What R-Value Is Good Enough?

Quote of the Day

If everything seems under control, you're not going fast enough.

- Mario Andretti, race car driver. I feel the same way about the fiber optic business.

Introduction

Figure 1: My New Garage Will Be Similar to This Building.

I plan on retiring at my lake home in Northern Minnesota. The first step in my retirement preparations is building a large garage on my retirement property that will allow me to work on my various projects – I have not mentioned it before, but I love doing auto body work. I am currently building a garage similar to that shown in Figure 1. Because northern Minnesota is quite cold in the winter, I needed to insulate and heat this structure. I also, however, will need to consider airflow in this calculation as otherwise there could be a risk of mold developing in the property, which would then require a mold remediation service to remove. With that in mind, this post will review some observations that I made as to the value of insulation and of using modern ventilation systems with heat recovery capability.

This blog post will show the impact on my January garage heating bill for different ceiling and wall insulation R-value options, subject to different ventilation assumptions. Whatever lesson you take from this post remember that no matter where you are living having the proper heating equipment is essential, this includes water heaters too as during the colder months pipes can freeze causing recurring issues, but with the correct services and installations from companies like Morris Jenkin homeowners like me can keep their homes toasty all the time.

The analysis presented in this blog post is captured in this Excel workbook. There is a small macro in this workbook that is used to select between the different ventilation scenarios. The model includes windows and can easily be modified to accommodate different size buildings and window configurations.

Background

Definitions

ACH₅₀

The number of air changes per hour with the home pressurized 50 Pa above outside air pressure. This measurement is usually made using a blower door.

ACH_Natural

The rate of air changes assume normal air home pressures. The value normally has to be estimated (example calculation). For this post, I will assume that the ACH_Natural is 1/20^th of the ACH₅₀ value, which is a common assumption. The Energy Star folks publish their calculation recommendations in this document, which have different factors for different conditions. Here is another good reference. I include the Energy Star table in Appendix A. Here is a simple web calculator that guides you through their table.

Technical Information

All the engineering needed to understand this analysis is presented in these blog posts:

• Home Insulation Math
• Effective R-Value of Common Wall Construction Methods

Analysis

Leaky House Case

Figure 2 shows my estimated monthly garage heating bill assuming (1) typical January temperatures, and (2) a very leaky structure. The key observation to make about this graph is that using extreme levels of insulation make relatively little impact on the heating bill because of all the air leaking away through the walls and ceilings.

Figure 2: Leaky Building (ACH₅₀=10).

I should note that I did not worry about ventilating the garage in this case because the structure is so leaky that no ventilation is necessary.

Moderately Air-Tight House Case

In Figure 3, I halved the leakage rate of the structure and observe that insulation levels have a bit more effect, but most of my money is still leaking through the walls and ceiling.

Figure 3: Moderately Tight Building (ACH₅₀=5).

Air-Tight House with Ventilation Lacking Heat Recovery

In Figure 3, my garage is so air-tight that the building code requires that I have ventilation. In this case, my ventilation level is less than the leakage levels of Figure 2 and 3, so my monthly bill drops and I can see that increasing my ceiling insulation level is certainly starting to save me money.

Figure 4: Tight Building (ACH₅₀=1) with Ventilator Lacking Heat Recovery.

Air-Tight House with Ventilation with Heat Recovery

Figure 5 shows that building an air tight garage with a heat recovering ventilator makes a big difference in the the monthly cost of heating the garage. The chart also shows that increasing the ceiling insulation levels of R38+ and wall insulation levels of R20+ make a big difference in the monthly cost.

Figure 5: Tight Building (ACH₅₀=1) With Ventilator Using Heat Recovery.

In fact, my garage will have a ceiling insulated to R38 and walls to R18. I would expect it to cost ~$250 to heat for the month of January in Minnesota.

During retirement, I hope to be living in Florida during December, January, and February 🙂 Northern Minnesota is too cold for me to consider living there year-round. During my absence in the dead of winter, I plan to keep the garage just warm enough to prevent frost heave.

Conclusion

This exercise made clear to me that need for heat recovering ventilators in a tightly-built home. Every home needs ventilation for the health of the occupants. Ventilation becomes the major source of energy loss when your home is well-insulated. Thus, it behooves you to do everything you can to recover all the energy you can from the stale air before you vent it outside.

I should mention that my wife has expressed concern that she will never see me again once the garage is built and all my toys are in there. She may be right ...

Appendix A: Energy Star ACH_Natural Estimation Method.

Figures 6 and 7 shows the ACH_Natural estimation method recommended by the Energy Star folks. Figure 6 shows how to evaluate the wind zone parameter.

Figure 6: Wind Zone Reference Map.

Figure 7 shows how to determine the divisor – Energy Star calls it the LBL factor. I used 20 for my rough analysis. I believe that LBL stands for Lawrence Berkley Labs, but I have not confirmed this.

Figure 7: Energy Star Calculation Recommendations.

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