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Drive-by Energy Audits

Posted January 18, 2011 08:30 AM by Alex Wilson
Related Categories: Energy Solutions
If heat loss through your roof is going to melt snow and cause ice dams, use a snow rake to clear the snow. Here's my wife raking our roof after last week's snowstorm. Photo: Alex Wilson. Click on image to enlarge

With all the snow we received in southern Vermont last week, it's a great time to be an energy nerd! Lots of snow on roofs means that it's easy to tell at a glance how energy efficient houses in the neighborhood are. I mean, it's not a thorough energy audit. But it's a good way to quickly get a sense of how buttoned up these houses are.

The principle is pretty simple: the less insulation in a house attic or at the rafters, the more heat escapes through the roof. That escaping heat melts the snow. As I'm driving into town, if I see that most of the nearly two feet of snow we received is still sitting there and the depth even, I can be pretty sure I'm looking at a well-insulated and tight house. Unless it's a fairly steep metal roof (in which case the snow slides off pretty quickly), loss of snow is evidence of poor insulation.

It's often possible to see exactly where the heat is escaping--because the snow is gone or the depth much less in that part of the roof. A wavy surface--like moguls on the snowboard slope--is a sure sign of wasted energy. In some conditions (especially with lighter snowfalls), it's possible to see the exact location of the rafters, because they provide some additional insulation so leave ridges or lines of snow while the rest melts. Usually, when excessive heat loss through the roof melts snow, you see ice dams at the roof edge. As the snow melts, it runs down the surface of the roof (under the insulating layer of snow) and freezes once it gets to the overhanging eaves--which are colder because they aren't receiving escaping heat from the house. That freezing snowmelt can form massive ice dams and thick stalactites of ice extending down toward the ground.

Another very common problem--even in situations where there's lots of insulation--is air leakage at the eaves. The juncture where the top of the wall framing connects with the roof framing and attic floor joists is notoriously difficult to air-seal, so leakage is common. Warm air flows up from the house below and melts the snow at the edge of the roof.

The superficial "fix" for these problems is to use a long-handled snow rake (a common Vermont tool) to pull that snow off the roof--at least the bottom four to six feet. It's best to do this shortly after the snowfall, before much melting from below happens and ice dams build up. (The photo is of my own house, with yours truly manning the camera from a safe distance while my wife carries out this important task.) With some of the snow left on the roof, you might still get ice dams (depending on conditions), but there's a greater chance that the snowmelt will run down the lower section of roof and evaporate without building up an ice dam.

The real fixes to these problems are much better done in good weather: dramatically boosting the insulation in your unheated attic or roof and carrying out air-sealing.

This blog isn't at all meant to suggest that we should skip a "real" energy audit as we think about how to improve the energy performance of our houses. A thorough energy audit by a weatherization contractor will include a blower-door test and, often, thermographic analysis (in which a special infrared camera is used to identify areas of excessive heat loss). But the drive-by energy audit after a snowstorm is a great way to get a quick sense of the need for a real energy audit.

In addition to this Energy Solutions blog, Alex contributes to the weekly blog BuildingGreen's Product of the Week, which profiles an interesting new green building product each week. You can sign up to receive notices of these blogs by e-mail--enter your e-mail address in the upper right corner of any blog page.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. To keep up with his latest articles and musings, you can sign up for his Twitter feed.

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1 I love that you're getting th posted by Ran3dy Bright on 01/18/2011 at 09:22 am

I love that you're getting this out! I'm an energy auditor from way back, and am even teadlching my 11-year-old daughter to to do this kind of drive-by audit! Nice job!

2 If you are extremely well ins posted by dfhoughton on 01/18/2011 at 07:33 am

If you are extremely well insulated you may need to rake before heavy rain. The deep snow that builds up over a cold roof becomes a huge sponge. If the pool of water held by the snow gets deep enough it will get under your shingles and soak all that nice insulation. Or if you have another cold snap after the rain it can form the ice dams not formed by snow melt.

3 I call it the Yogi Berra Ener posted by Lloyd Alter on 01/18/2011 at 07:45 am

I call it the Yogi Berra Energy Audit: He said "you can observe a lot, just by watching!

4 I always thought the reason f posted by Virgil on 01/18/2011 at 05:13 pm

I always thought the reason for ice dams was the opposite of what you describe - heat loss through the main body of the roof causes snow melt, then when it hits the overhang, it re-freezes (no heat from below). Thus, don't understand why you say air leakage at the edge of the roof is bad? If it keeps the snow in liquid form, so it flows over the edge and it won't re-freeze on the roof and cause a dam. That's why people use defroster cables at the roof edge, to keep the melted snow moving on over the edge. Surely leakage at the eaves accomplishes the same effect?

One thing we found makes a huge difference - the ceiling in between the 1st floor and attic is insulated, AND the finished attic is also insulated (unheated and unused in winter except for a couple of guests for a week or so). Thus, heat from the main house has to go thru' 2 layers of insulation - our roof always has snow left on it when all the neighbors' roofs are melted.

5 Virgil, What you describe in posted by Alex Wilson on 01/19/2011 at 06:20 am

Virgil, What you describe in the first sentence of your comment is exactly the process I'm referring to--so I think we're in a agreement there; I don't see the contradiction. Warm air leakage from the house is another source of heat that melts snow. Often that warm air hits the roof sheathing just a little ways up from the overhang, so the same process occurs: snow melts from the underside (against the roof) and runs down to the roof overhang, where it cools off and freezes--causing the ice dam. In both of these situations, the snow on the roof provides some insulation, so the melting occurs from the bottom (next to the roof).

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