Weatherization/Sealing bypasses

A bypass is any crack, gap, or hole that allows conditioned air to leak between your home’s conditioned and unconditioned space. This includes, for example, warm air escaping from the living area into an unheated attic, or cold air entering the living area from the basement or from the outdoors. You should focus your attention at the points where the conditioned space in your house meets unconditioned space. It doesn't matter if there are bypasses between two heated rooms.

Loose-fill, batt, blanket, and open-cell foam insulation will not stop air from flowing through bypasses. If you don’t seal bypasses, the insulation will only filter the warm air as it escapes into the attic, resulting in dirty smudges on the insulation.

Having bypasses:

Puts moisture into the attic, where it condenses and causes damage.
Allows insects, spiders, and small rodents to enter the house, travel between floors, and travel between rooms.
Wastes energy and money. If you seal as many bypasses as possible, your house will need less insulation, because the house will be doing much of the insulating itself by preventing convective heat loss.

If you are lucky enough to buy a house that is still under construction, you can take steps to make sure the house is as tight as possible. In the case of bypasses that pass through the foundation and exterior walls, you should, if possible, seal the bypass from both the inside and the outside before the contractor backfills. On the inside of the house, seal bypasses after the ducts, pipes, and wires are installed, but before insulation is installed. Going back to find and seal the bypasses is extremely difficult and costly once the contractor has installed insulation, seals up the walls, and installs floorboards.

If you buy a house that was built a long time ago, and if you feel drafts in the house, you can sometimes locate the source of the drafts with a candle. Light a candle, put the candle down in different spots in your house, step away, remain as motionless as possible, and see if the flame distorts to one side.

Materials to use for sealing cracks, gaps, and bypasses:

Caulk. (Pick the type of caulk that is appropriate for what you are doing. For example, use swimming pool caulk to seal bypasses in the concrete foundation. Alternatively, you could use hydraulic cement or an expansive mortar.)
Putty or hydraulic cement, for big holes.
Closed-cell foam, such as “Great Stuff” - especially good for rough, irregular holes, narrow gaps, and difficult-to-reach areas such as sill plate. If you’re not sure if a foam is closed-cell, check the manufacturer’s website. For example, from Dow Chemical’s website:

"Latex foams are typically “open celled” and, as a result, can take on water. In fact, the same properties that allow you to wash latex foam off your hands with water also mean that the cured foam can absorb water. This can cause wood rot or deterioration in areas where wet latex foam is next to wood, such as a window frame. In contrast, GREAT STUFF is closed-cell foam. It forms a water-resistant outer coating when cured."

Sheet metal and furnace cement, or Roxul [1] products, for sealing around fluepipes. If you’re not sure how to do this safely, check your local fire codes.
Rigid foam insulation or gypsum board, cut to fit, for sealing large gaping holes. Caulk around edge. Cover the rigid foam with gypsum board if it is in an exposed location, to protect from toxic fumes in the event of a fire.
Electrical receptacle and light switch insulating gaskets, designed to fit in between the electrical box and cover plate.
Holes in electrical boxes, and drafty oversized holes through fire-rated walls (i.e., for sprinkler pipes or wiring conduit) can be sealed with intumescent (fire stop) caulking or putty.

Where to look for bypasses:

In a home with an unheated attic, you should start looking for and sealing bypasses between the living area and the attic. Most of the heat that you are losing is probably escaping through the ceiling, because hot air rises. Look for gaps around the fireplace chimney, furnace fluepipe, and vent pipes that extend through the attic, and seal any gaps that you find.
To protect from fire, don’t pile insulation against fan motors and non-IC-rated recessed lights.
Don’t forget to seal and weather-strip the attic access door or hatch (usually in the form of pull-down stairs). An enormous amount of heat can escape through gaps around this door.
After focusing on the attic, the next most important place to look is around exterior doors and windows. Be sure to check between the exterior window trim and the siding (especially where the siding butts against the corner of the trim).
Apply caulk and spray foam, and then install weather-stripping on the bottom, sides, and tops of doors and windows. There are many types of weather-stripping, including rubber strips, foam strips, vinyl tubular gaskets, and metal spring-type weather-stripping. Choose whichever one(s) you think are most appropriate.
While you’re outside, seal any other gaps in the siding, especially around outdoor electrical receptacles, hose bibbs, and points where walls meet.
If your house is made of brick or stone, or if you have a chimney, check for gaps, and repoint with mortar, if necessary.
Now go into the basement and seal the sill plate (where the basement ceiling meets the foundation wall) if it hasn’t been sealed already. Seal around all pipes and conduits that pass through the foundation wall. Seal any cracks between concrete blocks. Seal around any pipes, conduits, wires, or cables that pass between floors.
Finally, go back upstairs, and seal any gaps around electrical outlets and switch boxes, and around any pipes or ducts that pass through floors, walls, or ceilings. Be especially attentive under sinks in the kitchen and the bathroom.
Sometimes, pipes are hidden behind access panels for safety or cosmetic reasons.
Don't forget to check for roof damage. For example, patch holes with roofing cement.
Now and then, check if the sealants have dried up and stopped sealing the gaps properly. Reapply as necessary.

After you seal all of the bypasses that you possibly can, make sure that your house is not too tight. A house without sufficient ventilation will trap moisture, odors, chemicals, dust, and microbes (such as bacteria and fungi spores). You lose some money paying for heat and air-conditioning, but that is the price you pay for not breathing stale, unhealthy air. Open some windows, operate the whole-house fan, install energy-efficient mechanical ventilation [2], or install more ventilation, if necessary.