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

house picture


by Paul Fisette


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.


Roof shingles wear out and need replacing about every 20 years. Builders in the mid-70s devoted very little attention to insulation and airtightness in the design of their building envelopes. So it's safe to assume that if a home needs a new roof, it also needs new, or at least better, insulation. High energy bills, drafty or sweltering indoor climates, and the formation of ice dams on roofs are all costly symptoms of poor energy design (see Out, Out Dammed Ice, November/December 1996, p. 21). In hot climates, reflective roofing materials can help keep the heat at bay (see White Roofs for Cool Homes). Insulation and air sealing are appropriate in all climates, but they are essential in very cold areas subject to ice dams.
roof with bare spot caused by leaking heat

roof with ice dam


Homeowners should plan ahead for the winter months when embarking on a reroofing project. The ice dams on the edges of these roofs, and the bare spot created by leaking heat in the top photo, show that additional weatherization is needed. Through adequate insulation, air sealing, and attic ventilation, existing homes can be effectively retrofitted.
In some ceilings, an energy retrofit is easy. Climb into the attic, block all air leaks connecting the living space to the attic space, and increase the thickness of insulation on the attic floor. However, in some homes--those with shallow-pitched rafters or sloped or cathedral ceilings--this plan is not so easy to follow. Reroofing provides an opportunity to gain access to these tight spaces.

Shallow Rafter Pitch

It is difficult to tighten and insulate ceilings in homes with shallow-pitched rafters. Near the eaves, the space between the bottom of the roof rafters and the top of the ceiling joists is just too narrow for workers to access. Also, there isn't room for proper insulation. Twelve inches of fiberglass or cellulose insulation is needed to deliver the R-38 values recommended in much of the United States. Even if you shoot for 9 inches (R-30) of insulation, a roof with a 4 1/2 pitch does not have the required 10 1/2 inches of clearance (9 inches of insulation plus 1 1/2 inches for roof venting space) until you get in more than 2 feet from the building's edge.

When blowing in cellulose and fiberglass, insulation retention baffles must be positioned over the outside walls at the eaves to prevent the insulation from blocking soffit vents. The narrow space at the eaves makes proper installation of retention baffles difficult or impossible. 

Reroofing makes insulating under a shallow-pitched roof easy. Strip the roof covering and sheathing from the lower edge of the roof. If the roof is sheathed with plywood, remove the first row of plywood from the bottom edge of the roof. If the roof is covered with boards, remove them for the first 3 or 4 feet. Carefully remove the sheathing and save it for reuse. The difficult-to-reach section of the ceiling now lies exposed and can easily be worked on.

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.

Fig 1. diagram of best insulation installing

Figure 1. The best method for installing insulation at the edge of a shallow-pitched roof.

Cathedral and Sloped Ceilings

Homes 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.

continuous soffit vents

Continuous soffit vents form an inlet for attic ventilation, while keeping insects out. 

Foam-filled cavity. Filling the cavity with foam may be a good (although pricey) choice for existing roofs where the framing members are shallow in depth. In the Northeast, we need a minimum roof value of R-38. It is difficult to achieve the required minimum R-value when an existing older house is framed with 2 x 6 or 2 x 8 rafters. And you can bet that fussy air sealing was not part of the original design.

Remove the existing insulation and completely fill the 2 x 6 rafter cavities with foam-in-place urethane. This will air tighten the ceiling nicely and bring the roof close to the minimum acceptable insulation level. Level the foam, and use 16-penny nails spaced 8 inches on center to attach 2 x 3s to the top of the 2 x 6 rafters to provide a vent space for continuous soffit-to-ridge ventilation. Next, replace the roof sheathing, roof trim, and roof coverings, and install soffit and ridge vents. There is enough room in 2 x 8 construction to provide 6 inches of foam, leaving a 2-inch air space for roof venting. 

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:

  • Strip roof shingles.
  • Remove roof sheathing.
  • Remove insulation.
  • Air seal.
  • Refill rafter cavities.
  • Install baffles over exterior walls for fiber .
  • Install plastic chutes (if using fiber insulation).
  • Install 2 x 3 furring over rafters' location.
  • Install structural roof sheathing.
  • Install trim.
  • Install roof venting system (if needed).
  • Apply roof covering.


Roof Venting

Roof vents are required by all building codes. Always vent the air space above the insulation in an attic or a cathedral ceiling. If there is a vapor barrier on the ceiling, a minimum of 1 square foot of net free vent area (NFVA) is required for every 300 square feet of ceiling area below the roof. (NFVA is the total area of air spaces in a vent screen, excluding the screen material.) The minimum requirement jumps to 1 square foot NFVA for every 150 square feet if no vapor barrier is in place.

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.

A baffled ridge vent. These vents create suction regardless of wind direction, and exhaust more reliably than other types, including power vents, turbines, and gable louvers.


There's nothing like new siding to dress up an old home. At first glance, the process seems simple. Strip the old siding from the walls of the house, replace failing trim, and put up new siding. But this limited vision may define lost opportunity.

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.

siding around window trim
Proper installation where new siding meets window and door trim is vital to prevent water leaks and damage. The added foam insulation extends the thickness of the wall, so the window must be padded out to make the back of the window trim even with the back of the siding. The siding needs a solid nailing surface at the joint where it butts against the window trim, so the piece of strapping that goes behind the window trim should extend out behind the siding as a nailing surface--half of the strapping should be behind the trim, half hehind the siding. It is also important that the window head flashing extends all the way back to the structural sheathing.

Adding Rigid Foam

But even a fully insulated wall cavity does not provide as much insulating value as is needed in a cold climate. Residing presents the opportunity to install the new siding over a layer of rigid foam insulation, boosting the wall's R-value. The process is somewhat complicated and labor-intensive, but when properly done, it will provide a tight, dry, warm structure for many years. Careful detailing and an understanding of the forces at work will ensure satisfaction.

Strip the walls bare, down to the sheathing. Carefully remove all trim, including corner boards, frieze boards, rake boards, window casings, and door trim. Save any that is good enough to reuse. Bring the house down to its bare bones, so you can see if it needs any structural improvements. Renail loose sheathing. Replace rotted elements. Air seal or patch all holes, gaps, seams, and cracks. Seal around window and door openings.

Then wrap the exterior walls with rigid foam sheathing (see Figure 2). Foil-faced polyisocyanurate and extruded polystyrene are good choices. Fasten the foam boards to the structural sheathing with broad-head nails and/or an adhesive caulk that is compatible with the foam. Make the foam wrap continuous and tight. Tape all the joints with contractor's tape (such as 3M Contractor's Tape)--don't use duct tape. The layer of foam will create an exterior air barrier and improve the wall's R-value.

The next part gets tricky. Nailing wood siding directly over plastic foam insulation is asking for trouble: the wood will tend to cup, crack, bow, split, and shrink abnormally. Foam doesn't provide a solid nailing surface, so nails have to be extra long to reach through the siding and foam to a solid surface. Since the nails are large, they split the siding. Plastic foam doesn't transmit heat and moisture like wood. As the sun heats the wood, the foam doesn't let the heat pass on. The siding overheats, dries, and cracks. Foam is also less permeable to water vapor. When the sun drives moisture from wet siding inward, the back of the siding stays wet, while the front dries. As a result, the siding cups, cracks, and sloughs its coat of paint. 

Figure 2. Wall with rigid foam on exterior of 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.

One more bit of bad news for the sidewaller. The ends of the roof will have to be extended to cover the built-out gable wall if the gable ends of the house do not have overhangs.

Will the Added Insulation Pay Off?

Why do all this work if it doesn't provide a big advantage? Obviously, the homeowner will save some energy. But how much? It is very difficult to predict even a simple payback period. The list of variables is long: climate, existing airtightness and level of insulation, fuel cost, size and shape of house, heat gain benefit, and on and on. To provide a sense of perspective, here is a hypothetical case:

The walls of a 48 ft x 28 ft one-story ranch house are upgraded from R-13 to R-18 by installing 1 inch of polyisocyanurate or extruded polystyrene foam (the costs are similar). The house is in a 6,000 heating-degree-day (HDD) climate, where electricity costs 10¢/kWh and oil costs $1/gallon. The retrofit improves the home's airtightness by 0.1 air changes per hour (ACH). The cost of the energy upgrade alone is 80¢-$1 per square foot wall surface area, making the approximate total cost of labor and materials $1,000-$1,250.

If the house is electrically heated, the simple payback would be as little as 5 years. However, it might be 20 years if the house is heated with fuel oil and the walls are upgraded from R-19 to R-24. Cost effectiveness improves significantly in colder climates, and where the house is tightened beyond the 0.1 ACH improvement assumed here. 

soffit vents

While most people focus only on aesthetics when choosing a siding, such as these redwood boards, the elements under the siding are equally or even more important to the comfort and energy efficiency of the home. Note the soffit vents visible to the left of the photo, which provide roof ventilation and reduce the possiblity of damaging ice dams. 

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

  • Improves comfort.
  • Improves airtightness.
  • Reduces conductive heat loss and thermal bridging.
  • Reduces condensation in walls by raising cavity temperature.
  • Reduces rain penetration into wall cavities through pressure equalization and drainage.
  • Helps block unwanted sound.
However, there are costs to achieve these benefits:
  • Complicated detailing.
  • Labor-intensive process.
  • Significant cost of materials.
  • Potentially long payback period.
  • Ants may nest in foam.
Wrapping a house is not a universal solution. It is an option. A poorly insulated, leaky house in a cold climate is clearly a good candidate for this type of energy retrofit. The decision gets more difficult as the climate moderates and the condition of the existing house improves.

Paul Fisette is director of Building Materials Technology and Management at the University of Massachusetts in Amherst.


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