the miracle of insulation
It's a brisk morning here in Central Texas, down to 33 degrees Fahrenheit (what qualifies as a brisk mornin' 'round here...). Peering through our window toward our neighbor's roof, this is what I see: The miracle of insulation!
This photo is beautiful for several reasons. First, notice the lighter whitish colored areas on the brown composition shingle roof. Those are varying degrees of frost (I told you it was brisk!). The large squarish area of white is over a finished-out garage, completed sometime after the house was built in the 1950s. The roof in this area was clearly insulated since heat from the house hasn't melted or partially melted the frost. The rest of the house is either not insulated at all or poorly insulated (a reflection on 1950s construction, not the neighbors, who are fine folks).
Now notice the lines running toward the top of the roof: These are the underlying rafters, the 2x4s (2x6s?) holding the roof up. The frost level above the rafters is about the same in the insulated and poorly insulated parts of the roof. In the poorly insulated part of the roof, the rafters are serving as a wee bit of insulation since they preserve some frost compared to the space between the rafters. However, in the insulated part of the roof, the rafters are the weakest link, providing a comparative thermal bridge between the batts of insulation. In some parts of the roof, the rafters giveth; in other parts, the rafters taketh.
Since our roof is white, it's hard to see the frost levels and thus the amount of thermal bridging.
What does your roof look like?