What R-Value Is Good Enough?

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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.

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. 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.

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.
ACHNatural
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 ACHNatural is 1/20th 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:

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 (ACH=10).

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 (ACH=5).

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

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: Tighter Building (ACH50=1) With Ventilator Using Heat Recovery.

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 ACHNatural Estimation Method.

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

Figure M: Wind Zone Reference Map.

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 M: Energy Star Calculation Recommendations.

Figure 7: Energy Star Calculation Recommendations.

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4 Responses to What R-Value Is Good Enough?

  1. Douglas Ruisaard says:

    If you are looking for a major cost savings aspect for your constructions with regard to heating (and cooling) then you should strongly consider geo-thermal systems. I have totally repaid the cost of the system over the past 10 years (Approx $15,000) and more! My monthly heating and cooling bill is about $20 / month. Perhaps you should do one of you excellent blogs on this technology!

    Cheers!
    Doug

     
    • mathscinotes says:

      Good suggestion for a post. My sister-in-law put a geothermal system in recently. I should check how that is working out for her.

      Thanks for the comment.

      mark

       
  2. Joe DeRenzis says:

    I have been involved in HVAC systems and installation in new homes for the last 10 years.
    The items you mentioned are all considered here also in Eastern Massachusetts.
    If you are building a new house, the builder needs to hire an energy audit company that helps design the varies insulation and air sealing requirements in the latest building code.
    One thing I noticed in garages of new homes, the garage door technology has not changed much. The garage doors basically cause a huge whole in the assembly where the (9 X 7) garage doors are located.
    The R value of the typical garage door is not much greater than say even 10 years ago.
    Check out the weather stripping in new garage door installation. Not much air sealing at the garage door opening compared to the recent past.
    I don't think you need to worry about ventilation in the garage if you are building a garage with the typical 2 door garage (9' X 7') and 25' X 25' perimeter with all the air leaking through the seams of the garage doors.

     
  3. Joe DeRenzis says:

    Douglas,
    I was involved with an installation of a geothermal system 5 years ago.
    The system was designed by a geothermal expert (supposedly an engineer).
    This particular closed loop system has 3- 500 feet deep wells that circulate glycol/water mix through the earth to 2- water sourced heat pumps. Using 2 large circulator pumps in the basement, the glycol mix travels about 200 linear feet through supply and return lines in the house and also through the 3ooo linear feet of 1- 1/4" well field piping.
    There are 2- 4 ton water sourced heat pumps in the third floor of the 3000 sqft house.
    If one is able to the analyze the 2 large circulators and the 2 two AC compressor ( at the water sourced heat pump), and not count the 2 air blowers motors because you would still need these in a convectional air handler or furnace,
    and the cost of the electricity in Eastern Mass, it's actually cost more to run this geo thermal system then say a convectional gas forced hot air system.
    This particular system costs my customer almost twice as much to run compared to his neighbor's home (which I also maintain) of the same size and insulation value which has a gas forced hot air system. He has complained to me about his monthly electric costs since it been installed.
    My customer has received other opinions on the overall installation and no one has found any issues.

     

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