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Calculation Example BBSR-Tool: Windows

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The following 2 examples are about the replacement of windows with uncoated double-pane windows (typical in Germany in the timeframe 1979 – 1995) against windows with low-E triple-pane glazing in course of a modernisation that is due anyway. For single- and two-family homes the equivalent energy price, the annuity and the amortisation period are determined using the BBSR calculation tool. as well as the costs and prospective energy savings per square meter replaced window surface. In the case of multi-family homes the typical costs from empiric studies are lower, but the effects shown in the following examples are similar.

Preliminary remarks

If older windows are due for replacement as a whole in course of a planned refurbishment, as minimum energetic quality of the new windows a double-pane low-E-glazing and a maximum UW-value of the window are demanded by the conditional requirements of the current EnEV.
This energy-performance level can be achieved without any additional costs since windows of lower performance are no longer offered on the German market (“state of the art”). Energy-related additional costs occur when the owner moves over to higher performing glazing (triple-pane low-E) or even to windows suitable for a “passive house”. In consequence, the installation of new windows with normal double-pane low-E-glazing can be interpreted as maintenance investment even when such a replacement is already connected with energy savings (in comparison to the state before the measure).
The BBSR calculation Tool shows the energy savings in comparison to the windows that are replaced. Within the calculation sheet for windows, the user can chose the share of investment assigned to maintenance. A pre-set value representing this share costs for replacement with windows of the same quality) is not foreseen, because cost-functions for windows of lower performance than currently required are not available. Costs for windows with double-pane low-E-glazing can only be taken as a first approach.
As an alternative approach, the share assigned to maintenance can estimated as the costs that would occur in case that the old windows are restored technically and optically to their original state (e. g. by replacement or retrofitting of the fixtures, seals and pane sealants, repair of damages to the frames, new coating). Whilst these costs can be determined, they will normally not be significantly lower than the costs for the installation of new windows with double-pane low-E-glazing and contemporary frame construction. The calculation sheets allow the determination of the cost-margin for restauration above which (in consideration of the individual conditions) a replacement is economically preferable to a restauration.
Energy price and efficiency of the heating system have a influence the economical situation significantly. The parameters therefore are varied in the following 3 examples. A common result is that a replacement using windows with triple-pane low-E-glazing is economically preferable to a replacement using standard windows (double-pane low-E-glazing).

Boundary condition used for all following examples

  • Period under consideration (“Betrachtungszeitraum“): 25 years
  • Interest rate (“Zinssatz nominal”): 4 % (means 3.0% real)
  • Energy carrier (“Energieträger“): see example
  • Gross current energy price (“Aktueller Energiepreis brutto“): see example
  • Future energy price increase (“Zukünftige Energiepreissteigerung nominal“): 2,4 % per year (means 1,4 % real)
  • Year of calculation (“Jahr der Berechnung”): 2018
  • Increase of prices since reference year 2015 (“Zunahme der Preise seit Bezugsjahr 2015): 2 % per year
  • Expenditure factor of heating system (“endenergiebezogene Aufwandszahl der Heizung“; demand of delivered energy per unit of useful heating energy): see Example

These boundary conditions match the default settings of the calculation tool. In order to properly depict the conditions occurring with partial modernisation, the degree days after the measure were assumed – other than the default setting – the same as before the measure:

  • Degree days - old – (“Gradtagszahl - alt –“): 3251 Kd
  • Degree days - new – (“Gradtagszahl - neu –“): 3251 Kd

The tool calculates the investment costs using the cost functions developed by IWU.

Example 1: Heat supply by condensing boiler (natural gas)

Installation of (in relation to EnEV-conform quality) advanced windows with low-E triple-pane glazing and “warm edge” of the glazing.

  • U-value before modernisation: 2.7 W/(m²·K)
  • U-value after modernisation: 1.0 W/(m²·K)

Specific boundary conditions for example 1:

  • Energy carrier: natural gas
  • Gross current energy price (“Aktueller Energiepreis brutto“): 5.9 ct/kWh
  • Expenditure factor of heating system (“endenergiebezogene Aufwandszahl der Heizung“; demand of delivered energy per unit of useful heating energy): 1.15

    Fenster Einfamilienhaus Ausgangs U-Wert 2,7 auf 1,0 Gas Brennwert Fenster Einfamilienhaus Ausgangs U-Wert 2,7 auf 1,0 Gas BrennwertSource: BBSR [Screenshot Berechnungstool]

Conclusion example 1

In example 1 with investment costs of about 465 €/m² and an average future energy price of 7.8 cents/kWh (nominal; calculated by the tool based on the boundary conditions; cell H 13 - not visible on screenshot) cost-effectiveness is accounted for if the investment costs contain a (not energy-relevant) share assigned to repair / reconditioning of 370 €/m² or more. The proportion of costs the example is based on

  • full costs (cell H 23): 465 €/m²,
  • share of repair costs (cell H 24): 405 €/m²

meet these requirements.

Per square meter modernised windows energy savings of 103 kWh per year in comparison with the initial state (cell C 24) and 11 kWh in comparison with the installation of a standard EnEV-conform window (Cell C 25) are estimated.

Method a) “Equivalent energy price”

Cost effectiveness is achieved for the installation of standard windows for an energy price of 3.12 ct/kWh or more (cell F 41). The future average energy price calculated by the tool as 7.8 ct/kWh meets this requirement. For the lower margin of the share of maintenance-related costs (370 €/m², see above) the equivalent energy price achieving cost-effectiveness amounts to 5.61 ct/kWh.

Method b) “Annuity”

For the chosen marginal conditions and the assumption that the costs for installation of standard windows equal the share of maintenance cost, the annuity amounts to 4.77 € per square meter modernised surface. For the lower margin of the share of maintenance-related costs (370 €/m², see above) the annuity amounts to 2.21 per square meter. The annuity shows the amount by which the annual costs for energy and investment is reduced in comparison to the state without implemented modernisation measure.

Method c) “Amortisation period”

For the chosen marginal conditions and the assumption that the costs for installation of standard windows equal the share of maintenance cost, the amortisation period amounts to 8.91 years. For the lower margin of the share of maintenance-related costs (370 €/m², see above) the amortisation period amounts to 17.04 years. If the amortisation period of a constructive modernisation measure does not exceed 25 years, cost effective is generally assumed.

Example 2: Heat supply by low-temperature boiler (natural gas)

Installation of (in relation to EnEV-conform quality) advanced windows with low-E triple-pane glazing and “warm edge” of the glazing.

  • U-value before modernisation: 2.7 W/(m²·K)
  • U-value after modernisation: 1.0 W/(m²·K)

Specific boundary conditions for example 2:

  • Energy carrier: natural gas
  • Gross current energy price (“Aktueller Energiepreis brutto“): 5.9 ct/kWh
  • Expenditure factor of heating system (“endenergiebezogene Aufwandszahl der Heizung“; demand of delivered energy per unit of useful heating energy): 1.21

    Fenster Mehrfamilienhaus Ausgangs U-Wert 2,7 auf 1,0 Niedertemperatur Gas Fenster Mehrfamilienhaus Ausgangs U-Wert 2,7 auf 1,0 Niedertemperatur GasSource: BBSR [Screenshot Berechnungstool]

Conclusion example 2

In example 2 with investment costs of about 465 €/m² and an average future energy price of 7.8 cents/kWh (nominal; calculated by the tool based on the boundary conditions; cell H 13 - not visible on screenshot) cost-effectiveness is accounted for if the investment costs contain a (not energy-relevant) share assigned to repair / reconditioning of 330 €/m² or more. The proportion of costs the example is based on

  • full costs (cell H 23): 465 €/m²,
  • share of repair costs (cell H 24): 405 €/m²

meet these requirements.

Per square meter modernised windows energy savings of 108 kWh per year in comparison with the initial state (cell C 24) and 11 kWh in comparison with the installation of a standard EnEV-conform window (Cell C 25) are estimated.

Method a) “Equivalent energy price”

Cost effectiveness is achieved for the installation of standard windows for an energy price of 2.96 ct/kWh or more (cell F 41). The future average energy price calculated by the tool as 7.8 ct/kWh meets this requirement. For the lower margin of the share of maintenance-related costs (330 €/m², see above) the equivalent energy price achieving cost-effectiveness amounts to 7.71 ct/kWh.

Method b) “Annuity”

For the chosen marginal conditions and the assumption that the costs for installation of standard windows equal the share of maintenance cost, the annuity amounts to 5.19 € per square meter modernised surface. For the lower margin of the share of maintenance-related costs (330 €/m², see above) the annuity amounts to 0.07 € per square meter. The annuity shows the amount by which the annual costs for energy and investment is reduced in comparison to the state without implemented modernisation measure.

Method c) “Amortisation period”

For the chosen marginal conditions and the assumption that the costs for installation of standard windows equal the share of maintenance cost, the amortisation period amounts to 8.44 years. For the lower margin of the share of maintenance-related costs (330 €/m², see above) the amortisation period amounts to 24.76 years. If the amortisation period of a constructive modernisation measure does not exceed 25 years, cost effective is generally assumed.

Example 3: Heat supply by direct electric heating system

Installation of (in relation to EnEV-conform quality) advanced windows with low-E triple-pane glazing and “warm edge” of the glazing.

  • U-value before modernisation: 2.7 W/(m²·K)
  • U-value after modernisation: 1.0 W/(m²·K)

Specific boundary conditions for example 3:

  • Energy carrier: electricity from public grid
  • Gross current energy price (“Aktueller Energiepreis brutto“): 26 ct/kWh
  • Expenditure factor of heating system (“endenergiebezogene Aufwandszahl der Heizung“; demand of delivered energy per unit of useful heating energy): 1.02

Fenster Einfamilienhaus Ausgangs U-Wert 2,7 auf 1,0 Elektroheizung Fenster Einfamilienhaus Ausgangs U-Wert 2,7 auf 1,0 ElektroheizungSource: BBSR [Screenshot Berechnungstool]

Conclusion example 3

In example 3 with investment costs of about 465 €/m² for every share assigned to repair / reconditioning cost effectiveness is achieved.

Per square meter modernised windows energy savings amount to 91 kWh per year of delivered energy (here: electricity from the grid) in comparison with the initial state (cell C 24) The installation of standard windows results in 10 kWh (cell 25) less energy savings. It should be kept in mind that the energy is significantly more expensive compared to the other examples.

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