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There really isn't much one can do to insulate the hood for engine heat retention.
The foil products R value is relative to the heat passing through and off the side with the foil facing an open space. The foil doesn't 'reflect' heat, it prevents emissivity (coming off). Heat from the engine is going the other way. In fact you are basically insulating against heat gain from the sun hitting the hood and radiating into the engine bay.
The R value of the heat going upward against the foil bubble stuff would be less the R 1.
And then when the vehicle is moving, the air flow is going to push that heat.
As well as the hood being metal and contacting other points of the car (conduction).
Apart from sound deadening, it is really futile.
A foil topped hood (outside) that is kept clean would turn the hood into an insulator as long as you reduced the contact points of the metal hood with the car or used a different hood material.
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