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.
To do that first we need to understand the forces behind infiltration; 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. In the next few paragraphs I’ll do my best to explain it.
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. The heavy cool air pushes itself out of the house through the crawl space, doors, windows, and gaps/cracks in the 1st floor’s building enclosure. Since the house can not remain in a vacuum, and pressures seek equilibrium, every bit of air that exits the house (exfiltration) must be replaced with air from outside the house (infiltration). The infiltrating air will enter the house where the pressure is lower, which in this scenario is high in the house. Since the attic has the highest temperature differential (a natural driving force), and the ceiling plane between the attic and living space is typically very leaky, this is where the majority of the infiltrating air comes from. This is a big concern because attic air is hot, humid, and carries pollutants like pollen, fiberglass, pest feces, etc.. Often reverse stack effect is the reason the 2nd floor is warmer than the first floor. Sometimes you can even smell the attic from the 2nd floor.
In the heating season the term used is stack effect, it’s literally the opposite of reverse stack effect, and because the temperature differential between inside and outside is greater, infiltration is greater.
Let’s go over the principles again in reverse; air is heated and rises because it’s less dense, it creates a high pressure zone high in the house while simultaneously creating a low pressure zone low in the house, air exits high in the house, and is replaced with air from outside low in the house.
You may have heard before 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 the air. (I recently installed a ventilation system in my non-vented crawl space with some interesting results. I’ll be sure to share the video with this group.)
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.
Attic infiltration is typically addressed in one of two ways; by keeping it vented, removing or moving the existing insulation, air seal the top plates, can lights, and plumbing/electrical penetrations, then put the insulation back or replace with new. Another method is converting to a non-vented, conditioned attic. This is usually done by applying spay foam to the roof deck and gable ends. This method makes 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.
The crawl space has two approaches as well, the most common being encapsulation (non-vented). In best practice encapsulations the existing insulation is removed, biological growth is cleaned, foundation vents are closed and sealed, a vapor barrier is installed on the ground and run up the walls and sealed, exterior walls are insulated, and a dehumidifier is installed. This makes the sense for most situations. There are times when it’s best to leave the crawl space vented and air seal and insulate the floor with closed cell spray foam. Typically that’s only done in areas that are at a high risk of flooding.
Back to my original statement. New windows and doors are great, especially if the old ones do not open properly. In most situations, even if you can feel a draft, they 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.
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 it’s R-value (resistance to heat transfer) is reduced significantly. Insulation will not stop hot humid, or cold dry air from entering your house.
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.
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.