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Snow-melt sleuthing

What melt patterns on your roof can  tell you about your’s home energy loss

If you’re looking for a winter pastime, try learning to read snow melt patterns on roofs.

Okay, you might be thinking it sounds a little nerdy, but not only is it a free and fun winter travel game, it may just help you to understand how your own home may be wasting energy.

For the first step in becoming a “Roof Melt Detective,” you need to be able to differentiate between snow patterns created by the sun, wind, or outdoor ambient temperatures. The easiest way is to compare nearby roofs of the same orientation. Chances are if the sun has evaporated snow from above or patterns have been created by a sweeping wind, neighboring roofs will have similar patterns. Melt patterns from inside heat loss will likely be somewhat unique to one roof in the neighborhood. They are also easiest to read a day or two after a snowfall, or even early in the morning after a frost.

The first photo shows a roof with an even, undisturbed snow pattern. You’ll typically see this in one of three situations: 1) the house has very effective levels of insulation in direct contact with an air barrier; 2) it recently stopped snowing and there hasn’t been time for melting; or 3) the building isn’t heated so the indoor temperature is the same or similar to the outdoor temperature — hence no heat transfer. The latter happens to be the case for the photo above.

When snow is melted by warm air rising from conditioned living space, the resulting patterns give clues to how the house is constructed, the effectiveness of the insulation and, more importantly, whether there is a continuous air barrier or not. Up to 40 percent of one’s heating bill can be attributed to air leakage — and often much of it through the ceiling into a ’cold’ attic. As the warm air continues to rise to the roof, it melts snow (or frost) at different rates, leaving signature shapes, such as roof rafters. Whenever you see a straight line bisecting a roof, such as in Photos 2 and 3, that line is more than likely marking where ceiling slopes transition to a flat ceiling. Note how the top of the remaining snow lines up with the top of the window in the gable, just where you’d expect the ceiling. Light fixtures, fans, drop down attic stairs or stairwells (as in the center of the roof above) are all common penetrations in the sheetrock or plaster, which, if unsealed, allow warm air to escape upwards. In Photo 3, snow has melted most quickly at the top of the roof above that ceiling plane, than on the lower slopes.

In Photo 4, we see the “wave” or “topographic” pattern a few days after a heavy snowfall, as snow melts unevenly — perhaps due to an attic full of belongings which slow or redirect warm air on its path upward. As the snow — solid water — melts from the warmed roof, it becomes liquid water and flows under the snow to the roof edge, where exposed to below freezing temps, it becomes solid as icicles or starts to form a ridge or dam. As more melt water flows, the dam builds. This will continue until ambient air temperatures rise enough to melt the snow and ice off the roof completely.

Vertical lines seen in Photo 5 are usually rafters. They will either melt first, if the slope rafter bays or cavities are insulated and the rafters themselves are of lesser R-value. Or they will hold snow longer as in the photo, because they are adding additional resistance to heat loss.

As seen in Photo 6, melting is often most prevalent under the roof edge, where warm air escapes between top plates, uninsulated attic floors, or though fiberglass insulation and / or improperly installed roof vents. While warm air rises, heat moves to cold and another condition which causes ice damns is if the top plate has less insulation value than the wall below it.

Photo 7 — a pattern which some of us might think of as Classic Charlie Brown — depicts using electric heat tapes for intentional roof heating as an easier way to remove snow from eaves than physically raking them. But if the intention is to removes snow before heat loss from inside creates icicles and ice dams from forming, it’s more costly than addressing the heat loss from inside.

I have written about effective strategies to eliminate ice dams and reduce heat loss through roofs in this column before, prompting an “Au Contraire” response opinion from a reader in a subsequent letter to the editor. To repeat my advice: Adding more ventilation was the conventional, 20th century approach which had varying degrees of success but continued to waste energy as it did nothing to prevent heat loss under the eaves. Advanced understandings of thermodynamics and its implications to building science have demonstrated that the more effective and energy conserving approach is to establish a continuous air barrier and insulate the top plate to a higher degree than the wall below. This is in conjunction with strategies to manage moisture. It’s always preferred to vent a roof if it can be done effectively, though that is often not the case. But adding ventilation without stopping air movement wastes energy, may not even solve ice dams. In many specific cases, careful detailing and consideration is required. And to that letter writer, if you are reading, this is exactly why architects and engineers hire me, an expert in building science, as the consultant on their designs.

Besides becoming an entertaining winter puzzle game, these types of roof melt patterns indicate that the building is losing heat in a way which can be slowed dramatically by creating an effective levels of insulation in direct contact with a continuous air barrier. If it’s happening on your roof, and a contractor wants to add insulation to your ceiling without doing air sealing, they will be selling you a partial solution which will be more costly to fix in the future.

In closing, can you interpret what’s happening in Photo 8?

Send your answers to me at mdillon@wildblue.net and I’ll let you know if you’re a successful Roof Melt Detective.

Margaret Dillon promotes high performance buildings through her energy consulting practice Sustainable Energy Education and Demonstration Services (SEEDS).

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