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I live in a 1950's single-story ranch-style with a vented attic space (with a roof ridge vent). I haven't turned on my central air yet for the summer, and I've noticed that the air temperature inside the house tends to increase between the time I get home and the time I go to bed, even though the outside air temperature is dropping during the same time.

I even jotted down some observations yesterday. High temperature outside was 85F at ~4:00pm (per weather app). At 6pm when I got home, it was 80 outside and 73 inside. At 11pm it was 70 outside and 77 inside. Had been keeping all doors and windows closed (until 11 when it was too hot to sleep comfortably), didn't use the range or oven for cooking, and only indirect sunlight coming in the windows during that time frame.

Might be more of a building science question than home improvement. I suspect it has to do with how air flows through the attic before and after the sun goes down, but don't really know why that would be the case. Mostly asking out of curiosity, but if there's something to help with the heat, I'm down to hear it.

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    Brick/stone/cement walls act like heat sinks/traps. They collect/hold on to heat in the day time and slowly release the heat when the temperature cools.
    – crip659
    Jun 2, 2023 at 20:29
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    Look up "Trombe Walls" (which work in areas of consistent sunshine - not so well where the sun is unreliable in heating season)
    – Ecnerwal
    Jun 3, 2023 at 2:43

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People think the need for A/C comes from the air outside being hotter than the air inside. No, it comes from direct sunlight. Sunlight is 300 BTU per square foot. And most roofs have an albedo in the single digits (reflect less than 10% of the energy). The moon is pretty light colored, but it's only 12% albedo (absorbing 88%).

But why evenings? For the same reason you don't need A/C prior to noon, even though 50% of the day's solarization has already happened.

Solar Gain meets building mass meets insulation

As mentioned just above, the lion's share of heat energy added to a building isn't transferred from the air. It's blasted directly by the sun, because the roof is non-white, so it absorbs most solar energy rather than reflects it). Albedo (the ability to reflect solar energy in any significant way) requires a near-white surface. You have to allow for the angle to the sun, but that means 300 BTU per square foot of roof or wall that is facing the sun.

This heat acts on the physical structure of the building. The roof materials, rafters, joists, exterior walls, studs. Materials store heat. It takes awhile for cold things to warm up and vice versa. The structure of the building is massive, and it is soaking up this heat. So this creates a considerable time lag between when the solar heat hits the building, and when it is felt.

4 hours before noon and 4 hours after noon, the sun is the exact same number of BTUs. But the difference is obvious: 4 hours before noon, the building has had all night to cool off. 4 hours after noon, it's been absorbing these BTUs all day and is quite hot.

I wish it was easy to stick recording thermometers in the roof's surface, inside rafter wood, inside attics, inside walls, etc. Because then it would be obvious.

After 4 PM or so, the sun's angle is more and more oblique, so less and less practical heat is being absorbed by the buildings. So why do we need the A/C only then? Because this accumulated heat is finally soaking through the insulation.

The insulation slows down heat transfer; it doesn't stop it. A freezer is very well insulated, but if you unplug it for a month in a 70F room, the freezer innards will be at 70F.

So at 4 PM, solarization is fading, but the temperature of the building's structure is probably at peak. And it's had time to soak through the insulation and start warming the parts of the building that are inside the insulation envelope.

When the sun actually sets, the "outside of the insulation envelope" parts of the building are still quite hot. And that heat is radiating out into the world, but it's also still radiating into the insulation. Where it continues to heat the parts of the building inside the insulation.

The thermal mass of all these components is why we need to run A/C til midnight :)

Functionally, the building mass is our enemy.

Nowhere is this more obvious than the dumb design of insulating attic floors. Now the entire attic, structure, and all things stored in the attic are added to that thermal mass which bedevils us and makes us run A/C later and later.

Change the insulation, change the game

So one strategy in Passivhaus and efficient design is to put as much building mass as possible inside the insulation envelope. Think about simple corrugated metal roof/walls followed by 8" of structural foam followed by 10" of reinforced concrete followed by studs and drywall or whatever. The metal skin has negligible thermal mass, and will closely match solarization - it will be about as hot 4 hours before noon as 4 hours after noon. No storage there. The insulation will greatly slow the skin's heat affecting the concrete. The BTUs which get through the insulation will find the huge mass of the concrete, and will have trouble warming it appreciably. So the concrete will act as a huge buffer, tending to hold the interior at a stable temperature.

This type of building turns the building's mass into our friend. It functionally provides thermal energy storage, which moderates temperature changes in the building - but also delays them tremendously.

But you don't need to go to such an extreme - many buildings without PassivHaus design can still benefit by using the bones of the building as energy storage. In fact, Technology Connections did a whole video on this, exploiting inside-the-insulation thermal mass to choose the time of day that A/C can run. The A/C starts almost exactly when your A/C shuts down, that just goes to show the extremes to which time-shifting is being done here. But it works because the insulation to interior thermal mass ratio is good enough. And this is just an average (though relatively recent build) home with some reasonable shading to escape some solar gain. (any house could do that with some white paint.)

But here's a more sensible plan: Solar panels respond to actual solarization. They output the same 4 hours before noon as 4 hours after noon. So, what if we just ran the A/C while the solar power is available? We'd have a bit of deviation - cooler than preferred in the mornings and warmer than preferred in the evenings, but not a lot of deviation depending on insulation. That combined with smarter insulation could get-r-dun for a lot of people.

We could also increase thermal mass of buildings by moving insulation layers outward - insulate the attic rafters instead of floor, or there are services which will add 2-6 inches of spray foam onto any roof, creating a new foam roof on top. Or skin the building with panel insulation.

There's a thing called the "Duck Curve" which reflects the difference between when solar panels generate power and when people want to run their A/C. There's such a glut of morning solar in California they had to pay Arizona to take it (at the time, their reservoirs were too low to use pumped storage like they usually do). Meanwhile they're spinning up peaking plants in afternoons to cover supplemental A/C load. I think Technology Connections is onto something.

For new construction, see also use ICF's or insulated concrete forms. They are single-use concrete forms, made out of foam, which stack up like LEGO blocks. Normally when the concrete cures, you demolish the forms, but in this case, you leave the foam there forever and it becomes part of the insulation.

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  • ICFs are an annoying design failure. They should have foam on the outside and not-foam on the inside. Instead, they isolate the thermal mass from both sides with foam.
    – Ecnerwal
    Jun 2, 2023 at 23:55
  • @Ecnerwal I wrote that, then deleted it for reasons I forget. But just demolish the inner ICF. Battleship Texas has a bunch of void spaces outside their armor, with thin steel prone to leaks; they filled them all with structural foam. Seems like a terrible idea, but it was so the ship doesn't sink and block the Galveston Ship Canal. At the dry dock they demolished the foam some astoundingly easy way - water jets, I think. It's going to be in that dry dock most of a year. Nick of time too, the Spanish are getting uppity :) Jun 3, 2023 at 18:56
  • Makes a lot of sense that even turning on my attic fans doesn't seems to help. The house has a brick façade, which I'm guessing holds a ton of heat. And it's an uneven grade, with the largest exterior wall facing south and getting baked the longest every day.
    – MikeyC
    Jun 5, 2023 at 15:28
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My hunch is the attic.

The attic is probably getting a lot hotter than the ambient outside temperature. During the evening hours some of that heat goes up and out, some of it goes down and out - i.e., into your living space.

The way to confirm this is to put a wireless remote thermometer in your attic and track temperature through the day and night. The ones I have are for refrigerator/freezer use so the temperature range is not right or I would recommend them. (At least, I don't think they give accurate readings at temperatures over 80 F.) But something similar, which I have selected for you at random on Amazon, is something like this indoor/outdoor thermometer:

Indoor/Outdoor Thermometer

Put the outdoor sensor in the attic. If you really want to get fancy, get an extra outdoor unit so you can track all three relevant temperatures:

  • Indoor
  • Outdoor
  • Attic

If the problem is the attic, there might be some adjustment you can do with passive cooling, but the usual solution is an attic fan.

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    In many cases an even better solution is extra attic insulation, because most houses this old have very little of it.
    – KMJ
    Jun 3, 2023 at 5:38

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