How to Invest Best

By Anber Rana and Piyaruwan Perera
PhD Candidates at UBC Okanagan 

Enhanced insulation and good airtightness result in impressive thermal performance of residential homes. Based on the two homes built for the Wilden Living Lab project, the UBC research team compared the cost and environmental impacts of different wall assemblies and airtightness options. Conventional batt insulation combined with excellent workmanship showed the shortest payback period while significantly enhancing the homes energy efficiency. Rigid/continuous insulation proofed to be even more efficient, but has a longer payback period due to its higher material cost.

A number of different wall assemblies have been studied. Read more about their energy performance.

The building envelope of a residential home defines the outermost shell that protects the conditioned indoors from heat, cold, moisture, noise and light from the outside. The main components are walls, roofs, windows, doors, and foundations. The excellent thermal performance of external walls is of particular importance to maintaining a comfortable indoor environment at low energy consumption (Ibrahim et al., 2014). An improved thermal wall system reduces heat losses, operating cost, and reliance on HVAC systems. Several methods can be used for improving the thermal performances of wall assemblies that include (Kosny et al. 2014):

• 

  Fig. 1 Heat loss through uninsulated walls

  Using thicker and wider insulation space in the wall cavity 

• Increasing thermal resistance of insulation materials 
• Using insulating sheathing 
• Controlling and reducing thermal bridges 
• Using airtight construction methods 

Two main problems associated with the poor thermal performance of wall assemblies are air leakage and inadequate insulation. Studies have shown that heat losses through poorly insulated walls can be as high as 35% compared to a code compliance wall assembly (Fig 1). Hence, higher levels of airtightness and wall insulation are required by modern building codes (Boudreaux et al., 2018). Similarly, the BC Energy Step Code (BC-ESC) introduced in 2017 and applicable to new residential buildings requires an even higher level of insulation and airtightness. Increased insulation levels in wall assemblies of residential buildings can help in meeting energy performance targets defined by the BC-ESC. 

What was tested?

In the Wilden Living Lab (WLL), Home of Today (HOD) and Home of Tomorrow (HOM) are built with wall assemblies with the effective thermal resistance (R) of 17.16 K·m2/W and 21.46 K·m2/W. The heat recovery ventilators are used to enhance the energy performance of both houses and to comply with the BC Standard building code 2015. The details of the exterior wall assemblies for the two homes are provided in Table 1. In this article, we employ energy data collected through the installed sensors in the two homes to develop calibrated energy models. The calibrated models were normalized to separate the energy and costs benefits associated with individual components of the WLL homes. The results of wall assemblies of the two homes are further tested for different wall assemblies comprised of multiple insulation types and thicknesses with different airtightness. The total combinations generate six (6) different scenarios, as shown in Table 2. The results of these scenarios can help in making decisions when selecting exterior wall assembly in Kelowna.

What was found? 

Savings in annual energy use, change in initial investment, reduction in carbon emissions, and the payback period associated with different wall system scenarios are provided in Fig 2. 

Fig. 2 Results for the six scenarios of wall systems analyzed

Conclusions and Recommendations

• Payback periods of the rigid insulated (EPS-based) walls are around 17 to 20 years; Payback can be lowered by incentivizing the purchase price and installation cost of rigid (EPS Styrofoam) insulation layers.
• Rigid insulation for wall assembly shows more appropriateness for buildings with high-cost energy-based HVAC systems (Such as electric heating and cooling at BC).
• Conventional Batt insulation with excellent workmanship is shown more viability for residential buildings in interiorBC (climate zone 5) due to the lower purchase cost and conventional installation methods. (Only considering building construction and operational cost within the first two years). Long-term costs need to be assessed to evaluate the long-term performance of upgraded insulation options.
• Excellent workmanship is needed to enhance the airtightness of a household and reduce natural air changes. This can be further assured with the mid-construction blower door test.
• Better payback periods can be achieved for the upgrades done for the envelope at the presence of electricity-based HVAC systems due to higher prices of electricity in Kelowna, BC.

Click here for more information on different insulation materials available for your house.

Stay posted for more real-life data results in the future. In the next article we’ll talk about WLL Research Findings to Date!

References

• Boudreaux, P., Pallin, S., Accawi, G., Desjarlais, A., Jackson, R. and Senecal, D., 2018. A rule-based expert system applied to moisture durability of building envelopes. Journal of Building Physics, 42(3), pp.416-437. https://doi.org/10.1177/1744259117750370
• Govt. BC., 2017. BC Energy Step Code – Province of British Columbia.
• Ibrahim, M., Biwole, P.H., Wurtz, E. and Achard, P., 2014. A study on the thermal performance of exterior walls covered with a recently patented silica-aerogel-based insulating coating. Building and environment, 81, pp.112-122. https://doi.org/10.1016/j.buildenv.2014.06.017
• Kosny, J., Asiz, A., Smith, I., Shrestha, S. and Fallahi, A., (2014). A review of high R-value wood framed and composite wood wall technologies using advanced insulation techniques. Energy and Buildings, 72, pp.441-456. https://doi.org/10.1016/j.enbuild.2014.01.004