Air sealing is essential for energy efficiency.
Insulation by itself is ineffective without good air-sealing (like wearing a thick coat with holes in it on a cold, windy day) and air sealing by itself is ineffective without good insulation (like wearing a plastic bag and nothing else on a cold, windy day).
To be effective the insulation should be in contact with the air sealing layer.
Good air sealing means that you have to pay careful attention to ventilation to protect the indoor air quality (which we will cover in Step 9).
In the Greeny Flat the roof panels create an effective air seal in the lid and the concrete slab creates an effective air seal in the floor. So we really just had to pay attention to air-sealing the walls and the connections between the different elements. For the walls we used the reflective insulation layer as the air barrier. We taped all of the joints and holes and paid careful attention to sealing around the windows, doors, and along the bottom edge. At the top edge we wrapped the reflective foam product up over the top plate and then laid the roof panels onto it creating a good seal.
Once the thermal boundary (insulation and air-sealing) was completed we tested the air tightness of the whole structure with a Blower Door.
A Blower Door Test measures the relative tightness or leakiness of a building in ACH50 (Air Changes per Hour at minus 50 Pascals). An extremely well-sealed building might have an ACH50 of less than 1.0 while an extremely leaky building might be over 20ACH50. We were hoping for 3.0ACH50 and came in at 3.25 so we were happy.
To achieve good air-sealing there should be a continuous air barrier around the entire thermal envelope. Particular attention needs to be paid to the places where different materials meet. Where the roof meets the walls, where the walls meet the windows, where the walls meet the floor, etc.
The simpler the shape of the building, the less complicated it will be to create an effective air seal. This is just one of the many reasons why the first rule of energy efficient design is to Keep It Small and Simple.