Attics are a popular starting point for homeowners looking to improve their home’s insulation. Although basements and uninsulated walls can lose more heat than a typical attic, upgrading the attic’s insulation can still greatly enhance the energy efficiency and overall soundness of your home. Even if your attic is already insulated, you can further improve it through air sealing and proper ventilation. Addressing air leaks around chimneys, vents, plumbing pipes, and electrical boxes can significantly reduce heat loss and prevent moisture-related issues. Here’s a detailed guide to help you enhance your attic’s insulation and energy efficiency.
General Considerations for All Attics
Regardless of the type of attic or ceiling in your house, examining a few key features before beginning work is essential. A thorough inspection will help you develop an effective retrofit strategy.
Accessibility
Most houses have an interior ceiling hatch for attic access, though some may have exterior roof or wall-mounted entries. The hatch should be large enough to bring in materials. If it’s not, or if there is no access, you can cut a hole in the ceiling in an inconspicuous place like a closet. Exterior entries, such as those in a gable end, reduce the number of openings needing air sealing and offer ready access for firefighters in case of a fire. Check for obstructions and ease of movement in the roof space. Attics with vertical clearances of less than 1 meter (39 inches) will be challenging to move around. Attics without sufficient workspace are covered later in this guide.
Structural Assessments
Inspect the roof framing, sheathing, finish, and soffit and fascia for moisture issues like leaks, stains, mold, flaking, or rot. Unresolved moisture problems can reduce insulation effectiveness and cause structural damage. If you find mold or rot, address the cause and fix it before adding insulation. Moisture can enter the attic from outside due to roof or flashing failures, or from inside the house as water vapor through air leaks. Common problem areas include poor flashing around hips, valleys, chimneys, and ice dams. Pay attention to water marks on the sheathing or rafters’ undersides. Check for air leaks around bathroom and kitchen vents, plumbing stacks, chimney chases, wiring penetrations, and pot lights. Discoloration of insulation is a sign of air leakage, so look under the insulation to locate leaks. During or after a cold snap, check for condensation buildup, which appears as frost in cold climates. After a rainstorm, check for moisture problems to determine if they are from interior or exterior sources.
Existing Insulation
Evaluate the type, condition (dry, wet, compact), depth, and coverage of existing insulation. Remove any damaged or contaminated insulation, but do not disturb vermiculite insulation, as it may contain asbestos. If the insulation is wet, address the moisture source and dry the insulation before adding new material. Ensure even distribution and full depth coverage, especially around the attic perimeter above wall plates to prevent cold spots that can lead to moisture and mold issues.
Air and Vapor Barriers
Houses should have a vapor barrier on the warm side of the insulation. Older homes may have wax paper, kraft paper-backed batts, or layers of paint as a vapor barrier, while newer homes usually have a polyethylene sheet. An effective air barrier is crucial to prevent moist indoor air from entering the attic and condensing. Locate any existing air barriers, repair holes or tears, and seal penetrations. If there is no air and vapor barrier, focus on comprehensive air sealing using caulking, gaskets, and weatherstripping to create an effective barrier. Proper air sealing is essential to prevent moisture issues in a well-insulated attic.
Structural Considerations
When renovating your attic or roof, consider upgrading the roof structure to support future photovoltaics or solar hot water panels. Adding a new layer of shingles over an existing one increases the roof’s load, so it’s typically recommended to remove old shingles before installing new ones.
Chimney Insulation
For masonry chimneys, check for charring, soot deposits, crumbling masonry or mortar, or deteriorating chimney lining. Do not insulate a suspect chimney; instead, have a specialist repair, seal, and insulate it. Air seal around chimneys using sheet metal flashing and non-combustible sealant. Maintain a 75 mm (3 in.) air gap between insulation and the chimney, constructing a barrier 75 mm (3 in.) higher than the insulation to prevent contact. For metal chimneys, maintain a 75 mm (3 in.) air space and install a non-flammable radiation shield to prevent insulation contact.
Electrical Installations
Work safely around electrical wiring by turning off the power. Consult an electrical contractor if wiring appears questionable or corroded. Standard interior recessed lights can be challenging to air seal and pose fire hazards if covered with insulation. Replace them with approved airtight fixtures or covers, or consider LED flush or flat mount fixtures for better energy efficiency and easier sealing and insulation.
Air Sealing
Seal around plumbing stacks and other pipes, using flexible gaskets or polyethylene sleeves for plastic plumbing stacks that expand and contract. Seal around wires, ceiling light fixtures, ducting, and at the junction of the ceiling and wall partitions. Ensure exhaust fans discharge outside, not below soffit vents. Air seal attic hatches and around chimneys using non-combustible fire stops. In attached housing, seal gaps between shared walls and the attic floor’s edge.
Ventilation
Proper attic ventilation reduces summer heat buildup, prolongs roofing life, and reduces air conditioner loads. It also prevents moisture issues by allowing water vapor to escape. Ensure existing vents are working correctly and not blocked. Use a ratio of 1 to 300 for unobstructed vent area to insulated ceiling area. Distribute vents to allow cross ventilation from end to end and top to bottom. Avoid electric exhaust fans for attic ventilation as they can pull house air into the attic, increasing heat loss and moisture accumulation. Wind-driven roof vents are no more effective than capped vents.
Insulation Installation
For accessible attics, use batt, blanket, or loose-fill insulation. Batt or blanket insulation fits snugly between ceiling joists, while loose-fill insulation is ideal for irregular or obstructed spaces. Ensure even depth and full coverage, especially around the perimeter. Use baffles to maintain airflow and prevent insulation from blocking ventilation. When using loose-fill insulation, install depth indicators and maintain even insulation depth throughout the attic.
Special Considerations for Half-Storey Houses
Houses with half storeys have several small attic spaces that can be challenging to access, air seal, and insulate. Follow the same air and vapor control principles as for accessible attics, paying particular attention to air leakage around knee walls. Seal critical air leakage areas with foam board or dense-pack loose-fill insulation. Ventilate attic spaces using gable vents and ensure proper airflow between insulated areas.
Increasing Thermal Resistance and Reducing Thermal Bridging in Attics
To improve the thermal resistance of your attic and reduce thermal bridging, you can insulate the wall section that adjoins a heated space in the attic. Treat this wall like a knee wall, filling the space between the rafters with insulation. However, ensure that all ceiling penetrations are sealed and comply with local building codes or standards. This process is part of creating a “hot roof,” which involves insulating the roof slope and knee wall space, and it can increase usable storage space in the attic. It’s important to remember that exposed foam insulation in accessible areas must be fire-protected. If space for insulation and ventilation on the sloping ceiling is limited, consider adding rigid board insulation directly to the ceiling, followed by air sealing and covering with drywall. This method significantly enhances comfort during both summer and winter, although it may reduce headroom. Some contractors specialize in high-density blown insulation, which helps reduce airflow while improving thermal values.
Insulating Dormer Windows
Many 1 ½ or 2 ½ storey houses have dormer windows, which can be insulated similarly to knee
walls using batts. Dormers typically have a lot of framing members that limit the amount of insulation that can be added. In addition to air sealing, consider using dense-pack blown-in insulation and adding rigid board insulation and drywall to reduce thermal bridging.
Insulating Attic Floorboards
If your attic has been floored over, even if it’s not used as living space, you can still insulate it by lifting the floorboards and treating it like an unfinished attic. However, some or all of the boards may need to be replaced to maintain ceiling stiffness. A contractor can fill the subfloor space with dense-pack loose-fill insulation through access holes. Ensure air leaks around plumbing stacks and open eave spaces are sealed first. Alternatively, spray foam can be used to air seal and insulate if accessible. Before insulating, check for wiring, recessed light fixtures, or other concealed heat sources beneath the floorboards. All heat sources must be protected from the insulation or removed. Achieving the minimum manufacturer’s recommended levels for attic insulation by filling this space alone is unlikely. If possible, add some insulation on top of the floorboards to keep them warm and reduce thermal bridging through the joists and floorboards.
Insulating Cramped Attics, Cathedral Ceilings, and Flat Roofs
Cramped Attics
For attics that are too cramped to work in, you have three options:
- Insulate on top of the existing roof.
- Add rigid board and drywall to the ceiling if there is enough headroom.
- Hire an experienced contractor to blow in dense-pack loose-fill insulation.
The last two options can be combined. For the first and last methods, inadequate headroom may necessitate not installing a new sheet-material air barrier. If no air barrier exists and there are no moisture problems, you can still insulate by ensuring humidity levels in the house are reasonable and sealing air leaks through the ceiling into the attic. Added protection can be achieved by painting the ceiling below the attic with latex vapor barrier paint or two coats of oil-based paint. Ensure the contractor prevents insulation from entering the eaves and blocking ventilation. If there are any recessed light fixtures, chimneys, or other heat sources in the attic, take precautions to avoid creating a fire hazard. Ventilating cramped attics can be challenging, but if the roof extends past the exterior walls, use soffit vents combined with roof vents or built-up ridge vents. If the house lacks eaves, ventilation should be approached cautiously. The best approach is to seal the ceiling below the attic from inside the house and insulate without installing additional vents. Check the roof space for moisture problems during or after a cold snap in mid-winter. Heavy frost buildup indicates the need for venting the space, thoroughly sealing air leaks, and reducing humidity levels in the house. Consult local building authorities for permitted procedures.
Cathedral Ceilings and Flat Roofs
Insulating a house with a flat roof or cathedral ceiling requires the services of a qualified contractor due to the limited space for insulation and ventilation. Renovation options involve some risk of moisture problems or thermal bridging, which can reduce insulation effectiveness. Ensure that any added material does not exceed the load capacity of the existing structure. One option is to install rigid board insulation directly under the existing ceiling and cover with a new finish like drywall. Discuss with a contractor or building official how to locate and install a new vapor barrier. Dense-packing the space with cellulose or glass fiber loose-fill insulation is another technique, but it eliminates roof ventilation and is generally not recommended without local building authority approval. The high-density insulation combined with comprehensive air sealing should reduce airflow sufficiently to avoid condensation problems. Extra care is needed to seal air leaks from below, which can be difficult due to wiring and plumbing puncturing the ceiling in various places.
Preventing Ice Dams
Ice dams are large masses of ice that form on the lower edge of a roof and in gutters, causing melting snow or rain to back up and potentially leak into the attic. They occur when the attic temperature is above freezing, causing the roof sheathing to melt the snow in contact with the roof. The meltwater runs down to the overhang, where it freezes again, starting an ice dam. To prevent ice dams, seal all attic air leaks and insulate thoroughly. Identify areas prone to ice dams by noting where snow melts first after the first heavy frost in fall or light snow. Common areas include where a knee wall meets the ceiling, beneath roof-ducted exhaust fans, plumbing vents, under skylights, and above leaky attic access hatches. Complicated roofs with many valleys, dormers, or large overhangs are more prone to ice damming due to more thermal bridging and air leakage.
Dealing with Cramped Attics and Ice Dams
Cramped attics with limited space for adequate insulation at the edge of the attic floor can promote ice damming. Use spray foam or extruded polystyrene to fill this space and maintain ventilation. Cathedral ceilings present more challenges for preventing ice dams, and it’s best to contract an experienced insulator or address the issue during re-roofing. When re-roofing, remove the roof finish and sheathing, air seal, fill cavities with insulation, and install new roofing materials, including an ice-shield membrane. For older houses with complicated roofs, multiple techniques may be necessary to eliminate ice dams. Preferred solutions include keeping house heat out of the attic by air sealing and insulating.
Renovations and Repairs
Renovations or repairs are opportunities to ensure a well-insulated attic ceiling and walls with an effective air and vapor barrier. Converting an unfinished attic into living space involves steps such as installing collar ties for structural support, ensuring continuous thermal envelopes with snug insulation, sealing any roof vents, and adding strapping for more insulation if the attic is not too cramped. Alternatively, you can secure rigid board insulation to the rafters and studs, seal or install a vapor barrier, and cover with an approved finish like drywall.
Adding a Dropped Ceiling or New Roof
For sufficient headroom, constructing a dropped ceiling can thermally upgrade a cathedral ceiling or flat roof. Options include constructing a new ceiling below the existing one, closing the space between exposed joists or beams, or mechanically fastening rigid board insulation directly to the existing ceiling. Ensure to prevent warm air from bypassing the insulation. For major exterior alterations like a new roof, adding insulation on top of an existing ceiling or roof is suitable for cathedral ceilings and flat roofs. Install rigid board insulation on top of the existing roof, air seal, insulate between joists, and install new roofing materials. This approach requires the services of a qualified contractor and must comply with local code requirements.