This has been bugging me for a while so I decided to undertake a bit of an academic study of thermodynamics, and now I feel like I may have a better grasp of what’s going on vis-a-vis the disappointing cooling performance of superinsulated attics.
The two measurements we need here are volumetric heat capacity: how much heat a material is capable of storing in itself; and thermal diffusivity: how fast heat is able to transfer through a material.
When we look at insulation, we typically look at its R-value; its total ability to slow down heat transfer. This is important, but it doesn’t give us anywhere near the whole picture. Insulation does indeed slow down heat transfer, but because insulation has mass, as heat moves through the insulation (slowly), it is stored in the material itself. This is a critical point that I have almost never seen discussed. As we’ll see, for heating performance, this is desirable, but it is bad for cooling performance, especially in an exposed application like an attic.
Consider the case of an uninsulated 1,200 square foot attic during the summer or in a hot climate. Heat transfers very quickly through the air to the ceiling drywall; air has a high thermal diffusivity value of 21.33 mm2/s. But air also has virtually no heat storage capacity; its volumetric heat capacity is 1.205 kJ/m³K. As a result, the summer performance is what we would expect: the ceiling gets very hot during the day, but it also cools down at night.
Now let’s superinsulate that attic and dump 24 inches of cellulose on top of the ceiling. Cellulose transfers heat very slowly (that’s the point); its thermal diffusivity is 0.811 mm2/s, or 26 times slower than air. But the cellulose is more massive too, with a higher heat storage capacity. This 1,200 square foot attic now has 5,520 pounds of material in it (2.3 lb/ft³ 2 feet deep * 1,200 square feet). And the volumetric heat capacity of cellulose is 53 kJ/m³K -- 44 times as much heat storage capacity as air!
This thick insulation will certainly slow the flow of heat radiating down from the roof sheathing (that’s the point of having it), but in slowing the heat down, it is also storing the heat in its own mass, and the bigger the mass, the more stored heat! As a result, the peak cooling load during the day is indeed reduced because the time that the heat takes to penetrate the insulation is increased, but the heat flow is also time-shifted; the stored heat in the insulation continues trying to enter the house even after the sun goes down. Why? Because a typical under-ventilated and shingled attic remains hot for several hours after the sun has gone down, so there is still a delta-T through the insulation, and all this heat continues to transfer through the insulation and charge it with even more heat. Because of the temperature gradient through the insulation, heat stored in the top and middle parts of the cellulose will continue to transfer down towards the ceiling drywall.
Even once the air in the attic itself has cooled off, there’s still stored heat in the insulation. The heat in the top layer of the insulation will soon get into thermal equilibrium with the attic air, but the temperature of the middle layer of insulation will still be much higher (remember there’s 24” of cellulose in this example!). And some of that heat in the middle is going to continue transferring down towards the ceiling. The ceiling drywall will not cool off because it will still be experiencing a delta-T; not with the attic, but with the hot insulation itself! And soon the sun will be out, heating the roof and the attic again, causing the same process to repeat. Increasing the insulation thickness will not address this problem as it simply provides more mass in which to store heat even as it further increases the time that the heat takes to flow through it.
So what is the net result of all of this insulation? The ceiling stays cooler during the day than if it was uninsulated, but it never cools off at night. It experiences small fluctuations in temperature around a point that is higher than the indoor temperature; it is constantly adding small amounts of heat into the house, never pulling any of it out. The house stays cooler during the day, but it’s warmer at night. Air conditioning use is time-shifted to the night, but total runtime may not be reduced by as much as expected. And if it is a very large air conditioner, it will be ill-suited to the new pattern of a small-to-medium constant heat influx rather than the old pattern of a large multi-hour heat influx, followed by a heat outflow at night.
Critically, the more insulation you have, the more it is going to take to pull heat OUT of it in the form of lower attic temperatures and longer periods of time where this is the case. This is why adding more insulation to an attic that gets hot doesn’t seem to help very much; this extra mass requires longer periods of lower attic temperatures to cool off the insulation than before. Unless you provide that, you're allowing the insulation to near-continuously absorb heat during the cooling season.
The solution to this problem, contrary to what I have frequently seen written on the subject, is not to futilely continue piling on more insulation, but to reduce the actual temperature of the attic itself, both during the day and during the night. That way, the heat flow will not only be lower, but there will be less heat stored in the insulation, and finally--critically--the direction of heat transfer can reverse during the night, which will actually pull heat OUT of the insulation, “recharging” it for the coming day, and potentially even pulling some heat out of the ceiling drywall too.
There are a couple of complementary ways that it seems feasible to do this:
1. Increase ventilation to cool down the attic faster. The outside air temperature is almost certainly bound to be lower than the attic temperature, both during the day and at night. Replacing hot attic air with cooler outside air will reduce the heat transferred to the insulation, lowering the heat flow. And in climates with high diurnal temperature swings, as soon as the ambient attic temperature falls below the interior temperature (quite possible during the shoulder seasons and even during the summer where I live), you get the awesome effect of the heat flow reversing direction, and the stored heat starts being pulled upwards out of the insulation and into the escaping air of the attic rather than down through the ceiling drywall.
2. Reduce the heat stored in the roofing material to reduce heat flow into the attic faster. With dark heavy shingles on the roof, they’re going to soak up a ton of heat during the day, and this mass will take many hours to cool off once the sun goes down. Even at 1 AM, the roof sheathing is still going to be hot and radiating heat down onto the attic floor insulation. With a lighter weight roof that has less heat storage capacity, there will be less mass to store heat, and the roofing and sheathing will cool off faster, reducing the heat flow into the attic more quickly. There is also a case here for eliminating the roof sheathing entirely to reduce the weight and turn the underside of a bare metal roof into an automatic radiant barrier, as long as there’s enough racking resistance added instead.
3. Use a radiant barrier and/or reflective roofing to actually reduce the amount of heat that enters the attic in the first place; lower attic temperature means a lower delta-T through the insulation, which means slower heat transfer, less stored heat, and faster cooling-off.
4. In a climate with no heating requirements, don’t use a large amount of insulation; Past the first couple of inches, it doesn’t really help you very much because it stores more heat. Instead focus on keeping the heat out of the attic in the first place. If you live in southern Florida, you want a ventilated metal roof, radiant barrier sheathing (if any sheathing at all), and strong attic ventilation. Coincidentally this is how they build roofs in hot climates anyway...
Finally, an interesting point is that in the winter, all of this heat storage in insulation is highly desirable, as it helps your house get through the night without cooling off too much. For example, after the sun goes down, temperatures drop and the attic air gets even colder, and it pulls heat out of the insulation on the floor faster than it was being pulled during the day. The more insulation you have there, the more heat that will be stored in the insulation and the slower this will happen. So the insulation right above the drywall will be at maybe 75 degrees, and it will take a very long time for the heat in that bottom layer to get sucked out by the cooling-off of the layers above it. As a result, your ceiling drywall will not really cool off during the night, which is exactly what you want in winter. But it’s the opposite of what you want in summer!
All of this makes sense to me but of course I could be completely wrong; I’m only a layman who has studied building science and thermodynamics in an amateur manner.
