Spray foam insulation in Vented Attic?

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I did a lot of googling to try and come up with an answer, but I couldn't find anything addressing my situation.
I have a standard early 1980s suburban house in Texas with a vented roof. The A/C unit ductwork is in the attic, but only the attic floor is insulated (blown fiberglass). Obviously, this isn't an efficient use of the A/C unit since the attic heats up the cool air in the ductwork before it reaches us. However, we have a very good A/C unit (home's original owner was an HVAC engineer) with a fully programmable thermostat and smart home layout (only 2 windows face west), so our electric bill never gets beyond $250 even in the nastiest summer. Even so, I'd like to improve the attic temperature so the A/C won't have to work as hard.
I've researched local insulation companies, and they push the spray on radiant barrier (cost ~$2500) that's applied to the roof deck. We can't use the foil radiant barrier because it would block our TV signal (antenna in attic space) & cell reception (no land line). I don't mind the price, but anecdotes that I've found range from "didn't do anything" to "dropped my attic temp 30degrees". The one insulation installer who came and looked at our house admitted that radiant barrier wouldn't have a great ROI for us -- it'd probably take ~7-9yrs to fully pay for itself.
However, I've been looking at open cell spray-on foam insulation for the roof deck -- specifically Sealection 500 -- and every review I've seen said that it works beautifully. Ideally, it's supposed to be used with an unvented attic, but I was wondering if it'd buy you anything in a vented attic. My semi-educated guess is yes, but I'd be interested in hearing educated opinions.
TIA, ami
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snipped-for-privacy@gmail.com wrote:

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: http://www.tigerfoam.com/ but I suspect you will be wasting money at best and creating a bigger problem worse case.
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The attic is vented, unheated, the floor is what needs more insulation, foams go to R7.5, some are only R 4
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snipped-for-privacy@gmail.com wrote:

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.
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Joseph Meehan wrote:

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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CJT wrote:

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.
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HeyBub wrote:

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.
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On 6/30/07 6:03 PM, in article snipped-for-privacy@news.supernews.com, "HeyBub"

What about if you use metal roofing?
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wrote:

CJT is definitely right on this. You NEVER NEVER NEVER insulate below your roof deck.
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...if the thermal envelope for the house is the attic floor. If the house was designed to use foam insulation from the start, it's possible that the thermal envelope will be at the roof deck.
But I agree that spraying foam under the roof deck when the house already has fiberglass insulation on the attic floor is a really bad idea.
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wrote:

A 30 year roof will last about 12 years on an insulated deck.
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ValveJob wrote:

saying. In some sense, almost ALL attic insulation is "below the roof deck." I have found that the addition of a (non-sealing) radiant barrier is an excellent addition to the system.
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CJT wrote:

rather than what some might think of as "insulation." You DO need to be careful about where the vapor barrier is, and not create a system that's susceptible to mold. I don't think foam is a good idea unless it's somehow made permeable to moisture.
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No, a 30 year roof will last about 12 years when you insulate the the roof deck from the attic.
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ValveJob wrote:

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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.
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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.
http://ducts.lbl.gov/
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snipped-for-privacy@gmail.com wrote:
<snip> We

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

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