been able to get the level down in using a mesh smart meter cover I bought, but I would like to have at least a 2' x 2' layer of protection located right behind the meter, in between the existing drywall and a new shiplap layer.
Note that the power levels used in these little transmitters is very low, typically 100 milliwatts or less, so they don't really pose any dangers. If you're just trying to attenuate that RF field, I would just put some aluminum window screen in the wall behind the drywall. You don't need to do anything fancy with the seams -- you don't need to tape or solder or even wire them together. Just overlap the pieces a few inches and staple it up before putting up the drywall and you're plenty good enough. You will get some reduction in signal this way, just not a quantifiable amount (which is a fancy way to say it's not really predictable, so you'd have to measure how well it worked).
I'll mention to that I'm an EE with a background in RF and microwave systems, and I work in the telecommunications industry where we have to mindful of these kinds of things. I wouldn't put the any effort into this kind of shielding in my own home. You'd be much better off investing in good lightning protection in terms of the potential for your money to buy you some protection for your health and safety.
Bill
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I'm not surprised you get some effectiveness, I just doubt it lives up to it's claims. There are, unfortunately, a lot of shady products out there for applications like this, so I tend to be suspicious of them. The projects I've been involved with where shielding was necassary had to do quite a lot to get to the required level of shielding, and we had to do a lot more than just paint. The shielding was expensive to install, and it was tested when complete to verify the required performance levels.
If they had a smaller can of that stuff, I'd order some to test myself.
Bill