Spray foam insulation in Vented Attic? Featured

Assuming the blown in insulation is up to spec, I'd bet you get a far better payback by making sure all the ac ducts are sealed and insulated and that the attic is well ventilated. Look into things like a ridge vent with lots more soffit vents. Worse case, you could put a power vent in there as well, but you still will need lots of soffit vents.

If you insist on spraying foam, you could look at this:

-- "Tell me what I should do, Annie." "Stay. Here. Forever." - Life On Mars

this:

The attic is vented, unheated, the floor is what needs more insulation, foams go to R7.5, some are only R 4

No. You don't want to insulate the roof deck. Doing so may shorten the life of your roof. The problem is the lack of or insufficient quality of insulation on the ducts. You also want to add insulation to the attic floor as I am sure the standards of the 1980's were not what they are today.

Insulate the ducts first and the attic floor next. Make sure you have at least minimum current code venting for the attic as well.

Are you saying the A/C unit itself is in the attic or just that the ducts run through it? If it's just the ducts, I'd make sure there are no leaks and then insulate the ducts themselves.

Insulating the attic won't help the A/C as much as sealing your ducts and making sure they have at least R-6 insulation. There are diminishing returns beyond R-6.

There is a lot of conflicting information on that. I seriously doubt it's true.

The problem is the lack of or insufficient quality of

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Do you have a cite for that? People use cell phones in their cars all the time, and they're more enclosed with metal than your house will be even if you install a foil radiant barrier in the attic.

Think about it.

1. Sun heats roof
2. Roof heats roof deck.
3. Deck heats attic
4. Attic heats duct work and living space ceiling.

Now if you interpose insulation between #s 2+3 (deck and attic), the attic will be a bit cooler but the roof deck will turn to toast. This, in turn, will cause the roofing shingles to melt (well, not really, but they'll get a lot hotter).

No, better to evacuate the heat from the attic instead of trying to minimize its infiltration.

snipped-for-privacy@gmail.com wrote: ..

Sorry I missed that part the first time I read it.

Hogwash. I have foil up there and it does not block my TV or cell phone.

I have thought about it. The devil is in the details. Think about it.

We can agree your 1-4 and still reach a different conclusion. If you interpose insulation between 2 & 3, the attic might be a _lot_ cooler, but the roof deck will turn only slightly hotter (there's a feedback mechanism at work -- the hotter it gets, the more it radiates).

I've actually measured roof temperatures with an infrared thermometer, and found a negligible difference with and without a radiant barrier (only a part of my attic has one -- but both parts get the same amount of sun and are otherwise comparable). In fact, the area with the barrier measured slightly cooler; I take that to be the same within experimental accuracy.

I suggest that other variables (e.g. color of shingle, parameters of their installation, etc.) are much more important. And weather patterns trump all.

I think you're right. My TV antenna is in an attic with foil and works just fine. Unfortunately, I don't have any good authority or personal "with and without" type evidence that I can cite for the assertion that there's _no_ negative effect.

I suspect a careful analysis of the propagation of the frequencies involved would show that a radiant barrier is transparent to them.

What about if you use metal roofing?

cars tend to have lots of glass, thats transparent to RF, radio frequencies,

homes with metal siding have worse cell and tv reception than the same home with vinyl siding

One data point does not make for proof.

It depends on the installation. The problem isn't necessarily "blockage" of the signal but multipath. The antenna catches the signal, but the foil can also reflect a piece of the signal back onto the antenna. Since the speed of light is not infinite, this second signal arrives a fraction of a second later and tends to damp out the first one (the waveforms are out of phase). The situation can be worse if there are additional reflections from other surfaces.

Depending on the location of the transmitter, antenna, and reflector, you may or may not experience multipath. If you do have a problem you may be able to solve it by moving the antenna to a different location in the attic or switching to a more "directional" antenna, i.e., one with more gain. A higher gain antenna does a better job of "ignoring" the unwanted reflections coming in the sides and back of the antenna.

In short, there are a lot of variables at play.

Back to the topic at hand, I would say applying insulation directly to the ducts themselves is the simplest and most effective solution for the original poster.

Again, do you have a cite? I haven't heard that before.

So if your neighbor has a foil radiant barrier in HIS attic, you would expect poorer reception? It would, after all, create the same sort of multipath.

I think citing some numbers and/or actual experimental results would be a lot more convincing than arguments like this.

It makes sense. Multipath as mentioned, as well as signal blocking, which occurs wherever gaps between metal are less than a tenth of a wavelength L = c/f, eg 3x10^8/900x10^6 = 0.333 meters (about 1') at 900 MHz. So 900 MHz cellphone signals can only sneak through non-metal paths more than an inch or so wide. And they can't penetrate much past the skin depth in aluminum, d = 3208/sqrt(f) = 0.107 mils at 900 MHz, which is thinner than most metal sidings or foils or paints.

Nick

All that is true, but it is unusual for the foil to cause a problem and in fact a neighbor's foil may cause a problem. I would not shy away from foil backed insulation for fear of a problem. It is just too rare an issue. From personal experiences I have had four homes with the foil and never had a problem.

CJT is definitely right on this. You NEVER NEVER NEVER insulate below your roof deck.