This article was originally published in the November/December 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online November/December 1996
When it's time to replace worn siding and roof coverings, recognize the opportunity to upgrade energy performance. Here are several options for incorporating energy retrofits into roof and wood siding rehabs.
The old adage says, Within every problem lies an opportunity. This is certainly the case when it comes to residing or reroofing a home. Replacing roofing and siding is expensive, so it's often a catastrophic event, like water dripping from the ceiling, that launches the project into motion. But there is much to be gained beyond fixing leaks and worn-out siding. A well-designed exterior retrofit will lower energy bills, improve comfort, and redefine a home's level of performance.
First, seal any air leaks in the ceiling. Use a can of foam to seal wire penetrations, cracks, or spaces that connect the attic to the living space below. Next, provide adequate levels of insulation. Since space is restricted at the roof edge, use an insulation material that has a high R-value per inch of thickness (see Home Energy's Consumer Guide to Insulation, HE Sept/Oct '96, p. 21). Polyisocyanurate foam board, such as Thermax, is rated R-7 per inch. Cut the insulation into strips and stack layers of these strips between the rafters and the ceiling joists directly over the outside wall. Cut the strips so they fit snugly against the framing members of the roof and ceiling. Be sure they extend 2 ft into the attic, and leave 1 1/2 inches above the stack to allow air to pass for soffit-to-ridge venting. Seal the strips to the framing members with canned spray foam to make the connection airtight (see Figure 1).
To finish the job, install insulation retention baffles to hold back loose-fill attic insulation. Reinstall the sheathing and begin the reroofing project. You may not get the full recommended R-value directly over the outside wall, but the airtightness and insulation will be greatly improved.
Cathedral and Sloped CeilingsHomes with sloped ceilings (like those found in Cape-style houses) or cathedral ceilings are difficult to retrofit because the rafter cavities are sealed by finished surfaces. Either the exterior surface of the roof or the interior surface of the ceiling must be retrofitted to block the flow of heat. It is painfully expensive to rip apart a new roof or ceiling to add insulation. An energy retrofit becomes more palatable and cost effective when it is combined with a scheduled reroofing project.
Each house is built differently. Some have insulation already in the rafter cavities; others are only partially insulated. Some have roof venting in place; others do not. Some are built tight; others are leaky. Peel away the outer skin of the structure to expose the roof cavity and frame. You can see what you are up against once the roof shingles and sheathing are removed. Check for the details listed in Table 1. Rot, degradation, and structural damage can be diagnosed and repaired and the energy envelope improved. Roof ventilation should be provided to help keep the roof sheathing cold (see Roof Venting). This is an important detail that helps prevent ice dams and control wayward moisture.
For all sloped-ceiling applications, the insulation process is the same. Carefully remove the roof sheathing using a nail puller and pry bar. Send the old roof shingles to a recycler and save the sheathing for reuse if it is structurally sound. Once the rafter bays lie open and exposed, decide whether to remove the existing insulation or add to it. Keep in mind that the goal is to increase conductive resistance and to block air leaks. Since the homeowner has invested considerable time and money to this point, removing the existing insulation and doing a little airsealing work will make sense in most cases.
Approaches that work well for insulating sloped ceilings involve filling the cavity with foam insulation, fiber insulation, or a mix of the two.
One word of caution: Plastic foam material, such as urethane or polystyrene, must be protected on the interior (living) side with a minimum covering of 1/2-inch gypsum wallboard to comply with fire codes. Exposed foam on the back side of an existing wallboard ceiling is no problem. Some products, such as Thermax, a foil-faced polyisocyanurate, are made with a fire retardant and are approved for exposed applications. Check this detail carefully.
Fiber-filled cavity. Where the existing framing members are deep, or fairly low R-values are needed, a good, less expensive insulation method is to fill the cavity with fiber insulation. Remove the existing sheathing and insulation. Then air seal gaps, cracks, and seams in the ceiling with caulk (good) or canned urethane foam (best). Reusing the old insulation after the air sealing operation has been completed is acceptable as long as the insulation is in reasonable condition.
Gauge the depth of fiberglass or cellulose insulation to match the required minimum R-value. A 2 x 10 rafter bay completely filled with fiberglass will have a cavity R-value of about R-31; a 2 x 12, about R-38. Dense fiberglass batts with higher R per inch are available. If the existing rafter cavity is completely filled with insulation, install 2 x 3 spacers on top of the rafters to create a roof ventilation chute.
Next, install a baffle at the bottom of each rafter bay above the exterior wall to keep air away from the insulation. Otherwise, air from the soffit vent can enter the insulation, degrading the effective R-value. Replace the roof sheathing, roof trim, and roof coverings, and install soffit and ridge vents.
While this retrofit works adequately in most cases, there are some trade-offs to consider. Loose-fill cellulose doesn't work well on steep pitches. It settles downward and blocks the ventilation air space, so plastic air chutes (such as Proper Vents) are needed to hold cellulose insulation in place. And call me paranoid, but I don't like installing cellulose in a roof where I can't inspect it regularly. Wet cellulose compacts and loses its effectiveness. It is only a matter of time before the roof leaks, and matted cellulose in a cathedral ceiling is hard to fix. Fiberglass fill is more forgiving in this regard, but it will allow air intrusion from soffit-to-ridge ventilation. If fiberglass is the choice, install plastic chutes to protect its top side.
Foam-fiber hybrid. Another option is to combine the two approaches described above to take advantage of high R-values and good air sealing with moderate cost. Remove the existing insulation and spray urethane foam into the cavities against the back of the ceiling to a depth of 2 inches. This gives good air sealing and a quick R-value jump start. Then fill the rest of the cavity with low-cost fiber. Follow the recommendations outlined above to protect against air intrusion and to provide roof ventilation.
In summary, these are the steps for a sloped ceiling retrofit with foam, fiber, or both:
The most efficient way to vent a roof is to use continuous soffit and continuous ridge vents. Continuous venting is the only system that moves air uniformly along the underside of the roof from the soffit intake to the ridge exhaust. Roof venting should be balanced: half of the NFVA should be located high, in the ridge, and the other half low, in the soffits, evenly divided between the two soffited sides of the house. Usually, ridge vents have an NFVA of 18 square inches per lineal foot and soffit vents 9 square inches per lineal foot, so they automatically balance.
Ridge vents that have built-in baffles are best. Baffles on ridge vents seem to create suction regardless of wind direction, and they exhaust most reliably.
In addition to improving a home's appearance and protecting it from the elements, residing should improve a home's energy performance. These days it's surprising to hear that some houses don't have any insulation in the wall cavities. But owners of old homes often have added insulation only to the easy-to-reach attic space, avoiding the more complicated enclosed wall installation.
Uninsulated wall cavities should be filled with blown-in cellulose, fiberglass, or foam. This process is made easy when old siding is removed from a house in preparation for new siding. Insulation can be pumped into wall cavities through holes in the exposed wall sheathing.
Figure 2. Wall with rigid foam on exterior of sheathing.
To avoid these problems, install vertical furring strips over the foam and then nail siding to the furring strips. This creates an air space between the back of the siding and the face of the foam. It is called a vented rain screen. Space the furring strips 16 inches on center and fasten them with screws through the foam and structural sheathing into the studs. Sixteen-inch spacing provides better nailing and stiffer, less wavy siding than 24-inch on-center spacing.
Carefully position additional furring strips to serve as nailers for all trim that you will have to replace, such as around windows and doors, corners, and frieze details. Extend the jambs on doors and windows outward to accommodate the extra wall thickness. Then reinstall or replace all trim members. Finally, nail horizontal siding to the vertical furring strips. Use galvanized ringlock nails for better holding power. You can apply vertical siding to horizontal strips too, but you should provide drainage paths down through the lengths of furring that serve as the nail base. Placing furring at a diagonal also works well for vertical siding, as long as you provide solid nailing for the end joints.
Planning the location of furring strips requires thought. The outermost surface of the furring becomes the new nail base. Siding must be fastened with solid nailing at its ends--for example, where it butts against the side of vertical trim, such as window casings or corner boards. Flashing details are more complicated too. All flashing should be carefully positioned so it extends to the back of the air space, spanning behind any furring strips. In fact, the foam sheathing should be notched 1/4 inch deep to receive the flashing, so that any water that happens to reach the foam won't have a pathway behind window and door flashings. The bottom of the air space should be open to the outdoors, but protected by a strip of insect screening.
Homeowners who measure the bottom line should also consider that wrapping a house with foam and building a vented rain screen has several benefits. It
Paul Fisette is director of Building Materials Technology and Management at the University of Massachusetts in Amherst.
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