This post first appeared on the Green Building Advisor Website.
As homeowners, builders, and/or designers, how do we choose the right products or assemblies that provide the best energy performance? How does a code minimum designed wall perform? How about changes in ceiling insulation R-values? What if we decide to increase the air tightness? How about triple pane windows? There are several ways in which product and assembly upgrades are chosen.
- First is experience. In having the opportunity to be in more than 100 homes a year performing energy assessments/audits and building investigations, I see both successes and failures.
- Being familiar with the different products on the market is important. As an example, selecting the right product for a given budget that can be installed over the board sheathing of an existing home to provide both bulk water management and air control.
- Using energy modeling software to quantify the decisions is helpful. The easiest way to decide which improvements to implement is by creating a baseline (code minimum) model of the project, then key in the proposed upgrades to see how the model changes.

I invested in a modeling software called REM/Design. The software isn’t overly expensive, but there is a software option that is free, BeOpt. There is a learning curve to mastering any of the software options, I’m continually learning tricks and tips in REM/Design.
Let’s go through how the software can help guide in decision making. I’ve used REM/Design to model a simple house build to the 2021 IRC energy code, we are starting with an already efficient house, we’ll see which tweaks make sense. The home is 30×40, slab on grade located in my market, climate zone 7. The home has 8’ ceilings, a high efficiency (92%) natural gas forced air furnace with a basic central air conditioner (14 SEER). Because of the slab on grade design, we have ductwork that leaves the conditioned space and is placed in the unconditioned attic. The ducts are insulated and sealed with the duct tightness test results at the code minimum of 48CFM, all the duct leakage is to the outside. We will model the home with an exhaust only ventilation strategy, later changing to a balance ventilation strategy just to show how much of a difference an ERV or HRV can make. The rate of exhaust only strategy has an exhaust rate of 40 CFM, operating continuously. The energy costs are $0.15 per kWh of electricity and natural gas is $1.25 per therm.
The foundation will have a slab edge R-value of R-10 with a layer of continuous insulation under the slab, also at R-10. Walls are 2×6, R-20 cavity insulation and also have a layer of R-5 continuous insulation. The vented attic is insulated to R-60. Window to wall ratio will be kept at 12%. The double pane windows have a U-factor of U-0.30 and a solar heat gain coefficient of 0.40. Air tightness is 3 ACH50. Given those metrics, the REM/Design software can calculate heating/cooling loads and costs, along with several other metrics.
The software estimates the heating costs for the model home at $732 per year, cooling adds another $69. The software also estimates the heating equipment load at 36.5 Kbtu/hr and the cooling load at 14.3 Kbtu/hr. (These loads are calculated using the 99% design temperature of –20°F and a cooling design temp of 83°F. The chart below shows the component heating loads.
Let’s start making some changes to the model to see how each change affects operational costs. We can start with an easy option, moving the ductwork out of the unconditioned and vented attic space and into the home. We can accomplish this by either designing a soffit system to hide the ducts or using a plenum truss, a truss with a designed notch that creates a space for keeping ducts inside the conditioned space without the need for dropped soffits. We also paid a little more attention to duct sealing and dropped the leakage rate from 48 CFM to 10 CFM. This change reduces our heating equipment load from 36.5 kBtu to 28.8 kBtu/hr. This also reduced the annual heating cost from $732 to $682. Not a lot of annual heating cost savings, but we did drastically reduce the size of the heating equipment.
The next easiest change will be to drop the air tightness level from 3 ACH50 to 1 ACH50. This doesn’t result in much of a cost savings, dropping the yearly heating cost to $661 and a reduction of the heating equipment sizing to 28.5 Kbtu/hr. What this change does do is provides more control as to how outside air enters the building, hopefully creating a healthier home.
Now let’s look at the change from an exhaust only ventilation strategy to balanced ventilation in the form of an ERV or HRV. I increased the ventilation rate from 40CFM to 50CFM (our natural air change rate decreased when we reduced our overall building tightness, requiring a few more CFM to meet ventilation requirements). I used an adjusted sensible recovery efficiency of 60%. This change sightly increased the heating cost to $714 and our peak load equipment sizing is up to 29.8 kBtu/hr.
Next up is a change in windows, moving from a dual pane, U-0.30 to a triple pane, U-0.18. The heating costs drop to $635, and the peak load equipment sizing is down to 27.1 kBtu/hr. The change from a two-pane window to a 3-pane in a very cold climate will not only reduce the operational costs, but it also increases comfort by helping to raise the average mean temperature of the different rooms in the home and allowing a slightly higher humidity level without the risk of widow condensation.
Next, let’s change the walls from an R-20 cavity and R-5 continuous to a R-20 cavity and R-15 continuous. This further drops the heating costs, now down to $580 per heating season and the peak load equipment sizing to 25.6 kBtu. This change also allows us to move from a class I or II vapor retarder to a class III, painted drywall, potentially saving the cost of purchasing a responsive or variable class II vapor retarder.
Our largest remaining load is the slab floor. We have R-10 on the exposed perimeter of the foundation and R-10 under the slab. Increasing both those values to R-15 drops our heating costs to $547 and our peak load equipment size to 24.9 kBtu/hr. Here’s our new graph showing the component loads.
As I said earlier in this post, we are starting with an already efficient home and then heated and cooled it with relatively cheap energy. I think most homeowners in climate zone 7 would be thrilled with an annual heating and cooling cost of less than $800. We managed to reduce these costs by 25% and reduce the size of the space conditioning equipment by roughly 30% with only modest improvements. Now, imagine if we double or tripled the energy costs, probably a reality for this home over its lifespan.
Steven Baczek uses this illustration in some of his presentations, tweaking one knob on the energy efficiency tool can affect other areas. We saw this when we reduced our air tightness level from 3 ACH50 to 1ACH50, which then required a little higher mechanical ventilation rate (40 CFM to 50 CFM). This, in turn, changed the heating and cooling equipment size needs. Better windows and a little higher R-value in continuous insulation brought the operating costs and equipment sizing requirements back down.
So, are the upgrades in our fictitious house worth the added investment? Do they add value to the home? They do if they are valued by the customer and are presented in a way where that customer can make an informed decision. We have to remember that energy cost savings is only one way to evaluate upgrades. Comfort in and longevity of the structure should also be considered. How do you sell energy design improvements?
Randy,
This is good stuff, I bought REM Design long ago and found it easy to use and quite accurate when compared to actual energy used (metered natural gas). To make your exercise even more informative would be to plug in different heating fuels and their approximate current cost. Natural gas is quite inexpensive compared to heating oil, propane or electric heat. Here in the hinterland natural gas is not yet available, I can have it trenched here from 7,000 feet away for approximately 150k according to Xcel Energy, said we would think about it. I think moderate or deep energy retrofits have been given short shift as to the ROI but they must be considered with the different heating fuels available. Not out of the question to spend 3k per winter to heat a house in MN off of the gas grid. This adds up to real money over time. Thank you for your efforts.
Doug