What is the Frost Phenomenon?

Winter is coming, and with it, many potential challenges for your home’s energy efficiency. In many parts of the U.S. and Canada, winter means heavy frosts and snowfall. Properly insulating your home, including your attic area, is of utmost importance to the overall efficiency of your home.

So how do you know if your home is adequately prepared to meet the demands of this winter’s severe temperatures and weather? You can start by being observant. After a night of heavy frost, go outside and take a look at your roof compared to those of your neighbors. Overlooking all external factors (such as direct sunlight to your roof that will melt your frost more quickly), what do you see?

If the first thing you see is a solid layer of frost over your shingles, you can breathe a little easier. Whereas, if you see no frost at all, or frost only along the rafter lines, it may be time to take action.

Frost Ghosting

The phenomenon you are experiencing when you see narrow vertical striping of frost on your roof is called “frost ghosting”. This is a very visual sign that you are losing heat through your roof. The lines where the frost has not melted is where the rafter system is touching the roof sheathing on the inside.

Typically, in stick-built homes, batt insulation is placed directly above the ceiling. This is done to keep conditioned air in the main living area. Problems arise when the conditioned air seeps through the batt insulation into the open attic space. Because there is no insulation on the interior side of the roofing plywood, the warm air from your home heats up the underside of the roof. The areas where the rafters butt up against the plywood roof take longer to heat up because they are thicker. Whereas the plywood, which is typically less than an inch-thick, heats up faster. So, when it frosts, the conditioned air in your attic melts the frost on your roof between the rafters causing frost ghosting.

Problems in Stick-Frame Roofs

Conventional House with Ghosting

Where many begin to have problems is when this melting on your roof leads to ice dams. Ever see a house or building with icicles on it? Well, the icicles are a byproduct of an ice dam. An ice dam forms when the temperature drops below freezing, accumulating frost, or in most cases, snow. Then, as the upper part of your roof heats up (hot air rises) from the inside, it melts the snow. The water then runs down to the colder eave, or edge of your roof, where it re-freezes forming a ridge. As more water melts from above and subsequently freezes along the ridge, water can seep under the shingles. It can even leak into your attic below, damaging drywall and insulation. At that point, mold and rot can grow and compromise both your roof and the health of your family.

Stick Frame Attic

Solutions with SIP Roofs

So, how can you solve this problem? The best option is to build your home with energy efficiency in mind and not experience the problem at all. This is where incorporating a SIP roof by Insulspan can potentially save you a homeownership headache. Take a look at how Insulspan SIPs roofs look in a frosty situation.

SIP Roof

Notice how the frost on this SIP roof is the exact opposite of the stick-built roof. In this SIP home, the visual phenomenon of frost ghosting of the rafters is minimal to non-existent. Instead of the frost melting in between the rafters, frost covers the majority of the roof. For SIPs, the frost only melts along the thermal bridges where the panels fit together; even then, SIPs minimize melting until it is hardly noticeable. The SIPs are keeping the warm air in, thereby not melting the frost on the roof.

The way SIP homes differ from stick-frame is how the builder thinks about the structural envelope and understands Building Science. Instead of insulating directly above the ceiling and hoping conditioned air does not seep through, SIPs become a part of the building envelope. The problem with the stick roof is that the interior side of the roof is not insulated. This allows the warm air to heat up the plywood.

Using SIP panels, the builder is directly insulating the roof itself. This means that any warm air that rises and tries to escape will be stopped short by the continuous foam core insulation. Rather than the whole space between the rafters heating up and melting frost, the only material that will heat up is the thin piece of lumber joining each panel. When the lumber heats up, the frost on the roof can only melt along those lines rather than the entirety of the roof, saving energy.

SIP Roof on Left Compared to Conventional Roof on Right

So, the next time you walk around your neighborhood after a thick frost, observe what the roofs look like. With your energy efficient roof in place, it is the perfect time to share your new knowledge with those neighbors who are experiencing frost ghosting.

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