I like looking at roofs, especially after snow storms. Roofs tell us all kinds of things. After this week’s snowstorm, many stories were slowly emerging as the sun melted the snow in varying patterns.
On most roofs facing south, the sun has melted all the snow. (See roof picture #1.) That’s to be expected. On some roofs facing north, the snow is still there. (See roof picture #2.)
We can see this south-no snow and north-snow on a garage. The garage is unheated, so these two snapshots #1 and #2 show the solar warming effect on a building in the absence of any other heat source.
Some north-facing roofs have no snow left and are just as bare the south-facing half of that roof. That suggests a home is not well air-sealed and under-insulated. In short, the thermal barrier between the heated space and the attic or roof is inadequate. That condition is very easy to solve! See Energize links below.
On some roofs facing north, part of the roof still has snow and part may not. (See roof pricture #3.)
That mixed pattern suggests something inside the home is warming the inside of the roof more above one room than above others.
In many cases, there is a perfectly logical explanation. The attic or roof deck may be partly sealed and insulated. Or it may be fully insulated, but a vent for a bathroom or kitchen may spread warm indoor air to the roof. Let’s call this the “reverse Oreo” condition (a dark center with white snow on either side).
Often, we may see strips of roof showing through the snow, as is the case on this north- facing roof. (See picture #4.)
Those dark stripes show where the roof joists are located. Let’s call this the “zebra” condition. The melted snow there indicates timber is connected to the heating space inside the home.
The good news here is the snow present between the joists proves there is good thermal barrier present in between the joists. The joists in this roof are conducting some small, but constant amount of interior heat to the outside. This can be furhter reduced by adding an adequate thermal barrier between the joist and the interior or the joist and the roof deck. But that is typically only practical during a renovation project.
Let's look more carefully at picture #3. Sometimes, we see the opposite condition of more snow on top of the roof joists than in between the joists.
We'll call this the "reverse zebra.” The reverse zebra condition may indicate the need for more insulation overall for the roof deck or attic in between the joists. That is again easy to solve.
Whether you have zebras or Oreos on your roof, it is an excellent time to get a free home energy assessment.
PS Visit EnergizeNY to find your community's Energize assistance, if you don't live in Croton or Ossining.