Airtight envelope – the ‘secret sauce’ of high performance construction
Airtight envelope construction is a good thing. A very good thing! Please do not let anyone tell you otherwise. It is impossible to build ‘too tight,’ provided you have proper mechanical ventilation, of course.
Air leakage is measured with a blower door test. The building is pressurized to 50 Pa, and then depressurized to 50 Pa, and sophisticated software calculates the best-fit line for the level of air leakage in your building. Air leakage is measured in air changes per hour. That is to say, how many times does the volume of air inside your building get exchanged in one hour at 50 Pa of pressure difference between the interior and the exterior. If you are not testing, you are guessing, and we do not guess when it comes to building science.
Most new construction varies between 3-10 ACH50. Passive House requirements for certification are set at <0.6 ACH50. Now that’s tight! To put that into a physical context, the amount of cracks and openings in a typical house with 10 ACH50 would represent the size of several sheets of paper. Now compare that with a blower door test of <0.6 ACH50, the air leakage of a Passive House, the total area of all the cracks and openings being the size a business card. That is what is referred to by Passive House veterans as the, ‘secret sauce’ of high performance construction.
Why is air leakage so bad anyways?
Air leakage in our buildings is a big no-no for many reasons. If a building is leaky, which most are, we are losing interior conditioned air that we have spent money to heat up, or cool down. Additionally, we are allowing exterior air into our buildings that we will have to spend even more money to heat up, or cool down. Basically, dollars are flowing through our envelope systems.
We cannot forget about interior comfort. No one likes to feel a cold draft whisping across your toes. Yes, slippers do help, but it’s unlikely you have slippers for all your guests. Yes, sweaters help, but who wants to rely on those for comfort from cold drafts inside our buildings? Leaky buildings are uncomfortable.
Structural durability of our buildings is significantly compromised with air leakage through the building envelope. This is a very serious issue. Air, and especially warm air, can hold incredible amounts of moisture in the form of water vapor. You cannot see it, but its there. Now, if we take our leaky building, and allow warm moist air to flow through all the walls, roofs, foundations, windows and doors, we have a problem – a big problem. When warm moist air that hits a cold surface, often exterior plywood or in the framing cavity, the moisture turns from vapor to liquid. This is one of the main reasons why mold and mildew form in our walls. People get sick from this. Additionally, air leakage is the main cause of ice dams and material decay leading to structural failure over the long term.
Where there are cracks in the building envelope, there is likely to be some nasty things growing in there. Maybe not today, maybe not tomorrow, but certainly in the future. It is not something you want to roll the dice with. Build it tight and ventilate right.
Interior air quality is a huge concern in our buildings. In leaky buildings, you breathe the air that comes through all these cracks and openings in the building envelope. As we know, these cracks in the building envelope are not a clean source of air. Who wants to breathe air that passes by mildew, mold, mouse droppings, material chemicals, etc.? Not a good idea.
To recall – leaky buildings are:
- Energy consuming pigs – why waste your money?
- Full of cold drafts and offer poor interior comfort
- At high risk of mold, mildew, and decay within the structure
- Poor interior air quality
Remember, you can never build your envelope too tight.
Please, Built it Tight & Ventilate Right.