Energy Efficiency Evaluation of Green Roofs as an Intervention Strategy in Residential Buildings in the Field of Spanish Climate

Round 1

Reviewer 1 Report

In this the annual energy behavior of a single family attached house type in 8 Spanish cities has been analyzed. Three hypothetical situations of complete conversion of the thermic envelope of a building without prior insulation, one of them with a traditional roof and two with green roof. English editing is well addressed. The topic of green roof is worth-of-interest. However, I require some comments to be addressed before considering again this paper.

My comments are:

1.      This paper carried out several simulations on the energy behaviour of green roofs. A paragraph underlying the limitations of the study is necessary.

2.      This study talks about evapotranspiration and energy savings of green roof while in the title the authors mention the energy efficiency. How are they correlated with?

3.      I suggest reducing the references without an analysis of their results is provided (see lines 58-66) more than 20 papers are cited without any description.

4.      Previous research on energy performance of green roofs by using simulation software, such as EnergyPlus, were carried out. The authors should describe this research and define the novelty of this paper compared to them.

5.      The authors are simulating the performance of intensive green roof on existing buildings. How this type of green roof is not suitable to be installed on existing buildings due to its high weight. The authors should analyse this aspect more in depth.

6.      Many mathematical models are more accurate in evaluating green roof evapotranspiration compared to Sailor’s model. Why did the authors choose this model to determine evapotranspiration? For example, the drainage layer contribution is neglected in this model.

7.      Differently from this paper, many previous authors showed that intensive green roof are more energy efficient than extensive green roof due to higher evapotranspiration and higher insulation properties due to the substrate depth. The authors should address this aspect.

Author Response

In this the annual energy behavior of a single family attached house type in 8 Spanish cities has been analyzed. Three hypothetical situations of complete conversion of the thermic envelope of a building without prior insulation, one of them with a traditional roof and two with green roof. English editing is well addressed. The topic of green roof is worth-of-interest. However, I require some comments to be addressed before considering again this paper.

My comments are:

1. This paper carried out several simulations on the energy behaviour of green roofs. A paragraph underlying the limitations of the study is necessary.

Heat transfer mechanisms in green roofs are complex. Variables such as vegetation, substrate, moisture content, etc. make it difficult to carry out a detailed study of the thermal behaviour of green roofs.

EnergyPlus uses a simplified model that only takes into account heat transfer in the main layers (vegetation and substrate). Despite being quite accurate, this implies ignoring certain more complex aspects. Lines 218-227 mention the limitations of the program.

Green roofs have been studied only from a thermal and energy point of view, but none of the environmental benefits they present have been assessed. In addition, it has been decided to study a specific green roof model (self-sustaining), whose evapotranspiration mechanism will be very limited due to water scarcity, as the survival of vegetation in cases of water stress.

At the end of the Introduction section we have added a new explanatory paragraph: lines 112-117.

 

 

2. This study talks about evapotranspiration and energy savings of green roof while in the title the authors mention the energy efficiency. How are they correlated with?

We understand the reviewer’s point of view. It could be considered to change the title to “Thermal evaluation…” instead of “Energy efficiency evaluation…”. However, as the research has focused on the renovation of existing buildings, we have considered that, in the title, the term “Energy efficiency evaluation” summarized the most important points. Energy efficiency is a more global term that can include different aspects such as improving the efficiency of air conditioning systems, design of solar control elements, installation of green systems (shade, evapotranspiration), etc.

We have focused on achieving this energy efficiency through the thermal improvement of the envelope and the installation of a green roof systems. Evapotranspiration is one of the main heat dissipation mechanisms of the green roof, and thanks to that heat transfer to the interior of the building are limited. Therefore, to achieve indoor comfort conditions, lower heating and cooling energy consumption is required. This energy saving implies that the existing building is more efficient in the consumption of resources and energy.

Added a paragraph at the end of the Materials and Methods section (lines 277-282).

 

 

3. I suggest reducing the references without an analysis of their results is provided (see lines 58-66) more than 20 papers are cited without any description.

References have been checked. In accordance with this suggestion, the references that did not include a clear description, such as lines 60-69, 103-104 and 226-227, have been reduced. In this way, the total number of references has been reduced from 70 to 58, being more focused about study.

 

 

4. Previous research on energy performance of green roofs by using simulation software, such as EnergyPlus, were carried out. The authors should describe this research and define the novelty of this paper compared to them.

Indeed, there are numerous investigations carried out with EnergyPlus. Our work, however, has focused on buildings object of renovation without thermal insulation in the envelope. 3 roof construction systems have been evaluated comparing traditional models and self-sustaining vegetated models, not only with the intention of knowing which one achieve better energy savings but also to see which climatic factors are more determinant in there energic behaviour. In the Introduction, lines 118-124 have been added to clarify the objectives, focused on the influence of climatic variables.

 

 

5. The authors are simulating the performance of intensive green roof on existing buildings. How this type of green roof is not suitable to be installed on existing buildings due to its high weight. The authors should analyse this aspect more in depth.

The limitations of the application of intensive green roofs in the field of building renovation, from the point of view of the added load to the structure, are briefly mentioned in lines 84-94. However, the explanation on this aspect has been improved. The load provided by each of the models has been calculated in the most unfavourable case (with greater insulation thickness), which corresponds to the city of Burgos (climatic zone E). It is also assessed whether the existing structure could support these loads, although it is always necessary to carry out a detailed structural study for each case (lines 179-198).

 

 

6. Many mathematical models are more accurate in evaluating green roof evapotranspiration compared to Sailor’s model. Why did the authors choose this model to determine evapotranspiration? For example, the drainage layer contribution is neglected in this model.

We understand the reviewer’s point of view. To carry out this research, EnergyPlus has been used. This calculation engine is open source, it is constantly being improved and, as it can be seen from the references, it is a widely used program to evaluate energy behaviour of buildings with green roofs.

Although other mathematical models may be more precise, in our research it has also been assessed that the calculation is simple, affordable and included in a program that allows the energy certification of the entire building to be obtained. In Spain, where any alteration to a building requires an energy certificate, EnergyPlus is considered as a calculation engine reference with proven accuracy. A brief explanation has been added in this regard in the Materials and Methods section (lines 209-213).

 

 

7. Differently from this paper, many previous authors showed that intensive green roof are more energy efficient than extensive green roof due to higher evapotranspiration and higher insulation properties due to the substrate depth. The authors should address this aspect.

We understand the reviewer’s point of view. Indeed, intensive green roofs are more energy efficient. The choice of model B (extensive green roof) to carry out a more detailed study of the climatic variables has caused confusion. This model is not the one with the lowest energy consumption, but rather the one with the greatest variability of behaviour among the 8 Spanish cities analysed. To clarify this aspect, lines 350-358 have been added where it is indicated that model C (intensive green roof) is the one with the lowest energy consumption. This is due, as the reviewer suggests, to the thickness of the substrate, which provides greater inertia to the construction system, and to the type of vegetation. This vegetation, with a LAI=5, generates a dense shade, less incidence of direct solar radiation on the substrate and is related to greater losses due to evapotranspiration in the vegetation layer. 2 new references related to this topic have been added.

Reviewer 2 Report

The topic is very interesting and the paper is well structured. Nevertheless, not having critical grammar errors, the English language will benefit from a proofreading in order to use more appropriate terms. 

In what regards to introduction, the objectives of the study should be clarified to improve the readability. Moreover, I suggest that Introduction ends with a summary of each section of the paper to give the reader an overview of the different sections of the paper.

It should be clearly stated how was the EnergyPlus reference model validated.

It should also be stated in methodology the main aspects of the model such as building envelope and glazing characteristics, occupancy profiles, mechanical/natural ventilation set-points, heating and cooling system characteristics and set-points, and a summary of the internal loads considered.

 

 

 

 

 

 

 

 

 

Author Response

The topic is very interesting and the paper is well structured. Nevertheless, not having critical grammar errors, the English language will benefit from a proofreading in order to use more appropriate terms.

The authors welcome comments about the terms in English. We have reviewed other published research and the following terms have been modified, better adapting to the context and clarifying the message:

Enclosure → Facade

Ground floor slab → Slab on grade

Common wall → Party wall

Rehabilitation → Renovation/Refurbishment

Complete conversion → Integrated renovation

Inherent weight → Self-weight

In addition, the entire document has been revised to correct errors.

 

 

In what regards to introduction, the objectives of the study should be clarified to improve the readability. Moreover, I suggest that Introduction ends with a summary of each section of the paper to give the reader an overview of the different sections of the paper.

The exposition of the objectives in the Introduction section has been improved (lines 118-124). Next, two paragraphs have been introduced that summarize the Materials and Methods, Results and Discussion and Conclusions sections, structuring and providing an overview of the content of the article (lines 125-135).

 

 

It should be clearly stated how was the EnergyPlus reference model validated.

In Spain, where any alteration to a building requires an energy certificate, EnergyPlus is considered as a calculation engine reference with proven accuracy. A brief explanation has been added in this regard in the Materials and Methods section (lines 209-213). This research has tried to solve a common problem in the case of the refurbishment of buildings. The reference model does not exist in reality, but the energy consumption of this reference model has been compared with the average energy consumption data of a single family house in Spain. Lines 299-305 have been added to clarify this aspect.

 

 

It should also be stated in methodology the main aspects of the model such as building envelope and glazing characteristics, occupancy profiles, mechanical/natural ventilation set-points, heating and cooling system characteristics and set-points, and a summary of the internal loads considered.

In the Materials and Methods section, two paragraphs and two tables have been added (Table 2 and Table 4) describing the main aspects to consider for modelling the reference model and subsequent renovations with EnergyPlus (lines 228-236 and 244-258) . The information that was already provided in lines 236-241 and table 3 is complemented and expanded.

 

 

We consider that all the suggestions have been taken into account.

We trust that the manuscript will meet with your approval, but should any doubt remain, please let us know.

 

Thank you for your attention.

Kind regards.

 

Júlia G. Borràs, Carlos Lerma, Ángeles Mas, Jose Vercher, Enrique Gil.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors addressed all my comments