Once again for those in the back.
A. Houses don’t need to breathe.
B. People need to breathe.
Yes, a house with low air leakage and no ventilation will likely have problems with humidity and poor indoor air quality.
No, that does not mean this approach is wrong. It means it’s incomplete.
You can’t make a house energy efficient without tightening up the building envelope. But when you do that, you need to address the whole house, not just the envelope. This is building science 101: the house is a system. A change to one part affects all the other parts. Building science includes everything: materials, assemblies, construction methods, and equipment. Oh, and the occupants.
Allowing your house to ‘breathe’ via a leaky envelope means you’re filtering fresh air through an ever-aging medium of construction materials, dust, dead insects, and assorted rodent turds. That’s not healthy.
It’s not enough to be energy efficient, the house has to also be comfortable and provide a healthy indoor environment, because occupants. When we lead with health and comfort, surprise! The best way to get a comfortable and healthy house also gives you an energy efficient one!
CONSENSUS: HOUSES DON’T NEED TO BREATHE, PEOPLE NEED TO BREATHE
In the building science community, there is consensus about tightening up the envelope. The heated discussions revolve around what materials and assemblies work best in which climatic zone, and how tight a house are we talking about?
There are many scholarly articles and research reports that point to this.
So many research reports.
Everything we do as an industry to make building enclosures healthy, durable, and energy efficient revolves around reducing air leakage. And why is that?
Because there is no viable way to strip heat and moisture out of air that is leaking into and through the building envelope.
Because water is always the enemy, regardless of which state it occurs in a house (vapour, bulk water, ice).
Heat loss associated with air leakage leads to high bills and occupant discomfort. Moisture flow associated with air leakage leads to condensation, and structural damage of the house. Moist, warm air flows lead to condensation, mold growth conditions, and poor indoor air quality.
In a leaky building envelope, when you have moist, warm air moving through a wall, it will condense out into the structure. If it’s moving through a ceiling, it will contribute to ice damming and moisture build up in the attic.
If you stop air moving into the building envelope, you remove the cause of many problems that cause damage to the structure. This is because, when you stop warm, moist air from moving through the building enclosure, you are keeping it in the house. And so now you can have problems with condensation and indoor air quality. Which means now, you need to add controlled air flow back into the house with a mechanical ventilation system.
Depending on the heating system, you could create problems with combustion air for gas or oil heating. When you tighten the building envelope, you change the pressure dynamic in the building. Often this can result in there not being enough relief or make-up air for the combustion appliance. The end result is combustion spillage - health and possibly life-threatening conditions.
STOP AIR FLOW, ADD AIR FLOW - WHAT?
At this point, it’s easy to think of throwing up hands and saying ‘I give up’. Yes, there’s lots to think about. No, it’s not rocket science.
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The assemblies that make up the house need to repel or shed accumulated moisture. They need to dry, not breathe.
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Mechanical ventilation allows you to control air exchange. It also allows you to temper and filter fresh air through a medium that can be cleaned and/or replaced.
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When you tighten a building envelope you have to add make up air for combustion appliances.
The house is a system. You can’t fight physics. There is no way to ‘win’ at trying to change or beat physics, you’ll just end up with problems that you can’t fix.
The goal is to minimize air, moisture, and heat flows to create and maintain a comfortable and healthy house that uses as little energy as possible.
There are 5 rules to follow to accomplish this goal:
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Drop the air leakage rate as low as is possible
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Insulate the building envelope to the highest degree possible
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House assemblies need to dry out, so different materials need different approaches
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A tight house needs mechanical ventilation
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Make up air needs to be provided to combustion appliances
Of course, the devil is in the details.
We use energy modelling and performance testing to ensure we’re playing by the rules.
PLAY BY THE RULES FOR HEALTH, COMFORT, AND ENERGY EFFICIENCY
To achieve Rule 1 and 2 we spend a lot of time and effort getting the right sequence of the right materials in different building components for different climates to adhere to Rule 3. Then we need to meet Rule 4 and 5, we need the right suite of mechanical equipment for that house in that place.
The challenge: what we know about ‘standard’ construction for Rules 1,2, and 3 does not always apply to high-performance housing practices. The physics don’t change, but the dynamics of air, moisture, and heat flows change, for example, when you have a highly insulated wall with no thermal bridging. The dynamics are different than a stick-framed wall with cavity insulation. The dynamics are different depending on the materials you choose. It’s a constant learning process.
More on Rules 1, 2, and 3 and high-performance construction in the coming weeks.
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