Retrofitting in the historical centre of Porto vs buildings thermal regulations and energy labelling

Francisco Craveirol, Vi’tor Leal2* and Eduardo Oliveira Fernandes2

1 AdEPorto, Porto Energy Agency, R. Gongalo Cristovao, 347, room 218, 4000-270 Porto, Portugal 2Faculty of Engineering of the University of Porto, Department of Mechanical Engineering, , Rua Dr.

Roberto Frias, 4200-465 Porto, Portugal

* Corresponding Author: Vitor Leal, vleal@.re. up. pt

Abstract

Since 2006 Portugal has a new thermal regulation and since 2008 an energy labelling scheme which applies to significantly retrofitted buildings the same requirements as for the new ones. There are requirements related with the envelope and with the energy supply systems, since the nominal primary energy consumption for heating, cooling and domestic hot water is evaluated. An additional requirement is the installation of solar thermal collectors for domestic hot water heating if the cover of the building has a suitable orientation.

Recognising the specificity of historical areas, the regulation foresees the possibility of exemption of full compliance for buildings located in historical areas if justified incompatibilities are found. This paper explores the issues arising when applying the regulation to six apartments of two buildings in the historical areas of Porto undergoing a retrofitting process. The results show that it is possible to comply with the energy requirements of the regulation without interfering with the building esthetical appearance and without using solar thermal collectors. However these seem to be crucial to achieve class “A”or “A+” labels.

Keywords: Buildings, Retrofitting, Energy Labelling, Solar collectors

1. Introduction

The recently published Porto Energy Matrix [1] revealed that buildings of Porto represent about 60% of the primary energy demand of the city. The urban management and the building design thus play a decisive role in the city energetic-environmental performance. This applies to new buildings, which are a golden opportunity for doing well, but also to existing buildings, which account for most of the building stock and are crucial to achieve large-scale effects.

In the city of Porto about 1/3 of the lodges are located in the historical area of the city [2]. Although most of this area is classified as world heritage by UNESCO, most of these buildings are very old and need a retrofitting process which upgrades their value and performance as buildings on their own but also their contribution to the recuperation of the old town as quality space. In order to foster this effort of retrofitting the old town, the municipality created a special office, the PortoVivo SRU — Sociedade de Reabilitagao Urbana [3]. This work was done in cooperation with the Porto Energy Agency [4] and the SRU with the objective of assessing the compatibility of the thermal regulations with the retrofitting process. It presents a first spot assessment of the initial status of compliance with the RCCTE in the retrofitting of the Porto downtown historical residential buildings, as well as the identification of some corrective measures and the quantification of its impact in terms of the energy labelling.

The thermal regulation in place in Portugal for residential buildings is the RCCTE [5] which was adopted in 2006 as part of the legislative pack that implemented the transposition of the Energy Performance of Building Directive [6]. The regulation itself foresees the possibility of exception for buildings located in historical centres, but only if a clear demonstration of incompatibility with the patrimonial values is made.

In terms of energy requirements, the fulfilment of the regulation implies the simultaneous fulfilment of the following requirements:

i) Minimum requirements for the envelope elements (U-value and solar factor)

ii) Heating needs (useful energy, Nic) inferior to a maximum level allowed (Ni).

iii) Cooling needs (useful energy, Nvc) inferior to a maximum level allowed (Nv).

iv) Hot water needs (final energy, Nac) inferior to a maximum level allowed (Na)

v) Total primary energy (Ntc) inferior to a maximum allowed level (Nt).

(kgoe/m[10].year)

Подпись: Ntc Подпись: 0.1 image418 Подпись: (eq.l)

The calculation of the total primary energy needs Ntc considers that the domestic hot water needs are satisfied at 100%, while the nominal heating and the cooling needs are only satisfied at 10% (due to use patterns). Ntc is computed as (eq.1) :

Where hi and hv represent the conversion factors from final to useful energy, while Fpui, Fpuv and Fpua represent the conversion factors from final to primary energy.

If (and only if) all the previous criteria i) to v) have been met simultaneously, then an energy class can be determined. The energy class is established through the quotient between the estimated primary energy use and its maximum allowed by regulation, with class transitions at each 25% improvement. The minimum allowed class for new or significantly retrofitted buildings is B — (table 1).

Table 1: Energy labelling as function of the relationship between the calculated primary energy use (Ntc) and

the maximum allowed (Nt).

0.75< (Ntc / Nt) < 1

0.50 < (Ntc / Nt) < 0.75

0.25 < (Ntc / Nt) < 0.50

(Ntc / Nt) < 0.25

Building energy class

B-

B

A

A+

Table 2 synthesises the main geometric and thermal properties of the buildings (as foreseen in the retrofitting design, i. e. the thermal characteristics are those previewed for after the retrofitting). Other important features of the buildings are the massive granitic walls and the fact that some of the surrounding streets are very narrow and do not allow significant solar incidence in some facades. Also, even in the windows with good solar exposition, the shadings are usually internal and the retrofitting designs usually try to keep this feature.

Table 3 shows the set of heating and cooling equipments considered for the calculation of final and primary energy.

Подпись: Figure 1: Case-study Buildings and its urban insertion

The first important note that was drawn from the analysis of the retrofitting design processes was that they stated right at the beginning that because the buildings where at an historical area they were exempt from compliance with the RCCTE, and therefore in most cases no effort was made to ensure compliance. The view of the authors of this article is that such an assumption cannot be taken a priory, and calculation will be made in section 3 to analyse whether there are in fact justified incompatibilities that may exempt the buildings from the application of the thermal regulation or not.

Table 2: General characterization of the buildings/apartments after retrofitting.

Building 1

Building 2

Fraction 1

Fraction 2

Fraction 3

Fraction 4

Fraction 5

Fraction 6

Number of sleeping rooms

1

1

1

0

2

0

Net floor area (m2)

54

46

48

49

120

62

External wall area (m2)

43

43

32

32

22

10

Window area (m2)

16

10

18

15

18

10

Roof area (m2)

0

46

0

49

0

62

Main orientation of windows

N

S

N

S

N

S

Envelope U value (W/m2°C)

1.9

1.9

0.41

0.41

2.34

2.34

Windows U value (W/m2°C)

3.0

3.0

3.0

3.0

3.0

3.0

Windows solar factor

0.37

0.37

0.37

0.37

0.37

0.37

Air exchange rate (h-1)

1.1

1.1

1.1

1.1

1.1

1.1

Table 3: Equipments considered in the base-case.

Equipment considered

Heating

Domestic boiler — n = 0.87

Cooling

Air conditioned COP = 3

Domestic Hot Water

Gas boiler, tank poor/ insul/. n = 0.65