How to Upgrade Your Building Enclosure for Better Comfort, Air Quality, and Energy Efficiency

The most significant comfort and energy efficiency improvement you can make to your house is upgrading its enclosure. Not windows, not insulation, not HVAC. If you minimize infiltration you can gain control of comfort, air quality, and energy usage.

What Is a Building Enclosure (and Why Does It Matter)?

A building enclosure is the barrier between your home’s interior and the outside environment. Gaps and cracks in this barrier allow uncontrolled air movement (infiltration), which drives up energy costs and degrades indoor air quality. 

To minimize infiltration, we first need to understand the forces behind it: temperature, pressure, and to a lesser degree (for us in Central Virginia), wind. Hot goes to cold, higher pressure goes to lower pressure, and both will try to find an equilibrium.

For example, wind blowing against one side of the house will be at a higher pressure than inside, and air will force its way in. Duct leakage can create a negative pressure in the house and accelerate infiltration. A temperature difference between inside and outside will also create a pressure differential — the bigger the temperature difference, the bigger the pressure differential. The process of temperature-driven infiltration and exfiltration is called stack effect.

What Is the Stack Effect? How It Affects Your Home Year-Round

The stack effect refers to temperature-driven air infiltration. In winter, warm air rises and escapes through the top of the house while cold outside air enters from below. In summer (reverse stack effect), the opposite occurs. Both scenarios pull air in from the dirtiest parts of your home, the attic and crawl space.

Reverse Stack Effect (Cooling Season)

Since the cooling season is still fresh in our minds, I’ll start with reverse stack effect first. When air inside the house is cooled, the cool air falls because it is more dense than the warmer air. The heavy, cool air creates a high pressure zone low in the house (from the weight of the air), while simultaneously creating a low pressure zone high in the house.

Here’s what happens:

1. Cool air becomes dense and sinks, creating high pressure low in the home
2. This simultaneously creates low pressure high in the home
3. The heavy cool air pushes itself out through the crawl space, doors, windows, and gaps/cracks in the 1st floor’s building enclosure
4. Since the house cannot remain in a vacuum and pressures seek equilibrium, every bit of air that exits (exfiltration) must be replaced with outside air (infiltration)
5. Infiltrating air enters where pressure is lower — high in the house
6. Since the attic has the highest temperature differential and the ceiling plane is typically very leaky, this is where the majority of infiltrating air comes from
7. That attic air is hot, humid, and carries pollutants like pollen, fiberglass, and pest feces

This is often why the second floor is warmer than the first floor, and sometimes you can even smell the attic from the 2nd floor.

Stack Effect (Heating Season)

In the heating season, stack effect is literally the opposite of reverse stack effect. Because the temperature differential between inside and outside is greater, infiltration is greater.

Here’s how it works:

1. Air is heated and rises because it’s less dense
2. It creates a high pressure zone high in the house while simultaneously creating a low pressure zone low in the house
3. Air exits high in the house and is replaced with outside air entering low in the house

You may have heard that 40–50% of the air you breathe comes from the crawl space — this is why. I’m not sure when the last time you ventured into your crawl space was, but most likely you do not want to be breathing that air. (I recently installed a ventilation system in my non-vented crawl space with some interesting results.)

Why Air Sealing Is More Effective Than Adding Insulation Alone

Air sealing should always come before adding insulation. Insulation works by trapping still air and if air is constantly moving through it, the R-value drops significantly. You cannot insulate your way out of an air sealing problem.

So what can be done to reduce infiltration, and more importantly, reduce it from originating from polluted sources like your attic and crawl space? The short answer: air sealing. Air sealing is always near the top of the list of any home performance project. Air sealing trumps insulation. Insulation will not work as advertised without air sealing. This is why insulating by itself, without air sealing, doesn’t always work as expected.

Simply adding more insulation to the attic and crawl space will not be as impactful without air sealing because insulation works by trapping air. If air is constantly moving across the insulation, its R-value (resistance to heat transfer) is reduced significantly. Insulation will not stop hot, humid or cold, dry air from entering your house.

It’s also worth noting: heating and air conditioning equipment can mask a lot of building enclosure flaws with sheer horsepower only to see those flaws quickly return after the system turns off.

How to Air Seal Your Attic: Two Approaches

Attic infiltration is typically addressed in one of two ways:

Option 1: Seal the Ceiling Plane (Vented Attic)

1. Remove or move the existing attic insulation
2. Air seal the top plates, can lights, and all plumbing and electrical penetrations
3. Replace the insulation or install new insulation to the appropriate R-value
4. Leave the attic vented as before

Option 2: Convert to a Conditioned (Unvented) Attic

1. Apply spray foam to the roof deck and gable ends
2. The attic becomes part of the conditioned space of the home
3. Ducts located in the attic are no longer in an unconditioned zone

This method makes the most sense from an energy efficiency standpoint if the ducts are located in the attic. Ducts in unconditioned spaces are a huge energy penalty and should be avoided whenever possible.

How to Air Seal and Encapsulate Your Crawl Space

Crawl space encapsulation is best practice for most homes. By sealing vents, installing a ground vapor barrier, insulating the walls, and adding a dehumidifier, you can eliminate the crawl space as a source of polluted air infiltration and control moisture that leads to mold and structural damage.

The crawl space has two approaches as well:

Option 1: Full Encapsulation (Non-Vented)

In best-practice encapsulations:

1. Remove existing crawl space insulation
2. Clean all biological growth
3. Close and seal foundation vents
4. Install a vapor barrier on the ground, running it up the walls and sealing at the top
5. Insulate the exterior foundation walls
6. Install a properly sized dehumidifier

This makes sense for most situations.

Option 2: Sealed and Insulated Floor (Vented Crawl Space)

There are times when it’s best to leave the crawl space vented and instead air seal and insulate the floor above with closed-cell spray foam. This is typically only done in areas that are at high risk of flooding.

What About New Windows and Doors?

New windows and doors are great, especially if the old ones do not open properly. But in most situations, even if you can feel a draft, windows and doors are not as leaky as the crawl space or attic. And when windows do leak, it’s fresh air — not crawl space or attic air.

So, if your goal is to have a comfortable, energy-efficient home with good air quality, reducing stack effect and infiltration by air sealing should be at the top of the list. Once that’s done, go ahead and buy some new windows.

Frequently Asked Questions About Building Enclosure Upgrades

Q: Why is the second floor of my house always hotter than the first floor? This is a classic sign of reverse stack effect. Cooled air sinks and pushes out at the bottom of the house, drawing hot, humid attic air in through gaps in the upper ceiling plane. Air sealing the attic is the most effective fix.

Q: Is it true that crawl space air makes up 40–50% of what I breathe indoors? Yes, this is a well-established building science principle. Stack effect draws air upward through the home, and in houses with vented or poorly sealed crawl spaces, a large portion of that air originates in the crawl space.

Q: Does adding more attic insulation help if I haven’t air sealed first? Not nearly as much as it should. Insulation works by trapping still air. If air is moving through the insulation due to stack effect or pressure imbalances, the effective R-value drops significantly. Air seal first, then insulate.

Q: What is the difference between a vented and non-vented (conditioned) attic? A vented attic uses soffit and ridge vents to move outside air through the attic space. A conditioned attic is sealed with spray foam on the roof deck, making it part of the home’s interior. Conditioned attics are especially beneficial when HVAC ducts are located in the attic.

Q: When does crawl space encapsulation make sense vs. leaving it vented? Encapsulation is the right choice for most homes. Leaving the crawl space vented with an insulated floor above is generally only recommended in flood-prone areas.Q: Can a bigger HVAC system fix comfort problems caused by a leaky building enclosure? Temporarily, yes but only while it’s running. Heating and cooling equipment can mask building enclosure flaws with sheer horsepower, but comfort problems return quickly once the system cycles off. The enclosure should be fixed first.