Insulation and Air Sealing, Why You Need Both

by | Last updated May 31, 2020 | Construction Practices, Houses, Sustainable Design

Air Sealing and Insulation are critical issues for energy conservation and sustainable design. Insulation prevents the transfer of heat and air sealing prevents the leaking of heat or air conditioning.

Everyone knows insulation is important in a house, but air sealing is also important. Here we will be touching on both subjects and their relationships. Insulation and air sealing are essential for building Sustainable Homes and is a core principle of Passive House Design.


Spray Insulation and Why Air Sealing Is Important.

All the information in this post is targeted for what we call cavity insulation. This is where you put insulation between wood or metal studs, joists, or rafters. I will not be discussing Exterior insulation and Finish Systems (EIFS). This is for stick built wood or metal framed houses.

We all know insulation is important to reduce your energy usage in heating and cooling. If you build a new home with a great deal of insulation but that also leaks air, you are not getting a great value for all the insulation in the house. You must insulate and seal your home from air leakage.

Spray Insulation for Air Sealing and Insulation

Spray insulation is a foam that is sprayed in between your wood or metal studs. This foam expands and gets into all the cracks, holes, and joints. It goes around windows, doors, or anything else in your walls, floors, or roof. Anything going through your insulated surface be it a door, window, pipe, exterior lights, exhaust vents create week points to your air sealed house. Cracks and construction flaws also account for air leaking. The goal is to make sure the conditioned air inside your home does not escape through your walls. Cellulose insulation and batt insulation (batt is the stuff that looks and feels like cotton candy) do not stop air from leaking though your house.

Spray insulation expands and fills all these places where air can escape. The Department of Energy says “Reducing the amount of air that leaks in and out of your home is a cost-effective way to cut heating and cooling costs, improve durability, increase comfort, and create a healthier indoor environment” I would emphasize that air sealing is a critical issue for home construction and one of the five basic requirements for Passive House Design. We have another article you can see to learn more about Passive House Design.

New Insulation Types

Insulation is becoming more important and we will continue to see new insulation types appearing on the market. Since becoming available spray insulation has grown in popularity. The value of insulation thermal properties is measured by R-Value. The higher the R-Value the better.

Spray Insulation

There are two kinds of spray foam insulation Closed Cell and Open Cell.

Open Cell Spray foam Insulation:

  • Cost: Open cell spray insulation is more affordable than closed cell insulation.
  • R-Value: Open cell insulation has an R-value of typically R-3.5 or a little more per inch.
  • Air seal: Open cell provides air sealing keeping the outside air out and the inside air in.
  • Vapor / Water Seal: Open cell is not water or vapor resistant.
  • Structure: Open cell has no structural value.

Closed Cell spray foam Insulation:

  • Cost: Closed cell spray insulation is more expensive than open cell spray insulation.
  • R-Value: The R-value of a closed cell insulation is typically R-6 to R-6.5 per inch (that is a very good R value compared to many other types of insulation).
  • Air seal: Closed cell provides air sealing keeping the outside air out and the inside air in.
  • Vapor / Water Seal: Closed Cell insulation is also vapor resistant water resistant and will not allow vapors, humidity, water to pass through it.
  • Structure: Closed Cell insulation is denser, heavier, and stronger. Closed cell is the only insulation that will actually add structural value to a wood or metal framed wall or roof. It adds rigidity and stability to the framing of the house increasing the entire overall structural value.

Economy is obviously an issue. Although closed cell is more expensive up front it will save you money over time by reducing your energy costs. It does have several advantages as we see in the list above.

Spray Insulation for new homes

Spray Insulation for new homes


Home Air Sealing Techniques

  • Use Closed Cell Spray Insulation on the walls and roof
  • Caulk all doors and windows
  • Use doors and windows with weather stripping.
  • Purchase doors and windows with low air leakage rates (found in the product specs)
  • Caulk and seal plumbing, duct, electrical and all other penetrations through walls, floors, ceilings, roof etc…
  • Use draft proof doors for your fireplace and flue dampers
  • Minimize penetrations through the exterior of the house


How do you know your home is air sealed?

Blower Door Test (Air Sealing Test)

Insulation and air sealing Blower Door Test

Blower Door Test being conducted on one of our houses.


We often test the homes we build using the blower door test to check that they are properly air sealed. A blower door test is a system for measuring air leakage. The person testing the house will put a plastic wrap on an exterior door with a fan in it. The fan blows air into the house and a pressure monitor detects the rate at which the air is escaping by the differential in the air pressure. This is a great way to know if your house is performing well in the air sealing department.

Our goal here is to make sure you understand that if you put a ton of insulation in your new home but the house itself leaks air you are not getting the full effect of the insulation. Be mindful of these issues as we are all trying not only to save a little money but pitch in anyway we can to help the environment. Higher quality insulation is more expensive up front but the long term savings are huge.

I hope this was helpful in helping you understand a little more about the importance of air sealing and insulation.


Insulation and Air Sealing

As an architect I study construction technology and materials closely. In this article we reviewed some of the basic concepts with regards to Insulation and Air sealing. This post does not assume to cover every possible issue or condition, but provide a general overview of the topic.


Thank You for reading our blog post on Spray Foam Insulation

Please leave questions and comments below. If you want to speak with an architect about a specific project you can contact us directly. Learn more about our services as house architects and sustainable design architects.


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Jorge Fontan
Jorge Fontan

This post was written by Jorge Fontan AIA a Registered Architect and owner of New York City architecture firm Fontan Architecture. Jorge Fontan has earned 3 degrees in the study of architecture including two degrees from the City University of New York and a Masters Degree in Advanced Architectural Design from Columbia University. Jorge has a background in construction and has been practicing architecture for 15 years where he has designed renovations and new developments of various building types.