What would be an Acceptable Attic Temperature compare to Outside\Inside ?
For Example; If Outside Temp. found is 75Degree F. What would be an Ideal Attic Temperature.
It is what it is .
Too many things to effect the heat.
Colour of shingles is there any wind how much ventilation is there .
How bright is the sun is it a cloudy day.
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Not 200°F 
Beyond that, what Roy said.
I thought I read somewhere that the temp should be within 20 degrees.
If you pass out and die, its too hot. If not, then its ok 
Here in central Missouri last summer on a 95 to 100 degree day it was common to see 140 to 145 degree temperature measured on the underside of the roof sheathing.
If you think the attic is going to be too hot open up the attic access and let the air flow up into it while you do some of your inspection on the main floor. Works well to remove the stale air also as you don’t really want to breath it anyways.
If you can’t use the AC open the window closest to the attic hatch.
That’s one heck of a litmus test.
To many factors to determine and never heard of an acceptable temperature differential. Shingle color, open attic space, ventilation to many factors and never heard of a ‘standard’…
So in my opinion the answer to your question is there is no real definitive answer.
The question is what is the** ideal** temperature of an attic when outside temp is 75.
The** ideal** temperature for the attic is whatever the thermostat is set to. 
Industry standards and Normal attic temperature difference should be within twenty degrees of the outdoors temperature. Inadequate ventilation cause a magnitude of issues with the home. Visit Lomanco website for a simple calculator you can use. Basic ventilation requires 50% intake air and 50% exhaust. More intake air is better. Regardless in Summer’s the attics get extremely hot and can be dangerous.
What Randy Mayo said would be common.
Too many factors. No set guideline. On a sunny day, your way over 20
Degree differential in most attics.
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The “IDEAL” temperature would be for the attic to be the same as the outside temperature. Now we all no that really never happens. Take ice dams for example…if the attic temperature was as cold as the outside air temperature, they would never form. That is why in a home with poor insulation and ventilation you are more likely to have ice dams. In the south when the attic temperatures become extremely high because of poor ventilation, it will literately cook the shingles off a roof. So I would say that the closer you get to equal the better.
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The ideal attic temperature would be the same temperature as the home interior, not the exterior. The same temperature as the interior would limit heat gain and loss to the living space and help limit heat-related damage to the roofing material where that was a concern.
But “acceptable”? There are no standards by which to establish limitations. It’s not a valid question and is a non-issue.
Time of day, year, sunny, cloudy…etc. I think you would have to shoot more for a daily and yearly average rather than an actual specific temperature. Almost like measuring radon as the temperature swings will be wide. Basically, many of the same factors that affect shingle performance also affect attic temperature.
& M Clark
There are no “standards” but there several considerations based upon how your viewing it. What type of roof material (could be damaged by heat). HVAC in the attic (is the air coming out sufficient to condition the IA to the thermostat set point). Weatherization (insulation and air temperatures affect heat transfer rate which y exceed the HVAC capacity).
The heat transfer rate is Q = U * A * Delta T
As C&S pointed out, if the Delta T is 0 degrees F (same temp in attic as indoors), heat will not pass in either direction.
It gets more complicated when you deal with the “U Factor” (insulation).
You can have a 20 degree hotter attic if the insulation is sufficient to slow down the heat transfer until the solar load diminishes.
If this doesn’t make sense, try going in the attic and if you can hardly breath it is too hot.
Note: if the attic is the same temp as indoors in the winter, you have too much moisture and heat leakage to be acceptable (so it doesn’t work for winter).
Refer to your Inspection Standards; you observe, not analyze anything.
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I have a different opinion on this. If your insulation and building envelope tightness are so bad that the unconditioned attic temperature is the same as the indoor temp throughout a variety of outdoor conditions, this is a very serious problem and the energy bills will be out of control.
Unconditioned Attics should be ventilated to/from the exterior such that the attic will be able to eject excess moisture and so that the attic space does not overheat in the summer time.
A conditioned attic would ideally be balanced with the rest of the interior space.
This is my understanding, if I understood the OP’s question.
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Bert this is a 5 year old post.