Tag Archives: convection

IT’S THE PERFECT WEATHER TO PLAY “READ THE ROOF”

by

0

White makes right: Snow shows that the right side is better insulated than the left.

White makes right: Snow shows that the right side is better insulated than the left.

With a fresh blanket of snow on the roofs, and the furnaces churning in the basements, it’s a fine time to play “read the roof.” This amusing/depressing game can reveal a number of secrets that your house might otherwise keep to itself.

Like, where is there insulation, and where is there not?

On the right side of this first example, you can see a clear line where the insulation ends, over the “knee wall” portion of the third floor. (See cross section below.) Escaping heat is melting the snow faster on the lower part than the upper. On the left side of that big dormer, there are a couple of rafter bays where escaping heat has melted the snow completely, suggesting no insulation; and between them, a pretty well insulated bay. Chimneys are almost always good at conducting heat from the inside to the outside, melting snow in a circle around their base.

 

Action shot: Thermal bridging under way.

Action shot: Thermal bridging under way.

Here’s a classic case of “thermal bridging.” On the left side of this roof, the wooden rafters that support the roof are allowing heat to escape faster than the insulated bays in between them. The migrating heat melts the snow over the rafters a bit faster, creating those parallel lines. It’s like an X-ray of the roof framing.

Extra credit: The overshot eave of the bungalow gets no heat from the interior, so see how the snow there is slightly deeper there?

 

Compare and contrast: An efficiency freak does not live in my neighbor's house. She may not have amazing insulation, but she has a life.

Compare and contrast: An efficiency freak does not live in my neighbor’s house. She may not have story-book insulation, but she has a life.

Can you spot the hot spots?

Can you spot the hot spots? (That line down the middle is ice on my windshield. Sorry.)

OK, that’s my house. I’m an energy efficiency geek. The home of my neighbor is more normal. In old houses, those rafter bays are only about 4 inches deep, which really limits the amount of insulation you can cram in there. Both these houses would have been built with no insulation, or perhaps a product from the “Yankee Ingenuity” line. (Newspaper isn’t uncommon; I have also seen sawdust and old clothing.) Updating of insulation tends to happen piecemeal, willy-nilly, as the years  and energy crises and remodeling projects go by. The result is usually not “blanket of snow,” but “crazy quilt of snow.”

 

This last one is frankly open to interpretation. It’s an odd pattern. We can see again that the cap of the house — the flat part of the third floor ceiling–is better insulated than the sloping part over the eave. On that lower part, those narrow white stripes suggest that the rafters under the roof are actually the most insulating element of the “roof assembly,” and the rafter bays between them are letting heat out faster. And both the chimney  and the sewer vent stack are acting as thermal bridges.

The puzzling part is that row of hot dots. Something about the insulation pattern directs rising heat to these distinct spots in the roofing structure. Those hot spots may not be very big, but once the black roof is uncovered, it heats quickly in the sun and enlarges the dot.

For reference purposes, a cross section of a basic roof. And the ice dams that occur when insulation is of the crazy-quilt variety.

Cross section. That swirly stuff is insulation, but only in new or really well renovated houses.

Cross section. That swirly stuff is insulation, but only in new or really well renovated houses.

ICE DAMS AND HOW TO NOT GET THEM

by

0

Figure-1

Image: UMN Extension

Looks like time to re-post this classic, after all: Last year, half the houses in the neighborhood developed ice dams. Part of the reason? This neighborhood has tons of bungalows.

The bungalow is native to sunny India, where the deep overhang of the eave acts like an awning.

Maine and India have little in common beyond the bungalow. Here, that overhang gets very cold, because it’s not heated by warm air rushing upward through the ceiling and into the attic.

That fleeing warm air does heat the roof over the attic, however, and melts the snow thereon. Water runs down the roof as god intended. But then it hits the unheated overhang and promptly freezes. Over time, a ridge of ice forms atop the eave. Now running water backs up behind a growing ridge of ice, seeps between the shingles, and enters parts of your house previously reserved for dryness.

The slanted ceiling of a bungalow’s second floor makes matters worse: You can’t add much insulation there to slow the flow of heat.

The outdoor angles compound the problem, as intersecting roof lines direct a lot of water toward the valleys.

What to do? Don’t do this: In a fit of freezing pique I once took a pickax to my ice dams. It tore that ice right up! And the roof under it, too!

Prevention, as usual, is the sensible thing. Maintain a clear path for water to take off the roof. Rake snow off the eaves before the cycle sets in.

And if you must build a bungalow in Maine, insulate the heck out of it.

ADJUST THE THERMOSTAT, OR STEADY ON?

by

1

Heat being heat. [PD] Wikimedia

Heat being heat. [PD] Wikimedia

Of course it’s not that simple. It’s all about mass. And the mass of one house can be arranged quite differently from the next. It’s particularly the hot mass you need to keep in mind.

The Umpteenth Law of Thermodynamics says every speck of matter  in the universe is trying to arrive at the same uniform temperature. Hot things are forever shedding heat, and cold things are forever absorbing it. We try to arrange our homes in a way that prevents heat from radiating out into the cold air, snow, trees, cars, and the black universe. But heat escapes nonetheless.

And the warmer your house is, the faster heat will pulse outside to achieve harmony with coldness. That’s why it saves energy to turn the thermostat down when you’re sleeping or working: The closer the inside temperature is to the outside temperature, the less ambitious the heat is about cooling off.

The big, fat caveat, especially here in Maine where many old houses still hiss and thump along with steam heat, is that mass messes up the thermodynamics (which weren’t particularly tidy to begin with). Steam has to heat up hundreds of pounds of cold, iron radiator before much heat can pass into the air and the walls and the toilet seat. Same goes for radiant heat in concrete floors: The heating mass takes a long time to cool off; then a long time to heat up.

Even so, turning the thermostat up and down for an old steam system doesn’t make the furnace work any “harder,” or burn more fuel in the long haul. It truly does save energy (money), says the Department of Energy–about 1% savings per degree if you turn down for eight hours a day.

The problem is that you may not love the sluggish changes in temperature that result from a massive heating system: By the time the bathroom gets warm in the morning, it’s time to go to work.

The cool news is that new thermostats are much better at physics than I am. Brainy new appliances can continuously calculate the ideal timing of your furnace’s bursts of effort.

 

ICE DAMS AND HOW TO NOT GET THEM

by

0

Figure-1

Image: UMN Extension

Looks like its time to re-post this: Last year, half the houses in the neighborhood developed ice dams. Part of the reason? This neighborhood has tons of bungalows.

The bungalow is native to sunny India, where the deep overhang of the eave acts like an awning.

Maine and India have little in common beyond the bungalow. Here, that overhang gets very cold, because it’s not heated by warm air rushing upward through the ceiling and into the attic.

That fleeing warm air does heat the roof over the attic, however, and melts the snow thereon. Water runs down the roof as god intended. But then it hits the unheated overhang and promptly freezes. Over time, a ridge of ice forms atop the eave. Now running water backs up behind a growing ridge of ice, seeps between the shingles, and enters parts of your house previously reserved for dryness.

The slanted ceiling of a bungalow’s second floor makes matters worse: You can’t add much insulation there to slow the flow of heat.

The outdoor angles compound the problem, as intersecting roof lines direct a lot of water toward the valleys.

What to do? Don’t do this: In a fit of freezing pique I once took a pickax to my ice dams. It tore that ice right up! And the roof under it, too!

Prevention, as usual, is the sensible thing. Maintain a clear path for water to take off the roof. Rake snow off the eaves before the cycle sets in.

And if you must build a bungalow in Maine, insulate the heck out of it.