Many people tend to point fingers at their HVAC system when they experience one room too warm while another is freezing. But in reality, it’s seldom the system that’s faulty. It’s the building envelope.
To comprehend why room temperatures differ, one must consider how heat, air, and moisture flow within the building, rather than the direction of the vents.
The physics behind uneven temperatures
Most comfort complaints in homes come down to air infiltration and convection.
Air infiltration is when your nice warm/cool air is slipping in and out of all the cracks in the building. Your HVAC is working hard to condition the space, while outside air is always seeping in and conditioned air is always seeping out.
Convection is all that warm air rises, cool air sinks stuff. It creates loops of natural circulation inside your house. Bad insulation and air sealing in a two-story home will give you a scenario where your upstairs is an oven and your downstairs is a fridge. The heat naturally rises right up to the top floor and then can’t go any further, so the upstairs heats up. This isn’t a thermostat issue – it’s a physics issue. Forcing more warm air into the room won’t help if the warm air is getting trapped in the floor and ceiling.
And then there’s the stack effect – warm indoor air rises up and out of the building in winter. This draws in cold air at the bottom, through leaks in the foundation. Voilà, your cozy home is acting like a chimney.
Why the insulation material matters as much as the R-value
R-value is a measure of how well a material resists conductive heat flow. The higher, the better, but R-value alone doesn’t give us the full picture.
For instance, fiberglass batts can have a respectable R-value on paper and still let air flow right through them. They are aerated. If there are gaps, the material gets compressed, or certain areas are left unprotected (all scenarios that are common in real-world insulation installations in existing homes), they basically don’t provide any air barrier benefit at all.
Closed-cell spray foam doesn’t work that way. It rises to fill a cavity entirely and creates a contiguous air seal at the application point. For folks living in regions with significant thermal variations between seasons, the kind that has your heating or cooling system roaring non-stop to keep up, the air-sealing aspect of the insulation product is critical. Contractors who do spray foam Phoenix installs are familiar with this situation. In their region, the air-tightness of the insulation product is just as important as the R-value, if not more so. This is because you are constantly trying to maintain a comfortable inside environment while fighting off the extremes of the outside one.
Homeowners can save an average of 15% on heating and cooling costs by air sealing and adding insulation in attics and floors over crawlspaces (U.S. Environmental Protection Agency). That’s the combined savings of the insulation (thermal resistance) and the air seal material (infiltration control).
Finding where your envelope is failing
Before you go spending big money on insulation or new equipment, first you need to locate where the losses are coming from.
A simple way to find air leaks is to hold a lit incense stick near potential infiltration sites on a windy day – window and door frames, attic hatches, recessed lighting, and the meeting point between floors and walls. Any movement in the smoke indicates a draft.
A thermal imaging camera will provide a much more detailed picture. Many energy auditors will include this with a pro assessment. Thermal images will show you thermal bridging through wall studs, cold spots from missing insulation, and where exactly conditioned air is leaking out.
Start with the attic. Inadequate insulation in the attic is one of the most common reasons HVAC systems short-cycle, meaning they turn on and off frequently without ever properly cooling or heating your home. In the summer, the roof deck absorbs radiant heat and, without sufficient insulation to stop it, your attic becomes extremely hot. That heat travels through your ceiling into conditioned space. In the winter, the opposite problem occurs.
Pairing insulation upgrades with smart system controls
Better insulation in your house changes the way your heating and cooling system behaves. If your building holds temperature for longer periods of time due to being “tighter” and better insulated, your system will be on for longer cycles that are more efficient than the short repetitive cycles most systems run on warmer days or with poor insulation. This extended cycling means that, overall, your equipment will last longer, as will the parts associated with heating and cooling (including water pumps, compressor motors, etc.), while requiring less energy to stay at your desired temperature in the first place.
Smart thermostats (room thermostats) can take another step out of the process. When placed in the rooms that are actually occupied, reflecting your living pattern rather than in a hallway near the only air handler in the home, they give the system actual temperature data and can reduce the lag between what the whole home is doing and the system itself.
There are maybe three things you want to make sure you prioritize if you are looking at comfort issues first:
Attic floor insulation and air sealing before anything. It’s the highest leverage location in most homes. If the air seal is less than perfect, the height of the insulation should be even higher.
Rim joists and crawlspaces are the easiest to overlook for a huge return on insulation value in terms of reduced forced air cold air infiltration at the foundation level.
Wall insulation has a lesser impact in many climates, but the closing of the thermal bridge through your studs can often be between the rooms that are effectively on the cold outside and the rest of the interior of the building.