Air Leaks -- Sealing Priorities

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With air-sealing, as with any job, it’s useful to know what your priorities are. Our strategy is first to always find the big leaks; sealing them makes the biggest difference in com fort levels. Then we look for the smaller leaks.

Air movement and leakage areas are not always intuitively clear, but the most important thing to remember is that air does not move through solid objects. Air leaks through connections and spaces between building materials.

First find the big leaks

Although it may sound obvious, the most important holes to seal are the largest ones. Not every house has every type of hole, but as you read through this chapter, you may recognize some of these big leaks—called bypasses—where indoor and outdoor air mix. I’ve actually seen bypasses that are big enough to crawl through!

Sealing just one of these large bypasses may make a greater difference than replacing every leaky, rattling window in your house. Most bypasses are small but still big enough for a cat to crawl through. When you head up into your attic to hunt for them, make sure your cat doesn’t follow you, or he may get lost.

Sealing the Attic Keeps You Warmer Down Below

Fig. 30-0

Warm air pushes hardest to escape at the attic level. If you can keep the air from leaking out from the top, you will prevent cold air from see ping in from the bottom. A glass under water (see inset) demonstrates the same principle: If the lighter air is prevented from leaking out from the top, the heavier water will not be able to get in from the bottom.

The air barrier is most important at the attic level, where the pressure is greatest.

Warm air leaks out the top.

Leaks near the bottom are also important because the pressure is greater there.

The pressure is less in the middle of the house.

Cold air seeps in from the bottom.

(Gold arrows = Warm air) (Gray arrows Cold air)

(Green dotted line = Air barrier)

(Red arrows = Air inside glass) (Blue arrows = Water outside glass)

The upside-down glass is a good air barrier, preventing exchange between the inside air and the outside water.

Seal the high and low leaks

Remember the stack effect, which pushes out air at the top and pulls in air from the bottom during the winter? The greatest pressure differences are those at the highest and lowest points in your house, so your top priority should always be the attic. Doing a complete job of sealing leaks between the house and the attic will help prevent cold air from seeping in from the bottom of the house (fig above).

When we sealed the leaks in our own attic, we were amazed at how much more comfortable our living room became. Sealing the attic also helps keep out moisture, reduce ice dams, and prevent hot air from leaking in from the attic and roof in the summer. Sealing leaks in an attic is hard work, but

it is the best place to access the thermal boundary at the top of the house.

Once you have sealed the attic, the next priority is the basement, crawl space, or slab—the home’s foundation. Sealing leaks there will help prevent cold air from coming in during the winter and cool air from escaping during the summer. Then you can spend some time with the sidewalls and smaller leaks in other parts of the house.



To create a comfort able environment quickly, find the big leaks first and seal them. Then look for smaller leaks.

What Can Go Wrong

Furnace ductwork located in an attic can be a major cause of ice damming on the roof. Warm air leaking from air handlers and ducts, as well as heat getting through uninsulated ducts via conduction, can melt snow on roof sheathing. As the snow melts, water runs toward the eaves, where it freezes again and creates a dam of ice that can push water underneath the shingles. All attic ductwork should be air-sealed and insulated to help prevent ice dams.

Shhhh -- Trade Secret

The biggest air leaks in the attic typically occur at places where the ceiling plane is interrupted. This can occur wherever there is a change in ceiling height or where there is a dropped soffit. Duct, plumbing, and chimney chases are also prime suspects when hunting down large avenues of air leak age.


Ice Dams

If you live in snow country, you’ve seen houses with ice dams clinging to the eaves. The most common solutions to ice dam problems are self-stick rubberized membranes and/or roof ventilation, such as soffit and ridge venting. Other fixes include mounting electric heater cables, shoveling snow off the eaves area, and installing strips of metal roofing near the eaves. Unfortunately, none of these fixes deals with the cause of the melting snow. Most ice dams form when heat loss into the attic warms the underside of the roof sheathing. This heat loss occurs when insulation is compressed or poorly installed near the eaves; it is also due to air leakage, one of the major causes of ice dams. To help prevent ice dams, thorough attic air-sealing should always be a top priority.

Fig. 31-0

How Ice Dams Form

Air leakage is the biggest source of the heat loss that causes ice dams; poorly installed insulation comes in second, Ice dams form when snow melts and runs down the roof to the eaves, where it refreezes and backs up into the eaves or the house, causing damage.

As ice builds up, additional snowmelt can back up under the shingles.

Cold air from soffit vents washes through the insulation (blue arrow).

Heat loss from compressed or poorly installed insulation melts the snow (red arrows).

Warm air leaking into the attic melts the snow (gold arrows).

Prev.: Understanding Heat Transfer
Next: Attic Air-Sealing

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