Air Sealing vs. Adding More Insulation
Adding insulation to an attic without sealing the air leaks first is a common mistake. Here is why order matters.
| Comparison | Air sealing | More insulation |
|---|---|---|
| What it stops | Air leaking through gaps, carrying heat and moisture. | Heat moving through the ceiling by conduction. |
| Effect on ice dams | Large: warm air in the attic melts the roof snow. | Helps, but leaks bypass it. |
| Effect on attic frost | Large: leaks carry moisture into the attic. | Little on its own. |
| When to do it | First. | Right after the air sealing. |
Why air sealing comes first
Once insulation is blown in, the leaks under it are buried and hard to reach. Sealing the top plates, wiring and plumbing holes, chimney chases and the hatch first means the insulation can do its job.
Signs your attic is leaking air
- Ice dams or long icicles along the eaves in winter.
- Frost on the underside of the roof sheathing or on nail tips in the attic on cold mornings.
- Dark stains on insulation, which often mark where warm, dusty air has been leaking through.
- Upstairs rooms that are much colder in winter and hotter in summer than the rest of the house.
Where the leaks usually are
In New Hampshire attics, the biggest leaks are usually at the tops of interior walls, around chimneys and flues, at recessed lights, where plumbing vents and wiring pass through the ceiling, and around the attic hatch or pull-down stairs. In older houses, open chases beside chimneys and above kitchen soffits can leak a great deal. Read more in air sealing in New Hampshire.
Doing both
Our attic jobs do both: air sealing first, then vent baffles at the eaves, then blown cellulose to R-60. See the attic cost guide for example ranges.
Measuring it
A blower door test shows how leaky the house is before and after, so the result is measured instead of assumed.
Related pages
- Air sealingHow we air seal attics.
- Ice dam preventionWhy ice dams form.
- More resourcesGuides, glossary and comparisons.
Common questions
Will air sealing make my house too tight?
Not from attic air sealing alone in an older house. If a house becomes very tight, mechanical ventilation is the answer, and we will tell you if yours needs it.