Energy efficiency measures - Residential - Building envelope - Windows / Canada
Case study assignment
You have been hired by a local home designer to prepare a feasibility study on her behalf. The designer wishes to compare different strategies for the window insulating glazing units (IGU's) for a custom house on a rural lot south of the town of Renfrew, not far from Ottawa. You have been requested to compare the energy and financial performance of standard double-glazing units; double-glazed, argon-filled units with a low-e coating on one pane; and triple-glazed, argon-filled units with a low-e coating on two panes.
Site information
The house is sited in a dense forest of conifer and deciduous trees. The entrance to the house is on the southwest, and the front is nominally southeast. Both the summer and winter shading of the house are considerable. Winter shading is 65% at the front, 65% at the left side, 65% at the right, and 55% at the back. Summer shading is 80% at the front, 80% at the left side, 75% at the right, and 65% at the back.
The heating system is an electric forced air furnace augmented by a ground-source heat pump. The system's seasonal efficiency is 300%. The house is air conditioned by the same heat pump system at a COP of 3.0.
The table gives window sizes, orientations and number of units.
The house has a conditioned area of 170 m²; the basement is not conditioned. The house is very well insulated, but has little thermal mass. As it is built for a working couple, the internal gains are quite low and are estimated from previous electricity bills to be 12 kWh per day.
Case study assignment
You have been hired by a local home designer to prepare a feasibility study on her behalf. The designer wishes to compare different strategies for the window insulating glazing units (IGU's) for a custom house on a rural lot south of the town of Renfrew, not far from Ottawa. You have been requested to compare the energy and financial performance of standard double-glazing units; double-glazed, argon-filled units with a low-e coating on one pane; and triple-glazed, argon-filled units with a low-e coating on two panes.
Site information
The house is sited in a dense forest of conifer and deciduous trees. The entrance to the house is on the southwest, and the front is nominally southeast. Both the summer and winter shading of the house are considerable. Winter shading is 65% at the front, 65% at the left side, 65% at the right, and 55% at the back. Summer shading is 80% at the front, 80% at the left side, 75% at the right, and 65% at the back.
The heating system is an electric forced air furnace augmented by a ground-source heat pump. The system's seasonal efficiency is 300%. The house is air conditioned by the same heat pump system at a COP of 3.0.
The table gives window sizes, orientations and number of units.
The house has a conditioned area of 170 m²; the basement is not conditioned. The house is very well insulated, but has little thermal mass. As it is built for a working couple, the internal gains are quite low and are estimated from previous electricity bills to be 12 kWh per day.
Financial information
The feasibility study, development, design/engineering, and miscellaneous costs are part of the basic design and construction costs of the house and do not add to the cost of the energy performance features. Based on a quotation from a major window supplier, the installed cost of the triple-glazed high performance windows is $460/m², of which $110/m² is installation. The double-glazed unit with argon fill and low-e coating is $395/m², of which $100/m² is installation. The base case window is estimated to be $365/m² with an installation cost of $100/m² included.
The retail cost of electricity in the area is $0.085/kWh. The local utility does not levy peak demand charges on residential customers. The inflation rate is 2%, and the assumed fuel cost escalation rate is 5%.
The design life of the structure is anticipated to be 100 years, but the windows are expected to last only 30 years. The house is being financed with a conventional mortgage, with a down payment of 26%. The term of the mortgage is 20 years at an interest rate of 6.5%.
Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.
Solution
The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.
Teacher's notes
Results
In 1998 the Lauriers approached architect Richard White to prepare a design for a new house on a heavily wooded site near the Madawaska River in the Ottawa Valley. Large windows were desired, both to bring the woods into the house and to keep the large interior volume from feeling cave-like. Although the house had a good distribution of windows for passive solar performance, the amount of shading and size of the south-facing windows raised a question regarding the best strategy for the glazing units: Would maximising solar gain to compensate for shading be preferable to maximising the insulating capacity of the windows to ensure good comfort in front of a large area of glass?
An energy and financial analysis compared a double-glazed low-e and argon-filled IGU with a triple-glazed double low-e, double argon-filled IGU. The analysis showed that with standard double-glazed units as the base case, the high performance double-glazed IGU had a very strong internal rate of return (IRR) - over 30% - with a simple payback of 8 years. The triple-glazed IGU achieved an IRR of 12%, with a simple payback of 17 years. Both strategies produced a better financial return than the mortgage rate. The energy savings ranking for the two systems was reversed, however. The triple-glazed IGU brought a renewable energy contribution to the house of 11.5 MWh/yr while the double-glazed IGU contributed 7.5 MWh/yr.
The house was started in the spring of 1999 and closed in before winter. In the completion of the project, the Lauriers did not hesitate to choose the triple-glaze strategy, preferring the additional comfort to the faster return on investment. The house came through its first winter in a semi-finished but comfortable state.
System description
The base case window that was used as the reference point for the study was a standard double-glazed clear IGU in a good quality wood frame, as manufactured by Loewen of Steinbach, Manitoba and supplied by a local distributor. The test model of the base case window has a U-value of 2.86 with a SHGC (solar heat gain coefficient) of 0.65. The alternative IGU's were also Loewen products and had the same wood frame. The modelled double-glazed low-e argon-filled IGU had a U-value of 1.67 and SHGC of 0.35. The modelled triple-glazed IGU had a U-value of 1.02 with a SHGC of 0.30. These figures are for fixed units; the figures for casement windows were slightly different.
Lessons learned
The big picture
Once the IRR for the window technology is better than the bank interest rate on the mortgage, the incentive to maximise window financial performance may decrease. With the financial report showing a positive contribution over an acceptable time, other agendas like improved occupant comfort may take a higher priority than improved financial performance of the window system alone. Energy assessment for residential construction is not common. Although whole building assessment tools have been available since the beginning of the R-2000 Program, mechanical sizing and the energy impacts of envelope design are usually handled by rules of thumb. The ability to "tune" windows for different exposures and thermal conditions is a new skill and not well appreciated outside of high performance building circles. The ability of these techniques to significantly lower the operating costs of new or renovated housing suggests that they will become more common over time.
Photo
Wood Window Corner Section, Ontario, Canada
References
The feasibility study, development, design/engineering, and miscellaneous costs are part of the basic design and construction costs of the house and do not add to the cost of the energy performance features. Based on a quotation from a major window supplier, the installed cost of the triple-glazed high performance windows is $460/m², of which $110/m² is installation. The double-glazed unit with argon fill and low-e coating is $395/m², of which $100/m² is installation. The base case window is estimated to be $365/m² with an installation cost of $100/m² included.
The retail cost of electricity in the area is $0.085/kWh. The local utility does not levy peak demand charges on residential customers. The inflation rate is 2%, and the assumed fuel cost escalation rate is 5%.
The design life of the structure is anticipated to be 100 years, but the windows are expected to last only 30 years. The house is being financed with a conventional mortgage, with a down payment of 26%. The term of the mortgage is 20 years at an interest rate of 6.5%.
Prepare a RETScreen study, documenting any assumptions that you are required to make, and report on the significant conclusions from this analysis.
Solution
The worked-out solution is the data file selected from within the RETScreen Project Database. The user automatically downloads the Project Database file while downloading the RETScreen software.
Teacher's notes
- Because the windows are part of the house and not a separate "energy project," the discount rate is no different than the basic cost of borrowing for the mortgage.
- Compared with the base case windows, there are no additional overhead or contingencies: the design process is identical and there are no additional risks.
Results
In 1998 the Lauriers approached architect Richard White to prepare a design for a new house on a heavily wooded site near the Madawaska River in the Ottawa Valley. Large windows were desired, both to bring the woods into the house and to keep the large interior volume from feeling cave-like. Although the house had a good distribution of windows for passive solar performance, the amount of shading and size of the south-facing windows raised a question regarding the best strategy for the glazing units: Would maximising solar gain to compensate for shading be preferable to maximising the insulating capacity of the windows to ensure good comfort in front of a large area of glass?
An energy and financial analysis compared a double-glazed low-e and argon-filled IGU with a triple-glazed double low-e, double argon-filled IGU. The analysis showed that with standard double-glazed units as the base case, the high performance double-glazed IGU had a very strong internal rate of return (IRR) - over 30% - with a simple payback of 8 years. The triple-glazed IGU achieved an IRR of 12%, with a simple payback of 17 years. Both strategies produced a better financial return than the mortgage rate. The energy savings ranking for the two systems was reversed, however. The triple-glazed IGU brought a renewable energy contribution to the house of 11.5 MWh/yr while the double-glazed IGU contributed 7.5 MWh/yr.
The house was started in the spring of 1999 and closed in before winter. In the completion of the project, the Lauriers did not hesitate to choose the triple-glaze strategy, preferring the additional comfort to the faster return on investment. The house came through its first winter in a semi-finished but comfortable state.
System description
The base case window that was used as the reference point for the study was a standard double-glazed clear IGU in a good quality wood frame, as manufactured by Loewen of Steinbach, Manitoba and supplied by a local distributor. The test model of the base case window has a U-value of 2.86 with a SHGC (solar heat gain coefficient) of 0.65. The alternative IGU's were also Loewen products and had the same wood frame. The modelled double-glazed low-e argon-filled IGU had a U-value of 1.67 and SHGC of 0.35. The modelled triple-glazed IGU had a U-value of 1.02 with a SHGC of 0.30. These figures are for fixed units; the figures for casement windows were slightly different.
Lessons learned
- The practice of assessing the financial performance of building elements based on simple payback from saved fuel costs produces a restricted characterisation of the improvement, which favours short-term paybacks, generally between 5 and 10 years. Assessment of value by IRR demonstrates very strong economic performance for improvements with medium to long term time frames. The ability to review building improvements in a similar framework to other financial investments allows a more accurate value to be assigned to the multiple benefits of the improvement.
- Additional costs of high performance triple-glazed systems over doubled-glazed systems take a significantly longer time to recover than does the premium for choosing high performance double-glazed systems over standard double-glazed clear windows. Regardless of the cost premium, investment in high performance triples can improve occupant comfort and still produce a better financial return than the current bank rate on bonds, savings certificates, or mortgages.
- Unlike conventional energy technologies, windows in passive solar houses respond to a number of agendas simultaneously. The best financial rate of return for the windows may not be the deciding factor in window selection. It would be more reasonable to assume that decisions are made on a composite judgement weighing occupant comfort, whole house performance and financial costs. To date, this balancing of factors has been done on a personal and informal basis. Use of more sophisticated financial measures like the IRR will help formalise the decision-making process.
The big picture
Once the IRR for the window technology is better than the bank interest rate on the mortgage, the incentive to maximise window financial performance may decrease. With the financial report showing a positive contribution over an acceptable time, other agendas like improved occupant comfort may take a higher priority than improved financial performance of the window system alone. Energy assessment for residential construction is not common. Although whole building assessment tools have been available since the beginning of the R-2000 Program, mechanical sizing and the energy impacts of envelope design are usually handled by rules of thumb. The ability to "tune" windows for different exposures and thermal conditions is a new skill and not well appreciated outside of high performance building circles. The ability of these techniques to significantly lower the operating costs of new or renovated housing suggests that they will become more common over time.
Photo
Wood Window Corner Section, Ontario, Canada
References
- Loewen Window Centre, Website: http://www.loewen-window-centre.com.
- Pope, Stephen, "Personal communication," Stephen F. Pope Architect, 2000.
- White, Richard, "Personal communication," White & Gilbride Architects, 2000.
