Spray foam attic ceiling ! What is going on here?

David, you’re saying that it’s not the heat transmitted from the interior… but heat absorbed from the home exterior and retained by the roof framing, and the subsequent release of that heat that is causing the shingles to thaw faster than other parts of the roof that didn’t retain any heat. Those parts are still cold (kinda like a cold roof overhang creates ice damming).

Because the rafters absorb and store heat from being in direct contact with the roof during the day, it makes sense that they would give off that heat when the roof temps drop, causing the striping seen. Being covered in insulation actually would amplify the effect.

Time (which we did not discuss) is also a factor.

The time for the thermal mass to give up it’s heat makes the lines more apparent.
If you look at the second house, you can still see lines, but because they reached the frost point about the same time, they are not as obvious.

When we talk about insulation, time is always left out and never considered, but it matters. When water pipes freeze, people say they need more insulation. More, only increases time before freeze, but it will freeze unless the weather changes.

Insulation will not stop freezing. It just slows things down.

Other factors such as the type of shingles, angle and direction of the roof to the sun will affect the amount of time it takes one roof warm up over the next one. I agree with David, this is about the roof warming from the outside.

Actually it is not warming, it’s the cooling rate that occurred before the frost started to form.

More energy is stored due to mass and it takes longer for the frost to start forming. (Which will also melt faster when it warms up because there is less frost built up).

The second roof has melted all the frost. Except in some areas where the trusses or rafters are if that is what I see at the upper part. These boards are slowing the transfer of house heat from penetrating the decking. The first home the rafters are the weak link the envelope of insulation. They have less insulation covering them. Hence home heat will penetrate at a higher level.

No, the second roof is Totally covered in frost…

The second roof doesn’t have foam.