Passive House Course Day 5 - Airtightness
OK, so we’ve established that to maintain a consistent indoor air temperature we can’t afford any unwanted air leakage from the building. To ensure this we need a continuous airtight barrier around the home. Let’s break this down into the different components;
Floors: A concrete slab is airtight, as is most flooring, however, the junctions to the walls will need to be taped.
Walls: Airtightness of a wall buildup can vary depending on the components. Be sure to understand whether the specified product is classified as airtight. Minimising penetrations is a must, so using plasterboard as the airtight barrier is not a good idea. Best practice is to create an airtight barrier on the inner surface of the thermal envelope (see below for suitable products), batten out the plasterboard wall finish allowing for a small services cavity for plumbing and electrical works, ensuring the airtight envelope is not compromised.
Windows & Doors: We can assume that a suitable window manufacturer will be engaged for a passive house, and have airtightness covered, however, the builder must ensure airtightness around the perimeter, usually taped.
Roofs: Again the airtight envelope is more easily achieved from the inside surface of the roof build-up, minimising penetrations if possible.
Passive House is quite specific about airtightness, and as such requires the home to be pressure tested to understand the number of air changes per hour (ACH) of the total air volume within the home. PH has a very strict tolerance of up to 0.6 ACH at 50Pa of pressure. (50Pa is supposedly the equivalent of 32kmh winds on the building).
To give some perspective, your average Australian home ranges between 10-15 ACH. In other words, a Passive House has 15-25 fewer air changes per hour!
Just think about that for a second…imagine its winter and there is a storm outside. Your heating system is roaring, trying to keep up with the amount of air that is being lost from the home. Or in contrast picture, a hot northerly wind in summer and your AC is at its threshold trying to compete with the lack of airtightness.
Now assume your home lasts for 100 years, picture the amount of energy that will be wasted in that period…. Astounding!
Multiply that against all of the inefficient homes in your suburb…. state.... country… We are so resource hungry - there is an opportunity for a rant, however, I’ll save that for another day ;)
Does it not seem a no-brainer to make the building more airtight?
No, you won’t suffocate! Read my next blog on ventilation to understand how Passive House combats air quality within a home.
For practical assessment of airtightness, Air Pressure Tests are conducted - commonly known as ‘blower door tests’. It is common for builders to engage 1-2 blower door tests during construction to a) help find leaks in the envelope and immediately rectify, and b) ensure that the house will meet Passive House standards at completion.
The recent update to the NCC makes reference to airtightness within a building, setting objective measures and alluding to the fact that in the not too distant future we will have mandatory measures for airtightness, just like Europe already. Why wait until we’re told to make change, we’re taking action on this topic now…maybe you should too.
Here is some fantastic reference info. Efficiency Matrix is at the head of the game in Victoria when it comes to understanding Passive House and providing blower door tests. Check out this YouTube video of a very cool CLT House in Gippsland by Aphi Projects, one of the best Passive House certified builders around. Amongst other passive house features, it demonstrates the final blower door test in action.
Pro-Clima seems to be ahead of the game in this department with wall membranes and tapes suitable for the lifetime of the building. Check out some good info here.
Even Bradford Insulation has experimented with methods of installation of their building wraps to understand the effects of airtightness. It’s a small article and a must-read.