What should be the difference between outside/inside temperature of an attic?

How Hot Is Too Hot When It Comes to Attic Temperature?

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Dear NH,
I am very interested in knowing what is a "bad" temperature for my attic. What should be the difference between outside/inside temperature of an attic on a summer day versus a winter day? Should there be a difference between a 78-degree day and a 98-degree day?

I have recently added a ridge vent to my roof and have also been taking temperature readings. I hit 130 degrees in my attic when the outside temperature was 83 degrees. Is this good? What's good? Any help in this matter would be greatly appreciated.
JB from Brown Deer, WI

I wouldn't get too obsessed over the temperature readings. Colder is better, but 130 degrees is not really that hot in that typical roofing materials can stand much higher temperatures. According to the Building Science Corporation at BuildingScience.com, a Boston-based architecture and building science consulting firm, the maximum acceptable temperature of an enclosed attic space near the sheathing is about 180 degrees. They have done much of their testing in Las Vegas and Phoenix, where conditions are the worst, and 150-degree attic temperatures are common.

Related: How to Insulate Your Attic for Winter

It's also been found that roofing design and materials have more effect on attic temperature than ventilation. Metal roofs reflect more heat than asphalt roofs, lowering temperatures, and lighter colored asphalt also lowers attic temperatures. Ventilation is important to give moisture within the attic a way to escape. Though most newer homes have extensive vapor barriers to keep living space moisture from getting into the attic, leakage around recessed light fixtures and bathroom vents still allow some moisture to enter the attic. This is not very destructive in the hot seasons, but can cause problems during winter freeze-thaw cycles when this moisture condenses on nails and other metal structures within the attic space.

For most homes, the biggest concern is the effect of attic temperature on living space comfort. The consensus seems to be that if you have the maximum amount of insulation in your attic floor (for your area), lowering the temperature of the attic by 10 or even 20 degrees will have a minimal if any impact of actual heating and cooling costs.

Reviewed March 2018

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