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