This article was inspired by a podcast episode about the house being the most underappreciated duct with Joe Medosch. You can listen to that podcast HERE.

We probably think about sheet metal, flex duct, or the ever-controversial duct board when we think about ducts. It’s probably shiny and out of sight in most homes.

But we’re forgetting about the biggest duct of all, which usually ISN’T shiny and is pretty hard not to see. We’re talking about the house itself. That may seem a bit odd to some people, but the house is indeed the largest duct (and probably the most overlooked).

What is a duct, anyway?

Here’s the basic Wikipedia definition of a duct:

Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air.

The building envelope can certainly qualify as a duct. It may not be a narrow conduit as we may tend to imagine, but air does move through it. 

As with leaky duct systems, air can also enter the home via infiltration when outdoor air seeps through cracks and other openings in the building envelope. Exfiltration is the opposite, which happens when the air leaves the building through openings in the structure.

Why should we care about the house as a duct?

When the A/C performance isn’t up to par, we may check to see if the ducts are leaky. 

Decreased air quality is a common symptom of leaky ducts. When a customer complains about a home being extremely dusty or smelling like the attic or basement, that should prompt us to check for leaks in the ductwork. A leak in the ductwork would allow for unconditioned air from the attic or basement to enter the ducts and get distributed throughout the home.

The home is also a duct with similar leakage concerns. If you live in a leaky house, then nasty attic or basement air is likely entering your living space. So, the infiltration decreases your indoor air quality. 

Whenever the house is under negative pressure, such as when you use mechanical ventilation strategies in a bathroom, you’re bringing in air through gaps in your home. The problem is that you can’t really control the quality of the air you bring in through undesigned holes in the building envelope.

How to apply good duct design principles to the building envelope

When contractors design the HVAC system, they may rely on Manuals J, S, and D for load calculation, equipment selection, and duct design, respectively. 

However, many contractors overlook the infiltration rate of the building envelope, or they simply estimate it instead of measuring it. It’s difficult to estimate the infiltration rate accurately without taking measurements.

When you do Manual J load calculations, part of the load will come from infiltration. In some cases, especially in historic homes, infiltration could be responsible for upwards of 40% of the load! So, just as we would seal leaky ducts, we can seal the building envelope.

It would be best to seal the envelope and then do Manual J calculations to give the HVAC system the most control over your comfort.

How to quantify the leakiness of a home

The best way to wrap our heads around a leaky home is to record data. We typically do that by performing a blower door test.

A blower door simply replaces a major opening in the house (like the front door), and it has a large fan that brings the home under negative pressure. We use a gauge to measure that pressure. 

We seal the home as best as possible (under “wintertime” conditions) and make sure all interior doors are open. We basically isolate the inside from the outside. Once we’ve done that, the fan pulls the house under negative pressure, and it can measure the building envelope leakage in cubic feet per minute (CFM). Then, we can figure out the air changes per hour (ACH). 

We can calculate the ACH by multiplying the CFM value by 60 to get the cubic feet per hour (CFH). Then, we divide that product by the volume of the space to get the ACH.

We generally pull a house down to -50 Pascals, so under those testing conditions, we call the ACH value ACH50. However, some especially leaky homes won’t even allow the house to go under that much negative pressure; the infiltration rate is so high that the outdoor air provides a significant positive pressure source.

Sealing the building envelope

We can seal the home when we finally understand the true extent of leakage. 

Some of the most common sources of leakage come straight from the attic or basement. For example, several wires could be running through an opening in the attic that’s far too big for them. Air can slip right through that opening, so we can seal it up to reduce infiltration.

In other cases, there are often gaps between the drywall and the home’s wooden structure. Air can easily pass through those openings when there’s a crack, so we can seal those up to make the house a lot tighter.

When you have gas appliances in the home, you need to consider a few extra things. Tightening a home allows carbon monoxide to linger within the building, so you need to make sure you’re doing combustion analysis BEFORE sealing to ensure that the house is safe for the occupants. 

After sealing a building with combustion appliances, make sure the fireplaces, furnaces, and stoves are still drafting as they should.

We’ve sealed the house. What do we do now?

After sealing the cracks in the building envelope, we’ll have less nasty outdoor, attic, or basement air coming into the living space. Great! But where will the replacement air come from?

We need to have a strategy to bring “fresh” replacement air into the home. Commercial HVAC markets already use fresh air intake systems to introduce outdoor air to the building envelope. This air is controlled and provides a necessary source of ventilation.

ASHRAE Standard 62 was revised in 2019, so we can expect ventilation to become much more important in the coming years. We would also be wise to start considering fresh air intake in the residential market. 

When you’ve sealed the home and blocked off undesigned air paths, you can create a new path that allows you to control the air coming into the home. Nowadays, we’re also seeing the rise of balanced mechanical ventilation strategies; as you reject air from your bathroom or kitchen, you also bring in some supply air.

In any duct system, we want air transport to be designed. We don’t want nasty crawlspace air to seep into the ductwork at poor connections, so we also wouldn’t like that air to infiltrate our living spaces directly.

Like the shiny, narrow ducts in the attic or basement, the house needs to have undesigned air paths sealed to give us the most control over the indoor air quality.