There are many signs that an attic is wet. Prolonged wetness will rot out the roof sheathing. Often this is first noticed when re-shingling. If you have ceiling leaks only in the spring, it may be that ice has been forming on the sheathing all winter and it suddenly melts when a warm spell arrives. You may see water stains or evidence of mold on the sheathing, rafters, or trusses when you are inspecting the attic. You may find the insulation has been packed down or stained by water or ice. The smell of a moldy attic will enter the house under certain weather conditions, usually in summer.

The usual response is to increase attic ventilation. This is the wrong approach. In some cases, adding ventilation will actually pull more moist house air up into the attic and make the problem worse. The best way to fix a wet attic is to stop air movement, or leaks, from the house. Once this is done, the existing ventilation is usually more than enough to keep the attic dry.

It is important to stop air leaks because a heated house is much like a chimney. Both a house and chimney are containers of warm air surrounded by cold air. Both tend to draw air in at the bottom and expel it at the top. All winter, a heated house is trying to push air through the top floor ceiling into the attic. Block up those air leaks and keep the warmth in the house to save both energy costs and damage to your attic.

Air leaks are usually found at penetrations or discontinuities. Safety regulations prevent sealing of many types of pot lights in top floor ceilings. House air is dumped into the attic through them. Choose sealed pot lights or avoid them on the top floor.

Bathroom fans need to be ducted outside. Make sure that they are properly vented. If the ducts are located in the attic, ensure that they are solid metal rather than flex duct, insulated and sloped to the outside. Do not wrap the insulation in plastic as this will trap moisture. Taping the duct joints, or sealing them with mastic, is helpful for controlling leakage.

Plumbing stacks and chimneys are often sources of air leakage. Seal these where they pass through the attic floor. For metal chimneys inside a chase or for old masonry chimneys, you may need help from an expert to ensure proper sealing and avoidance of fire hazards. Seal holes made for electrical wiring and cable installations.

There will be little air leakage in the middle of sheet of drywall or in the middle of an unbroken plaster ceiling. There may be many air leaks where partition or bearing walls meet the ceiling or around the perimeter of the house where the attic floor (or top floor ceiling) meets the outside walls.

All discontinuities should be inspected and sealed if necessary. Look for bypasses. They are major air passages from any floor into the attic. Dropped ceilings in the room below will often conceal a direct connection to the attic. Concrete block party walls between row houses often move house air into the attic.

There are several ways to check for these large and unexpected leaks. The blower door tester can pressurize the house with a big fan and amplify the leakage. Searching the attic at night for lights from below can be helpful. Scanning batt insulation for dirty areas which have been filtering the air from below is also useful, although such straining seems to occur less frequently with blown insulation. Sometimes the holes are so big that you can see into the house below. Some houses have heating or air

conditioning ducts or equipment in the attic. These can be the major source of air leakage and heat loss in the attic. Leaks can be sealed with caulking, expanding foam, plastic, or other methods. There are a number of publications giving details on sealing methods, including "Keeping the Heat In" from Natural Resources Canada 1-800-387-2000.

In an older house, the most thorough way to air seal an attic floor is to clear away insulation from each joist bay and seal all discontinuities. If the attic joists are covered with floor boards, a thorough job includes lifting them to expose the ceiling below. Do this for the entire attic except for areas underlain by unbroken ceilings in a large room. It sounds difficult, but for most attics it should not take more than

a day for a two-person crew. Wear good dust masks. Do the work in the fall when the attic is not too hot. Doing only obvious discontinuities without lifting all the insulation can be effective but may result in missing some air leakage paths.

While you are up there, why not put some more insulation down? Make sure that you have at least 300 mm (10 in.) of loose insulation or batts. There are only minor differences in the insulating quality of fiberglass, rock wool and cellulose. They all work well in attics.

Where to look for leaks:

1. Around plumbing stacks or plumbing walls

2. Chimneys through the attic

3. Any light fixtures from the ceiling below

4. Electric wiring

5. Ducting for fans or heating systems

6. Perimeter walls

7. Partition walls

8. Party walls

9. Above pocket doors

10. Above lowered ceilings

11. Where the side of a cathedral ceiling meets an open attic

12. Split level discontinuities

13. Where additions meet an older section of the house

14. Above rounded corners or staircases

15. Balloon frame walls