When it comes to the building shell, there are two major issues to consider: air leakage and insulation. While these problems are often related, they are not the same thing and should not be treated as if they are interchangeable.
Many older homes suffer from insufficient insulation and are also leaky, allowing conditioned air to escape and outdoor air to enter through gaps and cracks in the building enclosure. Insulation slows the transfer of heat, while air sealing controls the movement of air. Both are important, but they solve different problems.
One of the most important principles in building science is to air seal before you insulate. If you add insulation without addressing air leaks, you may miss an opportunity to improve comfort, energy efficiency, indoor air quality, and the overall performance of the home.
Understanding the difference between air leakage and insulation deficiencies is the first step toward developing effective solutions that deliver measurable results.