Thermal Inertia in Energy Efficient Building Envelopes

Francesca Stazi

• Publisher: Butterworth-Heinemann
• Year: 2017
• ISBN:
  9780128139714
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

The design and construction of the appropriate building envelope is one of the most effective ways for improving a building's thermal performance. Thermal Inertia in Energy Efficient Building Envelopes provides the optimal solutions, tools and methods for designing the energy efficient envelopes that will reduce energy consumption and achieve thermal comfort and low environmental impact.

Thermal Inertia in Energy Efficient Building Envelopes provides experimental data, technical solutions and methods for quantifying energy consumption and comfort levels, also considering dynamic strategies such as thermal inertia and natural ventilation. Several type of envelopes and their optimal solutions are covered, including retrofit of existing envelopes, new solutions, passive systems such as ventilated facades and solar walls. The discussion also considers various climates (mild or extreme) and seasons, building typology, mode of use of the internal environment, heating profiles and cross-ventilation

• Experimental investigations on real case studies, to explore in detail the behaviour of different envelopes
• Laboratory tests on existing insulation to quantify the actual performances
• Analytical simulations in dynamic conditions to extend the boundary conditions to other climates and usage profiles and to consider alternative insulation strategies
• Evaluation of solutions sustainability through the quantification of environmental and economic impacts with LCA analysis; including global cost comparison between the different scenarios
• Integrated evaluations between various aspects such as comfort, energy saving, and sustainability

Chapter One

High Thermal Resistance Versus High Thermal Capacity

The Dilemma

Abstract

The heating and cooling load of a building is mostly due to the heat transfer across its envelope and thus selecting an appropriate envelope is one of the most effective ways to achieve energy saving. For many years, improving the thermal performance of envelopes meant adopting a thick insulation layer, regardless its position or the presence of mass. These new envelopes act as thermal barriers causing summer overheating and attributing the regulation of indoor comfort to conditioning systems. Forty years of researches have demonstrated that thermal inertia is one of the most important parameters to improve thermal comfort as well as to reduce cooling energy demands especially in particularly dynamic external and internal (e.g., used intermittently) environments. On the other hand, the recent literature on sustainability highlights that, considering economic and environmental aspects, lightweight solutions should be preferred.

This chapter explains these new open issues on the recently introduced superinsulated envelopes also reporting items of previous research in the area. It outlines the book purposes and announces the main innovations and utilities.

Keywords

Optimal envelope; thermal comfort; energy saving; environmental impact; global costs; experimental study; dynamic envelope solutions; heat transfer across the envelope; selection of wall layers; EN ISO 13786:2007

1.1 Introduction

The heating and cooling load of a building is mostly due to the heat transfer across its envelope and thus selecting an appropriate envelope is one of the most effective ways to achieve energy saving. For many years, improving the thermal performance of envelopes meant adopting a thick insulation layer, regardless its position or the presence of mass. These new envelopes act as thermal barriers causing summer overheating and attributing the regulation of indoor comfort to conditioning systems. Forty years of researches have demonstrated that thermal inertia is one of the most important parameters to improve thermal comfort as well as to reduce cooling energy demands especially in particularly dynamic external and internal (e.g., used intermittently) environments. On the other hand, the recent literature on sustainability highlights that, considering economic and environmental aspects, lightweight solutions should be preferred.

This chapter explains these new open issues on the recently introduced superinsulated envelopes also reporting items of previous research in the area. It outlines the book purposes and announces the main innovations and utilities.

1.2 Background

1.2.1 The optimal envelope identification is still a challenge

A thermal mass exposed to the external and internal environments responds in a both immediate and time-dependent way, having a certain capacity for heat storage. If the temperature remains constant, the mass does not show its dynamic behavior, while for highly variable temperatures it strongly interacts with the environment. The layers relative position and materials adopted influence the time-dependent transmission. Hence, thick insulation layers placed adjacent to the mass inevitably modify this dynamic interaction.

New buildings are subjected to increasingly stringent standards of insulation, regardless the specific climate. The EU regulations on energy saving have been implemented in all Member States with the adoption of the North-European superinsulated model which, focusing on winter heating consumption, has led even in warm countries to the construction of buildings not much related to their climatic context and often designed with disregard for the occupants’ needs. Even in such climates, lightweight and superinsulated envelopes have been adopted in new constructions, while in existing buildings retrofit, insulation layers with considerable thicknesses were placed either on the external or internal side of the envelope, regardless of the relative position between mass and thermal insulation. This gave rise to problems of environmental control and to the consequent adoption of expensive systems to reach comfortable conditions in summer.

Indeed, in such hyperinsulated envelopes the opaque walls give small contribution on the thermal heat gains/losses, while the glazed surfaces are responsible for the main quote of internal gains for the greenhouse effect (Fig. 1.1). The glazed surfaces allow the incoming of short wave sun radiation that is then absorbed in the internal room components. Consequently, they reirradiate a long-wave thermal radiation, at which the glass is no more transparent so resulting in a rising of indoor temperature. The recent tendency to realize big transparent surfaces also combined with thermal blocking technologies for glazing (e.g., low-e and solar control smart coatings) and the increasing adoption of new airtight frames techniques, have even more increased the summer overheating risk. Then the heat accumulates inside and the new hyperinsulated envelopes, behaving as thermal barrier, obstacle its dissipation toward the outside.

Figure 1.1 Greenhouse effect in superinsulated buildings.

On the other hand, in the last years the achievement of high levels of thermal comfort has become a priority. Recently, the European Directives 2010/31/EU [1], 2012/27/EU [2], and Standard EN 15251 [3] highlighted the increasing proliferation of air conditioning systems in European countries and stressed the importance to return to an envelope design more strictly linked to the specific climate, also considering the indoor environmental conditions in order to enhance the comfort levels especially in summer.

However, the best solution(s) identification is still an open issue. Many authors have already shown that different insulation–mass configurations have unequal and often opposite effects on the various aspects among energy efficiency [4,5], comfort [6–8], environmental impact, or costs [9,10]. So that the best envelope could be: with internal insulation, in studies for cold climates or only focused on winter performance [11,12]; with internal mass and external insulation, in studies focused on summer performance [13,14–18]; with insulation placed on both sides of the wall [11,15,19,20] or a lightweight solution, in studies on the life cycle and economic assessment [21–26]. Very rarely studies addressed the multidisciplinary simultaneous evaluation of the different aspects.

Other factors complicate the debate on the envelope optimal choice. Firstly, the envelope performance varies based on specific building features, the considered operational conditions among intermittent use and continuous use [11,13,27] and the climate, extreme or with high temperature range [28]. Moreover, in the last decades the assembly techniques and the indoor environmental performance requested by the standards and expected by the occupants, the patterns of occupancy and plant operations have underwent to deep changes, making the identification of the best solution more difficult. These changes are still taking place. Finally, regarding the selection of wall layers, the authors until today [11–14] agreed on the choice of walls that strongly decremented the incoming heat wave thanks to the alternation of capacitive and resistive layers. However, the recent adoption of very thick insulation layers combined with new highly performing materials to reach the requested very high thermal resistances has introduced a new kind of building envelope, with too much elevated attenuating attitude, not achievable with the envelopes of the past. These new solutions have a strongly decoupled behavior between the external and the internal side and behave as thermal barriers, thus blocking not only the incoming but also the outgoing heat flux and creating a “thermos effect,” especially during the hot and intermediate seasons.

In all cases, the selection of the optimal strategy is very complex for the strong nonlinearity of the processes involved. This is due to the interaction of dynamic factors, such as the storage effect of massive layers and the strongly variable interior environment strictly linked to the particular (and not easily predictable) behavior of the occupants.

Hence the quantification of consumptions, comfort levels, and environmental impacts of new highly energy efficient envelopes for different ventilation paths, occupants behavior, and timetable for the heating plants also at the varying of the climates is still an open issue.

1.2.2 Comfort issues

The comfort issue in the last years has become a priority. In low-energy buildings, the small range between heating consumptions of the worst and best solution determines that the comfort in unconditioned period and environmental aspects prevail on energy issue [29].

Between these two aspects, the former becomes a priority in temperate climates with hot dry summer for the presence of extensive periods with high temperatures. During such periods, the requirements are different: in the daytime, during the hottest hours, it is necessary to reduce the thermal peaks that are mainly due to the internal loads (for occupants, greenhouse effect from glazed surfaces, etc.), while at night the heat stored during the day should be released toward the outdoor environment.

Therefore it is not advisable to adopt considerable thicknesses of insulating material, which creates a thermos effect impeding an outgoing heat flux. In many cases the use of these envelopes has led to overheating problems not only in temperate climates but also in hottest periods of the cold ones [30–33] and consequently to the need to install expensive cooling and mechanical ventilation systems in order to regulate ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Biography
• Preface
• Acknowledgments
• Symbols, Units, and Conventions
• Chapter One. High Thermal Resistance Versus High Thermal Capacity: The Dilemma
• Chapter Two. The Envelope: A Complex and Dynamic Problem
• Chapter Three. Retrofit of Existing Envelopes
• Chapter Four. New Envelopes
• Chapter Five. Passive Envelopes
• Chapter Six. Experimental Methods, Analytic Explorations, and Model Reliability
• Glossary
• Appendix A. Selected Examples of Buildings
• Appendix B. Details on Numerical Methods
• Index