Relatively easy access has made the attic a favourite starting point to insulate for many homeowners, despite the fact that most other areas, such as basements and uninsulated walls, lose more heat than the typical attic. Even if an attic is already insulated, there may still be an opportunity to improve the energy efficiency and soundness of the house through air sealing. Air leaks into the attic can account for substantial heat loss and can lead to a variety of moisture-related problems. The importance of air sealing your attic cannot be overstated.

Tips to Consider When Air Sealing Your Attic

General Considerations for All Attics

Types of Attics
Types of attics.

Regardless of the type of attic or ceiling your house has, there are a number of things to examine before beginning work. A thorough inspection of the following features will help you to develop your retrofit strategy.


Most houses with accessible attics have an interior ceiling hatch, although exterior roof or wall-mounted entries are not uncommon. A hatch should be large enough to allow you to bring in materials. If not, or if there is no access, you may be able to cut a hole in the ceiling in an inconspicuous place such as a closet. Exterior-mounted, gable-end entries represent one less opening that will have to be air sealed, which offers ready access to firefighters in the event of a fire.

Check the roof space for obstructions and ease of movement. Vertical clearances of less than 1 m (39 in.) will not allow you to move freely.

Air Sealing Your Attic
Some attics are easier to work in than others.


In addition to checking for structural problems, check the condition of the roof framing, sheathing, finish and the soffit and fascia for signs of moisture problems such as leaks, stains, mould, flaking or rot. Uncorrected moisture problems will reduce the effectiveness of insulation and can lead to structural damage such as wood rot or split rafters. Mould or rot are sure signs of advanced condensation problems. Identify the cause and correct it before adding insulation.

Moisture can come from the outside due to failure of the roof or flashing. Typical problem areas include poor flashing at a hip, valley or the chimney and ice dam leaks. Pay particular attention to water marks on the underside of the sheathing or along rafters.

Moisture can also come from inside the house, carried into the attic as water vapour by air leakage. Typical problem areas occur around bathroom and kitchen vents that penetrate the ceiling, around plumbing stacks and chimney chases and at wiring penetrations and pot lights. A tell-tale sign of air leakage is the discolouration of insulation, so do not hesitate to look under existing insulation to locate air leaks.

Check the attic during or just after a cold snap for condensation buildup, which will appear as frost in cold climates. Some frost buildup is to be expected, but if it is particularly heavy buildup (10 mm [2 in.] or more), look to make sure ventilation is present and not blocked with insulation. Checking the attic during or just after a rainstorm will help determine whether moisture problems are generated by interior or exterior sources.

Examine the existing insulation for type, condition (dry, wet, compact, etc.), average depth and coverage. If it has been damaged or contaminated by mould or vermin, remove it. However, do not disturb vermiculite insulation.

If the insulation is wet, do not cover it until the source of moisture is removed and the insulation is dry. If the insulation is dry, it will probably be all right to leave it in place. Generally, there is no problem in using two types of insulation. Check the depth of the insulation to determine its insulating value.

Make sure that the insulation is distributed evenly and that there is full depth coverage, especially around the perimeter of the attic above the wall plates. Uninsulated areas will cause a cold spot where the wall and ceiling meet, which can lead to moisture and mould problems.

Existing air and vapour barriers

Houses should have a vapour barrier on the warm side of the insulation. In older homes, the vapour barrier might have been provided by wax paper, kraft paper-backed batts or layers of paint. Newer houses usually have a polyethylene sheet vapour barrier, but overall, very few houses have an effective air barrier.

If there is an air barrier, locate it and determine its condition. Remember, an air barrier must be continuous; holes or tears must be repaired, and penetrations sealed. Do not hesitate to pull back existing insulation where leakage sites might be found and seal these areas. However, do not disturb vermiculite insulation as previously warned.

Increased insulation means a colder attic, which in turn means that any vapour escaping into the attic can condense before it can be vented. It is essential to air seal the attic to prevent moist indoor air from getting in.

If there is no air and vapour barrier, concentrate on comprehensive air sealing. Create an effective air barrier by using caulking, gaskets and weather-stripping to seal the joints between building components. Do not rely on batt, blown-in or poured insulation to reduce the need for proper air sealing.

Click here for a complete copy of Natural Resources Canada’s Keeping the Heat In guide.

Natural Resources Canada’s Office of Energy Efficiency offers this guide to educate on basic principles of building science and to provide guidance in home retrofit projects such as insulation and air sealing improvements.

Natural Resources Canada (NRCan) seeks to enhance the responsible development and use of Canada’s natural resources and the competitiveness of Canada’s natural resources products.

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