Air-sealing is Good, But Don’t Forget The HRV!

We all know what its like to work or live in a drafty building, or what it’s like to pay a larger-than-it-should-be energy bill due to extra heating or cooling. A common solution that is recommended to building owners is to seal the cracks in their walls, windows, doors, etc. It’s true that in many cases this air-sealing provides great “bang for your buck” in energy savings, but without proper understanding of the system that is your building you may end up with more problems down the road than just high energy bills!

It’s important to understand that even small houses are systems of interacting heat, moisture, air, water, building materials etc. and making a change to one aspect of the system often will impact others. In the case of air sealing, we are reducing the cold outdoor air from leaking into the building (or the warm humid air in the summer). This saves energy, but there is more that comes and goes through the cracks than your hard-earned dollars!

To understand this issue we need to understand what is important for a healthy indoor environment. Among the many variables are fresh air and appropriate humidity. An older building (or even a poorly constructed new one) will have significant air leakage around doors, windows and even construction joints such as where the wall meets the floor. These, although costly, serve a useful purpose – they allow fresh air to enter the building, and allow humidity and contaminants to escape.

In a commercial building these rolls are typically filled by mechanical ventilation systems, but in a house, there is often no ventilation other than opening a window. In these cases, when air sealing is applied, overall fresh air is reduced and humidity can become a problem. So, to ensure continued air quality, we need to compensate for these by adding a heat or energy recovery ventilator (H/E-RV), which brings in air from the outside.

You may now be asking: “But, why am I now bringing more air into the building when I just finished sealing it up!?”

To answer this it’s better to think of air sealing not as reducing the overall air entering the building, but taking control of the air entering the building. Instead of dozens cracks leaking cold air directly into your home, the air enters under your control via the HRV. Also, they are called heat recovery ventilators for a reason – they recover a portion of the energy as it leaves your building and uses it to pre-heat the air coming in. This reduces the amount of energy required to heat the incoming air.

Tightening the building envelope without accounting for appropriate flows of fresh air and humidity can lead air quality issues – so don’t forget the HRV!

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About Matt

Matt is a mechanical/building engineer who specializes in whole-building energy modelling, energy efficiency and solar buildings. He's worked for about 9 years analyzing the energy use patterns of buildings. He studied the energy performance of a low-energy, solar house for his Masters thesis. Matt has experience with EnergyPlus and eQuest.

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