Thermal Mass II: A Comparative Load Analysis of Identical Spaces

Introduction

In the previous post, we discussed what thermal mass is and why modern cooling load methodologies account for it. This follow-up moves from theory into application by examining two modeled spaces that were identical in every respect except for thermal mass—both in the building structure and interior contents.

The goal was not to rediscover first principles, but to quantify how much thermal mass actually alters cooling load behaviour, peak demand, and equipment sizing decisions when using time-dependent calculation methods.

Model Overview

Two enclosed, single-zone spaces were analyzed in HeatWise. The intent was to isolate thermal mass as the sole variable.

Constant Parameters (Both Models)

• Identical floor area, ceiling height, and geometry
• Same orientation, fenestration, glazing properties, and shading
• Same outdoor design conditions and weather data (Montreal, QC, Canada)
• Identical internal gains (occupancy, lighting, equipment)
• Same ventilation and infiltration rates
• Same envelope R-values (Wall: R-15, Roof: R-25, Windows: U-0.30)

Variable Parameters

Space	Construction Weight	Carpet	Wall Weight [lb/ft2]	Roof Weight [lb/ft2]
Light space	Light	No	10	7
Heavy space	Heavy	Yes	83	63

No changes were made to insulation R-values or U-factors—only material density, heat capacity, and exposed mass were altered.

Description of the Two Spaces

Low-Mass Space

• Lightweight steel stud walls with gypsum board
• Suspended acoustic ceiling
• Hard floor
• Light furnishings (workstations, chairs)

This configuration is representative of many modern commercial interiors where structural mass is isolated from the conditioned space.

High-Mass Space

• Concrete or masonry structural walls exposed to the zone
• Concrete floor slab with carpet
• No suspended ceiling (exposed slab above)
• Heavier wood and upholstery on furnishings
• Heavier interior partitions

The envelope thermal resistance was unchanged, but significantly more mass was directly coupled to the zone air and radiant field.

Both spaces had 10 occupants, 0.9 W/ft² of lighting, and typical light density office equipment. Equipment, lights, and people were assumed to be in the space from 9-5, but not during the evening or over night.

Results: Sensible Load Behaviour Over Time

Peak Cooling Load

Despite identical instantaneous heat gains, the low-mass space exhibited a higher peak sensible cooling load, occurring shortly after peak solar and internal gains.

The high-mass space demonstrated:

• A lower peak load
• A delayed peak, occurring several hours later
• A noticeably flatter load profile

This aligns with the fundamental behavior of thermal mass: radiant and convective gains are absorbed by the structure and furnishings, delaying their conversion to air load.

The table below is a summary of peak cooling loads for each space:

Space	Peak sensible load [Btu/h]	Peak latent load [Btu/h]	Time of peak load
Light Space	19,884	2,000	September, 3pm
Heavy Space	15,324	2,000	July, 4pm

The graphs below show the difference in overall profile in the loads experienced between each space:



Key Takeaways

This example illustrates two important points for equipment sizing and energy analysis:

1. Buildings with more thermal mass will typically have lower peak cooling loads, resulting in smaller equipment sizing.
2. Buildings with more thermal mass may require more total cooling, because cooling loads remain elevated longer into the night, requiring continuous cooling during hot days.

To properly understand total energy spent on cooling, you would need to add up the bar sizes under each hour in these plots. Doing this would yield the following results:

• Heavy Space: 226,788 Btu
• Light Space: 207,569 Btu

While the Light Space has a higher peak load, requiring larger equipment - and therefore higher CapEx - the Heavy Space requires more total cooling energy - higher OpEx.