Modelling Perimeter Heating Demand

A Function of Occupant Thermal Comfort



Thermal discomfort can occur in perimeter zones due to radiant heat loss from the occupant to a poorly insulated exterior wall. It is valuable for architects to be able to quantify this discomfort during early design stages when considering facade options. Current tools which facilitate this analysis ignore the perimeter heating systems which would normally compensate for radiant heat loss. Therefore, it is difficult to argue that one facade creates more comfortable conditions than another. This paper proposes an alternative approach for bringing thermal comfort and energy quantities into early stage design discussions. As part of the approach, the exact perimeter heating energy demand required to deliver thermally comfortable conditions is quantified. A Python model for producing this quantity is demonstrated using the example of a typical Toronto multi-unit residential building. The results showed that a unit with a High Performance facade would require 60% less perimeter heating energy than a unit with a Low Performance facade. Peak heating demand from a radiant ceiling panel ranged from 350 W to 1460 W for the highest and lowest performing facades respectively. Overall, the model is shown to be effective at assessing the performance of facades, and capable of building an argument for higher performance facades. However, future improvements should address radiant asymmetry and transmitted solar radiation.


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Jonathan Graham

Sustainability Analyst

KPMB Architects

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Geoffrey Turnbull, OAA, MRAIC, LEED AP, CPHD

Director of Innovation

KPMB Architects

Photo of David Constable OAA, LEED AP, CPHD

David Constable OAA, LEED AP, CPHD


KPMB Architects

Photo of Mike Reimer P.Eng, LEED AP

Mike Reimer P.Eng, LEED AP


First Light Energy Solutions

Photo of Joshua Vanwyck, CPHD

Joshua Vanwyck, CPHD


JMV Consulting


Introduction and Background

As the focus of high performance building design expands from a focus on mechanical systems to include a focus on envelope design, it is useful for architectural designers to be able to understand the performance characteristics of various facade configurations at an early stage of design. Indeed, emerging regulatory requirements for high performance buildings often require guarantees of overall building performance, thermal energy demands, etc. at early stages of design (e.g. conclusion of schematic design phase.) Moreover, it is increasingly important for architects to be able to understand the impact of facade design decisions on occupant thermal comfort, quality of daylighting, etc. at an early stage, when it is easiest to align the building massing, plan, section, envelope strategies, etc. to produce optimal outcomes.

Occupant thermal comfort is influenced by the indoor air temperature, mean radiant temperature, solar radiation, air velocity and humidity. Accurate modelling of these parameters requires knowledge of the mechanical systems used, internal heat gains, and the thermal conductance and capacitance of surrounding materials (Berardi and Soudian 2019). This information is typically unavailable during schematic design phase, which signals the need for simplified thermal comfort models.

A handful of tools have the ability to evaluate indoor thermal comfort using the limited information available during early stages of design (Hoyt 2016, Menchaca-Brandan, et al. 2017). These tools require basic information about the indoor and outdoor environmental conditions, the glazing geometry, and the target window U-value. These inputs are easily obtainable during schematic design. However, most of these tools assume the absence of perimeter heating. This muddles potential arguments that higher performance facades result in better thermal comfort, because a properly designed perimeter heating system should inevitably compensate for facade-induced discomforts (Berglund, Yoshino and Kuno 1997).

Despite this logical gap in efforts to link facade performance to indoor thermal comfort during early stages of design, it is clear that a lower performance facade should require more heating energy to maintain an acceptable level of comfort. The exact quantity of this energy, if it could be isolated, would be descriptive of both facade performance and indoor thermal comfort.

This paper aims to demonstrate how thermal comfort and energy quantities can be brought into discussions about facade performance during early stages of design. This is achieved through a novel approach wherein the perimeter heating energy demand is treated as the energy required to deliver thermally comfortable conditions. A purpose-built Python model is introduced and tested using an archetypal Toronto residential suite as an example. The results of heating energy demand, peak heating demand and peak radiant panel temperature are reported. Lastly, intricacies and limitations of the modelling approach are discussed.


Experiment Overview

A model was developed to quantify the heating energy required to maintain thermal comfort in the perimeter zone of a residential suite in Toronto, Ontario. The modelled space

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The view factors between the occupant and each of the surfaces is shown in Table 2. The largest view factor was to the ceiling, floor and interior walls (“other interior

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The experiment demonstrated the expected relationships between facade thermal performance and the heating energy delivered to the suite. Facades with low overall U-values and low WWRs required significantly less energy

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As high performance building design increasingly places value on the role of envelopes, it is useful for architects to be able to understand the performance implications of facade configurations at

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Rights and Permissions

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