Let's talk about HEAT load

For insulation, "heat load" refers to the constant transfer of heat energy into a cooler space, like a house on a hot day. Insulation does not stop this heat from entering; it only slows the process down. Over time, as temperatures remain elevated, the insulation continues to absorb thermal energy, and a significant amount of heat can eventually penetrate the interior.

How heat penetrates insulation over time

Heat is relentless and always moves from a warmer area to a cooler one. The higher the temperature difference, the faster this heat transfer occurs.

Absorption and delay: On a hot day, insulation first absorbs the heat energy from the exterior. A quality insulation will hold this heat and prevent it from immediately passing through. This absorption process is why the indoor space stays comfortable for a while.

Insulation as a capacitor: You can think of insulation as a thermal capacitor, similar to how a capacitor in electronics stores and releases electrical energy. The insulation "stores" the thermal energy, with its capacity limited by its material, thickness, and R-value.

The "slow leak": As the day progresses and outside temperatures remain high, the insulation material gets warmer and eventually becomes saturated with heat. The heat energy slowly but continuously leaks through to the cooler interior, even with high-quality insulation.

Cumulative effect: While insulation significantly slows down heat transfer, it does not stop it. A poorly insulated space will feel the effects of the sun's heat much faster, but even a well-insulated building will gradually warm up inside if the outside temperature remains elevated long enough. This is why a building will often feel hottest in the late afternoon and early evening, after the insulation has been absorbing heat all day.

The role of R-value

Insulation is rated by its thermal resistance, or R-value, which indicates its ability to resist heat flow.

R-value and effectiveness: A higher R-value means greater resistance to heat transfer. To handle heat load effectively, insulation must have a high R-value to slow the heat transfer rate as much as possible.

Factors that lower R-value: Over time, the effectiveness of insulation can decrease due to several factors:

Aging: Some foam insulations use special gases to improve thermal performance. Over time, these gases can escape and be replaced by air, reducing the material's R-value.

Moisture: Water absorbed by insulation dramatically increases its thermal conductivity, allowing heat to pass through more easily.

Compression: Insulating materials like fiberglass rely on trapped air pockets for their effectiveness. If compressed, these air pockets are eliminated, reducing the R-value.

Temperature's effect on R-value: Some studies indicate that the thermal conductivity of materials can increase as the operating temperature rises. This means that on the hottest days, the insulation itself might become slightly less effective at resisting heat than its standard R-value rating would suggest.

Strategies to manage heat load

Because insulation can only slow heat penetration, additional strategies are often used to manage the heat load on a building.

Reflective barriers: Radiant barriers, which are reflective foils, reduce radiant heat gain by reflecting heat away from the building, keeping it from ever reaching the insulation.

Ventilation: Proper attic and roof ventilation can remove the buildup of heat before it has a chance to soak through the insulation and into the living space.

Window and shade management: Shading windows and using curtains can prevent the sun's radiant heat from directly entering a building.