Reroofing and Residing To Save Energy
When it’s time to replace worn siding and roof coverings, you can recognize this as an opportunity to upgrade your home’s energy performance. Here are several options that can help you meet your goals for some roofs and wood siding rehabs.
The adage that says: within every problem lies an opportunity, is certainly valid when it comes to residing and reroofing a home. Roofing and siding a home is expensive. So, the decision to replace siding and roofing usually takes time to develop. Often a catastrophic event like water dripping from the ceiling 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.
On The Roof
Roof shingles wear out and need replacing about every 20 years. Choices about reroofing are often limited to appearance, cost and the ability of the roof to keep the homeowner dry. But it pays to do more.
It is safe to assume that if a home needs a new roof it also needs new, or at least better, insulation. Homes built 20 years ago are underinsulated by today’s standards. Fuel was cheap then. Builders of that period devoted very little attention to insulation and air tightness in the design of their building envelopes. High energy bills, drafty indoor climates, and the formation of ice dams on roofs are all costly symptoms of a sub-par energy design.
Ice dams cause millions of dollars of structural damage every year. But ice dams are not the disease. They are a symptom of a home’s energy sickness. Heat leaking to the roof deck from the living space below is the root cause of ice dams. The cure is to block the loss of indoor heat, keeping the entire roof surface cold and saving energy dollars in the process!
An energy retrofit in the ceilings of many homes 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. A good plan is one that provides a continuous air and insulation barrier. But in some homes this plan is not so easy to follow: It is difficult to tighten and insulate ceilings in homes with shallow-pitched rafters. There just isn’t enough room to access the space between the rafters and ceiling. Also homes with sloped ceilings like those found in Cape-style houses and houses with cathedral ceilings are difficult to retrofit because the rafter bays are sealed by finished surfaces. Reroofing provides an opportunity to gain access to these enclosed spaces. Here are some suggestions that may help in your next project.
Shallow Rafter Pitch
It is difficult to properly air-seal and insulate the ceilings of homes that have shallow roof pitches. While in the attic, workers can not crawl close enough to the intersection of roof rafters and ceiling joists. The space between the bottom of the rafter and the top of the ceiling is just too narrow. Also, because this space is narrow, there is not enough room to install the deep levels of insulation that are required.
Twelve inches of fiberglass or cellulose insulation is needed to deliver the R-38 values recommended in much of the country. Even if you shoot for nine inches of R-30 insulation – a roof with a 4”in 12” roof pitch does not have the required eleven-inch depth (9 inches of insulation plus 1.5 inches for roof venting space) until you reach a distance of more than 2 feet from the buildings edge.
Blown-in cellulose and fiberglass insulation are popular materials. But when these products are used, insulation retention baffles must be positioned over the outside walls at the eaves to prevent the insulation from blocking the soffit vents. The narrow space at the eaves makes the proper installation of retention baffles difficult or impossible. Reroofing makes this job easy.
Once the scaffolding is erected, 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 the first 3 or 4 feet of roof boards. Carefully remove the sheathing and save it so you can reuse it later. The difficult-to-reach section of the ceiling now lies exposed and can be easily worked on.
The first step should be to seal any air leaks in the ceiling. Use a can of foam to seal any wire penetrations, cracks or spaces that connect the attic to the living space below. The second step should be to provide adequate levels of insulation. Since space is restricted at the roofs edge, use an insulation material that has a high R-value per inch of thickness. Polyisocyanurate foam board, like Thermax, has an R-value of 7 per inch. So cut the insulation into strips and stack layers of these strips between the rafters directly over the outside wall. Cut the strips of material so they fit snugly against the roof- and ceiling-framing members. Be sure they extend into the attic 2-feet. Leave enough room (1 1/2”)above the stack of foam blocks to allow the passage of air for soffit-to-ridge venting. Seal the strips of foam board to the framing members with spray foam from the can to make the connection air tight.
To finish the job: Install insulation retention baffles if they are needed to hold back any 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 air-tightness and insulation will be greatly improved.
Cathedral and Sloped Ceiling Fix
A remedy for this condition requires more than a trip to the attic (since there is no attic) with a few bags of insulation and a foam gun. In this case either the exterior surface of the roof or the interior surface of the ceiling must be retrofitted to block the flow of heat. Roof ventilation should be provided to help keep the roof sheathing cold. The inclusion of roof ventilation is an important detail that helps prevent ice dams and control wayward moisture. It is a painfully expensive to rip apart a new roof or ceiling to add insulation. But an energy retrofit becomes more palatable and cost effective when it is combined with a scheduled reroofing project.
|shallow rafter depths||low R-value||improve R per inch of thickness|
|inadequate soffit-to-ridge ventilation beneath roof sheathing||low potential to cool sheathing and remove moist air from roof||install continuoussoffit-to-ridgeroof ventilation system, keep air channels clear with baffles|
|discontinuous air barrier||allows passage of heat and moisture to attic/roof||air-tighten, seal all penetrations into attic and roof frame|
|improper or incomplete insulation of rafter bays||allows the transfer of heat through conduction||add proper depths of insulation, protect loose-fill over exterior walls with baffles|
Each house is built differently. Some have rafter cavities filled with insulation; other houses have rafter bays that are only partially filled. Some houses have roof venting in place; others do not. Some are built tight, others leaky. But one thing is certain: if you are experiencing ice dams or your house is cold, something is wrong! Corrective action must be taken. An effective strategy involves peeling 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. Rot, degrade and structural damage can be diagnosed and repaired and the energy envelope improved.
For all sloped ceiling applications the 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 plan to reuse the sheathing later if it is structurally sound. Once the rafter bays lie open and exposed, you have to decide whether or not to remove the existing insulation or add to it. Keep in mind that your goal is to increase conductive resistance and to block air leaks. Since you have invested considerable time and money to this point, I think removing the existing insulation and doing a little air-sealing work makes sense in most cases.
There are certainly many approaches to improve the performance of an existing sloped ceiling or cathedral-frame roof. Here are few suggestions that I think work well. (I am always looking for new innovative solutions, so please share your experiences with me.)
1) Foam-Filled cavity approach may be a good (although pricey) choice for existing roofs where the framing members are shallow in depth. In the northeast where I live we need a minimum roof value of R38. It is difficult to achieve the required minimum R-value when an existing older house is framed with 2×6 or 2×8 rafters . And you can bet that fussy air-sealing was not part of the original design.
Remove the existing insulation and completely fill the 2×6 rafter cavities with foamed-in-place urethane. This will air-tighten the ceiling membrane nicely and bring your roof close to the minimum acceptable insulation level. Nail 2×3’s to the top of the 2×6 rafters with 16-penney nails spaced 8-inches on center, after the cavities are filled to provide a vent space for continuous soffit-to-ridge ventilation. Next replace the roof sheathing, roof trim, roof coverings, and install soffit & ridge vents. There is enough room in 2×8 construction to provide 6 inches of foam, leaving a 2-inch air space for roof venting.
One word of caution. Whenever you use plastic foam material like urethane or polystyrene, you must protect the surface on the interior (living) side with a minimum covering of 1/2-inch gypsum wall board to comply with building codes. So if you are applying the foam to the back side of an existing wallboard ceiling, you are all set. Some products like Thermax, a foil-faced ployisocyanurate, are made with a fire retardant and are approved for exposed applications. Check this detail carefully.
Roof venting is required by all building codes. You must vent the air space above the insulation in an attic or cathedral ceiling. If you have a vapor barrier on the ceiling, a minimum of 1 square foot of net free vent area (NFVA is the total area of air spaces in screen, excluding screen material) for every 300 square feet of ceiling area below the roof is required. The minimum jumps to 1:150 if no vapor barrier is in place. Having said this, if you install deep, airtight insulation in rafter bays, you can build unvented roofs. However, you must discuss this option with your building inspector to be sure your design is satisfactory. Roof rafter cavities filled with spray-in-place foam or dense-pack cellulose can perform nicely. These applications can be made so airtight that no moisture-laden air can migrate into the roof cavities of cathedral applications.
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: 50% high, in the ridge and 50% low, in the soffits ( 25% on each side of the house). NFVA is marked on each roof venting product. Usually ridge vents have a NFVA of 18 square inches per lineal foot and soffit vents are 9 square inches per lineal foot, so they automatically balance.
I recommend using ridge vents that have a built-in baffle. Baffles on ridge vents seem to create suction regardless of wind direction and it has been my experience that they exhaust most reliably. ShingleVent manufactured by AirVent, Inc. is one example of a baffled ridge vent.
2) Fiber-Filled cavity approach may be a good (and less expensive) choice for existing roofs where the existing framing members are deep or required minimum R-values are fairly low. Remove the existing sheathing and insulation. Then air-seal gaps, cracks and seams in the ceiling with caulking(good) or canned urethane foam(best). Reusing the old insulation after the air-sealing operation has been completed is ok 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 2×10 rafter bay completely filled with fiberglass will have a cavity R-value of about R-31; a 2×12 about R-38. Dense fiberglass batts with higher R/inch are available. Install 2×3 spacers on top of the rafters to create a roof ventilation chute if the existing rafter cavity is completely filled with insulation. Next install a baffle at the bottom of each rafter bay above the exterior wall to block air from entering the lower end of the insulation. Air from the soffit vent can enter into the fiber insulation at this point degrading the effective R-value. Replace the roof sheathing, roof trim, roof coverings, and install soffit & ridge vents.
While this retrofit works adequately in most cases, there are some trade-offs to consider: Cellulose insulation doesn’t work well on steep pitches unless it is dense-packed. You need an insaller who is experiences in dense-packing applications. Otherwise, it settles downward and blocks the ventilation air space. Plastic air chutes like Proper Vents are recommended 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 looses its effectiveness. It is only a matter of time before you get a roof leak and in a cathedral ceiling you can’t fix matted cellulose very easily. Fiberglass fill is more forgiving in this regard, but will allow air intrusion from soffit-to-ridge ventilation. So if fiberglass is your choice, install plastic chutes to protect its top side.
3) Foam-Fiber hybrid approach is another interesting option. You can combine the two approaches described above to take advantage of high-R and good air-sealing with moderate cost. For example: You can 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 you 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.
Summary of Steps
strip roof shingles
remove roof sheathing
refill rafter cavities
install baffles over exterior walls for fiber fill
install plastic chutes (options 2 and 3)
install 2”x3” furring over rafters location
install structural roof sheathing
install roof venting system (if needed)
apply roof covering system
On The Walls
So the siding on your house has seen its best day long ago? There is nothing like new siding to dress up an old home. At first glance the process seems mindless: 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.
First and foremost, any re-siding project should improve a home’s appearance and protect it from the elements. However, most older homes were not built with energy conservation is mind. Re-siding should also improve a home’s energy performance. I am always surprised when I hear that some houses don’t have any insulation in the wall cavities. Owners of old homes often add insulation to the easy-to-reach attic space, but avoid the more complicated enclosed wall installation. Obviously, uninsulated wall cavities should be filled with blown-in cellulose, fiberglass or foam. This process is made easy when old siding in removed form a house in preparation for new siding. Here insulation can be pumped into wall cavities through holes in the exposed wall sheathing.
Another promising scheme for wall cavities already insulated, is to install new wood siding over a layer of rigid foam insulation. This system is somewhat complicated and labor intensive; but when properly constructed, will provide a tight, dry, warm structure for a many years. Careful detailing and an understanding of the forces at work will assure satisfaction.
Strip the walls bare, down to the sheathing. Remove all the trim: corner boards, frieze boards, rake boards, window casings and door trim. Carefully remove and save any trim that is good enough to reuse. Bring the house down to its skin and bones so you can see if any structural improvements need doing. Renail loose sheathing. Replace rotted elements. Develop a good air-sealing plan: patch all holes, gaps, seams and cracks. Seal around window and door openings. And then, wrap the exterior walls with rigid foam sheathing. Foil-faced polyisocyanurate or extruded polystyrene are 2 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 (solvents in some adhesives eat foam!). Make the foam wrap continuous and tight. Tape all the joints with 3M contractor’s tape (not duct tape). The layer of foam will create an exterior air barrier and improve the wall’s R-value.
The next part of the plan gets tricky. You should not nail wood siding directly over plastic foam insulation. Siding applied directly to foam has a history of failures. This method causes wood siding to cup, crack, bow, split and shrink abnormally. Nailing wood siding directly to foam doesn’t work because foam doesn’t provide a solid nailing surface. Nails have to be extra large to reach through the siding and foam to a solid surface and the larger nails split the siding. Also, plastic foam doesn’t transmit heat and moisture like wood. The sun pumps heat into the wood siding and its transmission is blocked by the foam. The siding overheats, dries and cracks. Foam is less permeable to water vapor. The sun drives moisture from wet siding inward. The back of the siding stays wet while the front of the siding dries. As a result, the siding cups, cracks and sloughs its coat of paint.
It is best to 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.
Carefully position additional furring strips to serve as nailers for all trim that you will have to replace: around windows and doors, corners, frieze details, etc. Extend the jambs on doors and windows outward to accommodate the extra wall thickness. Then reinstall or replace all trim members. Apply the siding by nailing horizontal siding to the vertical furring strips. Use galvanized ring-shanked 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 nailbase.
As you might imagine, 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 like 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. 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.
Payback for Walls
It gets sticky here. Why do all this work if it doesn’t provide an advantage? Obviously, you will save 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, etc., etc., etc. To provide a sense of perspective, I offer is a guesstimate for a hypothetical case:
48’ x 28’ one-story ranch house in a 6000 heating degree climate
1-inch polyisocyanurate or XEPS foam (similar payback periods)
electric cost @ $0.10/KWH and oil @ $1.00/gal
improvement in airtightness = 0.10 ACH
cost of energy upgrade only = $0.80 – $1 per ft2 wall surface area ($80/square – $100/square upcharge) approximate cost of labor and materials $1,000 – $1,250.
If you upgrade an existing, electrically-heated house from R 13 walls to R 18 walls under the conditions listed above, you might expect a simple payback in as little as 5 years. However, it might take 20 years for a payback if the house is heated with fuel oil and the walls are upgraded from R 19 to R 24. Keep in mind that cost effectiveness improves significantly in colder climates and if the house is tightened beyond the 0.10 ACH improvement factored into the given example.
Pros and Cons
Philosophically, there is no question, saving energy is a good, healthy, enlightened policy. But consumers measure the bottom line. Wrapping a house with foam and building a vented rain screen achieves several things:
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
is cost effective in some cases
upgrade appreciates in value as energy costs rise
However, there are costs to achieve these benefits:
added cost of materials and labor
potentially long payback period
ants may nest in foam
Wrapping a house is not a universal solution. It is an option. In the end, I think it is clear that a poorly insulated, leaky house in a cold climate is a good candidate for this type of energy retrofit. However, the decision gets more difficult as the climate moderates and the condition of the existing house improves.
Last updated: April 1, 2009