Up on The Roof
The toolkit of an energy auditor is a fascinating collection of building science technology: infrared cameras, Wi-Fi-enabled digital manometers, blower doors, fog machines and combustion analyzer. It’s a lot of cool technology. Most of it runs on batteries or needs periodic calibration or has sensors that fail or software that glitches. However, there is one completely free, fail-safe method of seeing exactly where heat is leaking upward out of a house, and it should be used by any well-trained New England energy consultant. All you need is one-half inch to six inches of snow and a day or two of calm, cold weather. What is this method? Looking at the roof.
As heated air escapes upward from a house, most of it comes into contact with the underside of the roof sheathing, before it can make its way into the sky. Where it does so, it warms up the roof, and this means that the snow atop the leaky areas melts much faster than snow on well-insulated areas. The result can be a near-perfect ‘fingerprint’ showing where heat is leaking from the house below. I first heard about this method at the Better Buildings by Design Conference put on each year by Efficiency VT, and I’ve made use of it ever since. The pictures are examples from around northern Vermont:
Picture 1 – Most of this roof is well-insulated and shows even, consistent snow cover but melt patterns in the middle tell a different story: When the gable addition (on the right) was built, its roof was only insulated as far as the original eave (bottom of the roof at left). The ‘dead’ space between the two living areas is open to wall cavities below but has no ceiling or rafter insulation. Warm air from the house comes up through the wall (remember our old friend the convection loop?), has free access to the underside of the roof in that area, and consequently warms it up enough to melt the snow. You can even see the line of a rafter in the middle of the melted section. That six- or eight-inches-tall wood rafter provides more insulation to the roof than the one inch of wood roof decking around it, and therefore the snow directly on top of the rafter melts more slowly than the area around it, marking the rafter’s location. The level of detail that can be shown on roofs in this condition is amazing. When the weather is right, a quick walk around the outside of your house can show you a lot about where the air leaks are in your building envelope.
Take a look at the example in Picture 2: The bare section roof on the left is south-facing, and has been melted by sunlight. What’s the story being told by the melt pattern on the addition with the swooping porch roof? Line 1 corresponds to the top of the interior wall. Everything below line 1 is dead space over the porch roof, ‘outside’ the thermal envelope of the house. No heat from below means no snow melt. Line 2 corresponds with the ceiling of the second floor. Everything above it is attic space, and as you can see (especially on the left side of line 2), heat escaping into the attic is melting snow more quickly above that line. In between the two, is a sloped ceiling, with some insulation in the rafter bays. What about the big melted stripe in the middle? In this case, the culprit is an interior wall at Line 3. There is still fiberglass insulation across the entire sloped ceiling, but whereas most of the ceiling is air-sealed from inside relatively well by the painted sheetrock, the wall framing cavity was never air-sealed at the top so it acts as a chimney, bringing warm interior air right up to the roof plane where it passes through the fiberglass and melts the snow.
Nate Gusakov is a BPI-Certified auditor, home performance contractor, and energy consultant for Zone 6 Energy in New Haven, VT.