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Evgenii Konkin
Evgenii Konkin

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Thermal Mass: Why a Concrete Building Peaks 5 Hours After the Sun Does

#hvac #engineering #energy #building

In a metal building, the cooling load peaks within an hour of peak sun. In a heavy concrete building, the peak shifts to late afternoon. The difference — thermal lag — changes equipment sizing.

Thermal lag by wall type 

Construction                  Time Lag
──────────────────────────────────────────
Metal panel (lightweight)     0-1 hour
Wood frame + insulation       2-3 hours
4" concrete block             3-4 hours
8" concrete block             5-6 hours
12" poured concrete           7-8 hours
Brick veneer over block       6-8 hours
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Heavy materials absorb solar heat, store it for hours, then release it to the interior.

Why the peak shifts 

Lightweight metal building:
  Sun heats wall → heat enters interior in <1 hour
  Peak solar at noon → peak cooling load at 1 PM

Heavy concrete building:
  Sun heats wall → concrete absorbs and stores heat
  Heat enters interior 5-6 hours later
  Peak solar at noon → peak cooling load at 5-6 PM
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Real case: concrete office in Dallas 

Hour    Solar    Wall    Roof    Internal    TOTAL
────────────────────────────────────────────────────
Noon    peak     absorb  enter   80%         38 tons
3 PM    decline  release start   100%        44 tons
5 PM    low      release peak    100%        47 tons ← ACTUAL PEAK
7 PM    zero     still   still   dropping    35 tons
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Engineer sized at noon: 42 tons.
Actual peak at 5 PM: 47 tons.
Result: 11% undersized. West side 3°F above setpoint every afternoon.

Multiple components peak at different hours 

Component               Peak Hour (south/west exposure)
────────────────────────────────────────────────────────
East window solar       8-10 AM
South window solar      11 AM - 1 PM
West window solar       3-5 PM
Roof (metal deck)       12-2 PM
Roof (concrete)         4-6 PM
Wall (metal)            12-2 PM
Wall (concrete)         5-8 PM
People/lights/equip     9 AM - 5 PM
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The total cooling load = sum of ALL components at EACH hour. The hour with the highest sum is the design hour — not the hour with the highest single component.

Lightweight vs heavyweight 

Metal building:
  + Peak occurs near solar peak (predictable)
  + Load drops fast after sunset
  - Load spikes quickly with sun angle changes
  - Morning pre-cool needed less

Concrete building:
  + Morning stays cool longer (stored nighttime cool)
  + Load rises gradually
  - Peak delays to late afternoon
  - System must run later into evening
  - Nighttime "coast-down" still releases stored heat
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The rule

For lightweight construction, the noon load calculation is close to the peak. For heavyweight construction, run the calculation at every hour from noon to 8 PM. The peak is almost always in late afternoon — and it can be 10-20% higher than the noon value.

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