13. Save Energy on the Roof & Attic

 

Roof and Attic

From the roof experts:

  • Seal ducts in the attic (or crawl space) with mastic or UL-181b certified duct tape. Other tapes dry up and lose adhesion over time. Mastic is a messy paste, but it dries quickly and seals permanently.
  • Running an attic ventilator when the attic floor isn’t well sealed can suck the air out of the house and waste a lot of energy.
  • The roof-wall junction is a common source of thermal bridging. There’s no easy after-the-fact fix for this common structural and architectural element.
  • “Cool” roof colors (lighter, more reflective) lower the roof temperature by 50° to 60° F and reduce the peak cooling load by 10 percent to 15 percent. Look for shingles rated for high “solar reflectance” and high “thermal emittance.” (Vice versa in cold climates, where you want a dark, absorptive roof surface.)
  • Your house could receive a tax credit for installing a metal roof.
  • Chopped cellulose, treated to be fire-resistant, is an eco-friendly way to insulate the attic floor.
  • Be sure there is adequate insulation where the sloped ceiling meets the attic floor (don’t block soffit ventilation grills).
  • Buy asphalt shingles with a fifty-year guarantee (versus the usual ten- or fifteen-year warranty). They’re thicker and heavier and much more durable—excellent against hail and storm damage.
  • Any uninsulated area in the attic lowers the R-value of the overall roof assembly.
  • Install enough insulation in the attic to get three to four inches of insulation over floor joists, otherwise a thermal bridge.
  • Allow three to six inches of clearance between the chimney (and other potentially hot surfaces) and any insulation.
  • Is there a vapor barrier under the attic insulation? If not, paint the ceilings of the upper-floor rooms with a vapor barrier paint or primer.
  • Caulk around any electrical and junction boxes in the ceiling right below the attic (ceiling fans, lights).
  • High ceilings allow air to stratify in warm layers at the top. A cathedral ceiling can be 15° F warmer at the peak. A “heat harvester” fan can take that hot-air layer and (gently) move it down to where the people are below.
  • The attic is everybody’s go-to storage place. Don’t squash any ducts with boxes or rip any openings in the duct runs. Better yet, stay away from ductwork altogether.

 14.1 INCREASE ATTIC VENTILATION IN THE SUMMERTIME.

You may think the air in the attic is stagnant. However, a correctly designed loft always breathes, if at a rate unnoticeable to observation. Ridge vents along the roofline (or ventilators in the roof plane itself) draw air out. Vents along the end panels or soffit vents under the eaves then draw replacement air into the attic space.

The humidity level remains relatively constant inside the attic, i.e., the same as outside. There’s no vapor-pressure difference to draw moisture into the roof, through the attic walls and eaves.

14.2 INSULATE THE STAIRWAY TO HEAVEN.

The one spot in the attic that doesn’t usually get insulated is the back (attic) side of the pull-down stairs (or access panel, as the case may be). R-2 insulation value versus R-60 for the rest of the attic?

It’s kind of messy to insulate that panel with fiberglass batts (not to mention getting that itchy stuff all over your arms whenever you climb into the attic). One option is to buy a four-inch rigid Styrofoam sheet and cut it with a hand saw to match the backside of the attic access panel.

Or position the Styrofoam panel over the opening so that when the stairs go up, they gently push the insulation upward.

 14.3 YOU DON’T NEED TO AIR–CONDITION THE ATTIC.

The attic is the Number One worse place to run ductwork. The attic temperature can reach 150° F in the summer, versus 60° F air in the ducts. No wonder you get a blast of hot air out of the supply diffusers every time the AC kicks on!

Most AC ductwork has little or no insulation (basically, the pricier the house, the more exotic the ductwork and insulation). Older homes have just a thin, flexible plastic tube (R-0); the standard upgrade is square duct board (R-4 for 1” up to R6 for 1-1/2” thick boards), sometimes even commercial-grade sheet metal ducts (very expensive) with either internal or external R-6 insulation.

People use the attic for storage, especially if there’s plenty of headroom. “Storage” means squashed air-supply ducts, broken seams, and generally, cold air exiting the duct into the hot attic.

Please put this on your handyman’s list for their next visit: Check the ductwork in the attic and plug any leaks. For out-of-the-way duct runs, cover them with fiberglass batts. It’s even easier with loose, blown-in cellulose insulation, which is easily moved around and not itchy!

If you have to purchase a couple of rolls of R-13 fiberglass batts to cover the ducts, it’s money well spent. Just be sure, again, you don’t squash any of the ducts.  

 14.4 INFRARED BUILDING SCANS FIND HEAT LEAKS.

If the snow melts off of your roof faster than it does on all the neighboring houses, there’s a good chance you need more attic insulation. Go high-tech and hire a contractor to do an infrared scan of your whole house. (Always wanted to do that, but

I just add the insulation anyway.)

Areas in the outside shell of your house where heat transfer is more significant—windows, doors, and other penetrations—show up bright red. Who knows, if you live in a really old house that has extreme heating and cooling bills, you may not have any insulation at all in the outside walls.

Experiment with the infrared scan: close the curtains on one window, the blinds in another, both in another, and see how each shows up on the thermograph photos.

 14.5 WHAT ACTUALLY KEEPS WATER OUT OF THE ATTIC?

Roof shingles are only the first line of defense against rain. They slough off most of the rain, but quite a lot of moisture still seeps through. The real workhorse in a roof is the “tarpaper” underneath the shingles, applied in overlapping layers for a duplicity of protection.

Tarpaper is felt paper impregnated with asphalt. (The same material used on the outside walls to form the vapor barrier.) When roofers drive nails through the shingles to hold everything to the plywood underneath, the asphalt seals around the nails and makes the whole assembly watertight.

All the water that gets past the shingles drains down the roof, unable to penetrate the tarpaper. The shingles (or other roofing material) also protect the surface from hail, sunlight, and other damage. But, mostly, the roofing material is there to protect the tarpaper underneath.

 14.6 ROOFING ECONOMICS.

Some roofers might tell you, “You don’t always need to rip all the old shingles off to put a new roof on.” If the roof is fifteen years old, you don’t think the tarpaper might need replacing, too? Especially since that’s what does all the work?

Shingles are heavy, and heavier still when they’re soaked with water. Roof framing can only stand so much weight: twelve inches of snow in the winter, perhaps ice, and you’re asking for trouble by putting multiple layers of shingles on the same roof, just to save a few dollars?

As long as you’re spending all that money for a brand-new roof, do it right. Protect your house and everything inside to the greatest extent possible: Remove everything down to the plywood backing, and start with new “water barrier” tarpaper, then fifty-year shingles.

14.7 BUILDING CODES ARE MINIMUM STANDARDS.

You can always do better than the mechanical and building codes, which are minimum standards and not always focused on energy conservation. Besides, a thirtyyear-old house was built to thirty-year-old building codes.

The last house I lived in, in Mississippi, had almost no insulation in the attic. That’s all the code required when it was built. (When I did a roofing job, there were three layers of shingles—back to the original green shingles!) The first thing I did was to add ten inches of blown-in insulation in the attic to get R-30, the amount recommended by the government Energy Star standards. That cut my utility bills in half and paid for the investment in less than a year.

My current house in Western North Carolina had R-30 when I moved it (it’s only ten years old), which meets the current Energy Star recommendation. However, I added another six inches of fiberglass batt insulation, which, again, paid off the investment in less than a year.

14.8 DOES ATTIC INSULATION HELP MORE IN THE SUMMER OR  THE WINTER?

Heat rises. In the summer, you want the heat to rise and stay there. Antebellum houses in the old south have tall ceilings, twelve feet or more. That’s so the heat of the day rises and forms a thermal barrier there, a static isotherm that acts as extra insulation against more heat entering the house from the hot attic. (Because a warm air layer at the ceiling means a lower temperature difference across the ceiling assembly, slowing down heat transfer from the attic.)

In contrast, houses in cold climates usually have low, 8-foot ceilings, so that heat stays closer to where the people are. However, heat always moves toward cold, and the greater the temperature difference, the faster the heat loss.

Building codes require much more insulation in northern locations (R-60 versus R-30 down South) because it gets so much colder outside—down to 0° F in places. Relative to 65° F inside equals a 65° difference. (In the south, the attic can reach 150° versus 85° inside equals a 65° difference, too.)

The difference is that heat goes up, and the thermal gradient in summer is down (the attic is warmer than the inside of the house), so less insulation is needed by code. I’ve always maxed out on attic insulation in my southern homes, up to R-60 (16 inches), because insulation is cheap, and it lowers your utility bills all year long.

14.9 FORCED ATTIC VENTILATION KEEPS THE SUMMER TEMPS DOWN. 

The building codes specify precisely what to do for adequate ventilation. The operative word here is “adequate.” Contractors are bound by the code, so your attic is no doubt adequately ventilated (assuming all components of the ventilation system are still working). That only means the roof assembly is able to prevent moisture damage to the structure.

We’ve seen that the attic temperature can reach 150°F on a 90°F summer afternoon, even with the standard ventilation (ridge vents, etc.) in place.

A thermostat-controlled ventilation fan can draw 90°F outside air into the attic to replace that 150°F attic air. The minimal amount of electricity to run the fan is far less than the cooling load it relieves from the air conditioning unit. (If the house is empty for an extended period, disconnect the attic fan.)

 14.10 ROOFTOP SPRINKLERS ARE LOW TECH BUT HIGH SAVINGS. 

Buildings can take advantage of water’s high heat of vaporization, just like the human body. This thermodynamic principle is especially useful in areas with a ready supply of inexpensive water. Warehouses in the deep south, for example, can use this idea to cool down a metal roof. Installation and operating costs are a fraction of a full HVAC system or even a forced-air ventilation system.

A high-tech, specialized sprinkler system on the roof emits a fine spray from evenly-spaced sprinkler heads every time a temperature-controlled valve opens for a fifteen-second burst of water. You only need enough water to wet the roof uniformly, with no water flow down the incline. A thermostat attached to the roof surface opens the valve for another burst when the roof temperature rises to setpoint again.

With each cycle, the heat of vaporization is extracted from the surface of the roof as the water evaporates. Virtually no electricity is used (only a brief, low-voltage signal from the thermostat to open the valve), so water is the only cost.

The system lowers the working temperature of the roof, which extends the useful life of most roof surfaces because the heat of the sun inflicts less thermal damage.

I installed one of these commercial sprinkler systems on the roof of my home in central Mississippi. It was a straightforward installation and turned out to be quite effective. My neighbors called it a “redneck air conditioner.” Until I showed them my utility bills.

The City of Vicksburg has a demand charge for water plus a sewer disposal charge linked to the volume of water used. These fees lengthened the payback slightly, but the system still paid for itself in just two years, after which time the savings were all mine.

There was no attic under the roof area where I installed the rooftop sprinkler. The room underneath had a vaulted ceiling up to the rafters, the ideal kind of roof assembly for this system. It still works fine on a roof with an attic: if the roof temperature stays below, say 120°F, in the summer (versus 150°F without the sprinkler), then the attic temperature will remain low as well.

Note: These roof projects are interdependent. If your roof has a thermostat-controlled attic vent fan (which gets the attic temperature down to the 120°F), then the rooftop sprinkler won’t have as good a payback. And vice-versa: installing a vent fan if you already have the sprinkler system won’t save as much energy.

 14.11 INSULATING A VAULTED CEILING.

Sometimes there’s no attic above an occupied space in which to add insulation; for example, a vaulted ceiling that fronts on the roof rafters. If you need R-60 insulation (sixteen plus inches), but the rafters are only ten inches, you’re out of luck.

One option is to add insulation to the roof. You can “frame out” the roof above the vaulted ceiling with two-by-fours on edge, and fill that four inches with exterior-rated rigid foam. Build a new roof on top of that, and you’ve got your R-60 energy savings.

 14.12 DARN IT! FOILED AGAIN!

You may have heard the term radiant barrier applied to a roof treatment. This is a thin layer of aluminum (metal) foil that’s integral to the roof, typically under the felt paper and shingles. You can also install it from within the attic, to the underside of the roof.

A radiant barrier blocks the high-energy, short-wavelength part of the sun’s bandwidth. Absent the metal foil, damaging Ultra-Violet rays penetrate the attic and enter the occupied spaces to heat the building mass. (Notice that metal roofs are radiant barriers.)

If you live in a climate dominated by hot summers, it’s helpful to block that UV radiation because you eliminate part of the thermal load on the air conditioning.

If you live in a colder climate with harsh winters, however, you want that thermal energy to reach the living quarters, where it heats the furnishings, floor, and other thermal mass. (This implies that metal roofs have their drawbacks in northern climates.)

One note of caution: It feels extra cold in the winter if you have a radiant barrier, even if it’s a mild winter. At least that has been my experience living in houses with and without radiant barriers.

14.13 PASSIVE SOLAR ROOF CHOICES.

Sunlight streaming into your house from the sun, low on the horizon in the winter, heats a concrete floor nicely (or a concrete wall) to bring thermal mass up to a whole new level: as a radiant heater. This use of thermal mass is a critical element in passive solar architecture.

A light-colored roof in a warm climate saves cooling energy because much of the sunlight is reflected away before it can heat the roof assembly (and penetrate UV into the house proper). Likewise, for light-colored exterior walls.

A dark-colored roof in a cold climate attracts all of the sun’s energy and can also cut your utility bills. Ditto for dark-colored exterior walls.

Note: Changing the roof or wall color only to save energy isn’t a good project because it costs too much, and the payback is too long. However, roof color is definitely an important consideration when you need to replace the roof or paint the exterior.

 14.14 SEALED VERSUS VENTILATED ATTIC DYNAMICS.

Attics have vents to mitigate moisture problems, but they’re sometimes overwhelmed. If the roof leaks, a lot of extra moisture can intrude, often not noticed for a long time, until it discolors the ceiling below.

Some homeowners seal the whole attic: block the vents and spray the entire undersurface of the roof and the inside of the end panels with expanding foam insulation. This eliminates the whole ventilation-moisture problem, doubles the insulating value of the attic-roof-ceiling assembly, and gives you a dry, secure storage area.

The attic will still get hotter than the outside, but not excessively so. Sealed attics do have one new problem: the roof now gets much hotter than before, and that diminishes the useful lifetime of shingle roofs. It might even void the warranty on your shingles. If you have a metal roof, that’s not a problem.

If you do seal the attic, be sure none of the exhaust fans from inside the house still vent into the attic. Send that air outside.

14.15 PAYBACKS AREN’T HELL, AFTER ALL.

So far, we’ve discussed several roof projects: radiant barrier, forced ventilation, rooftop sprinkler, and adding more insulation. As you do each roofing project, the payback on the remaining renovations decreases. Consequently, you need to plan ahead and devote limed resources to the best projects.

The cost for a contractor to install a thermostat-controlled attic fan (roof work plus running the electrical conduit) is going to be about the same as the DIY (do it yourself) project, adding more insulation to the attic. The attic fan only saves energy in the summer. The insulation saves energy all year.

The radiant barrier project requires a new shingles job (or a metal roof, which can be very expensive but lasts forever). If you need a new roof anyway and you live in a warmer climate, at least get a light-colored roof (maybe with an integral radiant barrier), then see what the attic temperature is after that.

Or you can keep the old roof a little longer and try out a rooftop sprinkler (assuming a good, inexpensive water source), which will extend the life of your roof for a few years.

 14.16 HAVING TWO GREEN THUMBS

Having a home to practice on lets you crawl up onto the roof with a garden sprinkler to cover the roof every thirty minutes with a glaze of water to test out the idea of a rooftop sprinkler. (Be careful up there!)

Does it lower the attic temperature? Does the surface temperature of the roof remain much lower? Will the manufacturer tell me just how much longer my existing roof will last? (Which you can then factor into your payback calculations.)

 14.17 FROZEN SNOW IS ACTUALLY A DECENT INSULATOR.

Igloos are made of blocks of compacted snow, cut with an ice saw and stacked into a circular dome. Fiberglass insulation works because it traps air. Frozen snow does the same thing: ten inches of compacted snow has the same R-value as six inches of fiberglass—better than the outside wall of your house! (No thermal bridging from wood studs.)

If frozen snow on your roof wasn’t so heavy (not to mention ice underneath), it might be smart to leave it there. Except, a heavy roof can quickly become a collapsed roof.

Note: Frozen condensation inside the freezer is an insulator, blocking off the cooling coils. Make a habit of cleaning it off when it gets over ¼ inch.

Future Tech! 

14.18

If all the houses in California were required to install “cool” reflective roof colors for new construction and re-roofing jobs, they’d save so much energy there wouldn’t be any more rolling brownouts in the summer.

14.19

Roofing manufacturers need to develop “hot” roofs for cold climates that absorb solar radiation and store all that thermal energy.

14.20

Local building codes should require all ductwork to be located inside the building envelope and not in the attic or crawl space.

14.21

Mortgage companies can require a thermal scan of homes as a measure of their energy-efficiency.

14.22

None of the houses I ever occupied had a vapor barrier in the attic floor. That square footage is comparable to the total outside wall area, and there’s zero vapor barrier? That’s okay with all the current building codes?

14.23

Given new building standards concerning “tight construction,” there’s still scant attention to sealing the attic floor.

14.24

Homeowner’s insurance companies should give a healthy deduction for houses with fifty-year shingles, which are impervious to hail and storm damage.

14.25

Thermostat-controlled attic fans benefit the summer cooling load so much (and with so little power) that most warm-climate cities would do well to require them in all new houses (matched with “tight” attic floor construction).