Construction Design-Performance Metrics

Most new homes built today follow the prescriptive code path for energy efficiency.  Those requirements will depend on the code cycle of your area and your exact climate zone.  The requirement for my area are 3, 3.33, 10, 15, 20, and 49.  These are the code minimum performance numbers for different assemblies in the home.  Let’s discuss each of these numbers, and then I’ll give my opinion as to where I think they should be. I’ll start with the number three and go in the order of the list.

Air Leakage Rate.  3 is the maximum air leakage rate for a new home, 3 ACH50.  This is the only number on the list where smaller is better.  It is also the only number that often isn’t planned or set as a goal.  Most of the blower door tests results where I test a new home are a surprise for the builder or homeowner.  It’s rare I see one fail, but it’s also rare that the builder will try to lower the leakage rate.  As a matter of fact, in my market, the insulating contractor is usually responsible for the air tightness of the structure.

Window R-value.  The insulation requirement for a window assembly is rarely listed as an R-value, it’s much more common as a U-value.  Code minimums in my heating dominated climate are U-.3.  To calculate the R-value, use the formula 1/U, which gives us the R-3.33.  The window is the area of a wall that has the lowest insulation value, common wall to window ratios are 15-20%.  The window has a major effect on the overall wall insulation factor.

Slab Edge Insulation.  There are areas of the country where insulation is not required to be on the exterior edge of a slab.  This is not the case in a cold climate.  R-10 is needed and a good starting point.  In addition, R-5 is the minimum required under a slab when the slab is heated.  I have used several different insulation types below grade, all can work well if installed correctly.  There are times when I use more insulation, I will discuss when in a little bit.

Foundation Insulation.  R-15 continuous is usually the minimum required for below grade foundation insulation.  Interior cavity or a combination of the two can change the R-value requirements.  My preference on projects I’m involved in is to use an insulated concrete form or ICF, especially for basements.  This can get us more than R-15 continuous on both the inside and outside.

Above Grade Walls.  R-20 is the code requirement for cavity wall insulation, this can be reduced with the addition of exterior insulation.  My market uses mostly R-21 fiberglass batts with no exterior insulation, the State of Minnesota has eliminated the exterior insulation requirement.  Code officials in the state fear contractors will try to use a rigid plastic foam insulation on the exterior and a polyethylene sheet as the vapor/air control layer on the interior.  This will result in a “double vapor barrier” and add risk to the assembly.

Ceiling.  R-49 is the minimum insulation requirement for most ceiling and roof assemblies in a cold climate.  There are a few exceptions, for instance when rafters in a ceiling are not deep enough to accommodate the thickness of insulation to achieve R-49, less R-value is allowed.  This reduction is restricted to a specific size in square feet, for example, R-30 can be used but the maximum is 500 square feet or 20% of the total insulated ceiling area, whichever is less.

Those are the code minimum performance metrics for a new home in my cold climate.  Now, let’s discuss where I think they should be.  I believe these are the numbers where the codes are heading, and in one case, have already arrived at.  1, 5, 10, 20, 40, 60.

1 ACH50 Air Leakage Rate.  This would be a challenge for most builders who aren’t concentrating on air sealing their own projects.  With some training, this metric isn’t that hard to hit.  There is an option for those who don’t want to try, it’s called Aerobarrier.

R-5 or U-.20 Windows.  As I said earlier, windows are the weak point of every wall assembly.  Getting to R-5 will require moving to a triple pane unit, which increases the cost.  There are a couple big advantages to triple pane windows, comfort and comfort.  Decreased heat loss and the ability to increase indoor humidity levels during the heating season.  If you live in a cold climate, you’ve seen condensation on windows during cold temperatures.  Triple pane windows are less likely to act as dehumidifiers when humidity levels are a little higher.  This increase humidity makes the home more comfortable.

R-10 Slab Insulation*.  R-10 isn’t usually a bad resistance to heat flow for slab edge installation.  This is because the slab edge is typically not a large area of exposure.  I will add more insulation when a slab is heated though, I prefer R-20 in that case.  I would also use R-20 sub-slab insulation with the heated slab.

R-20 Foundation Insulation.  As I said earlier, I prefer to use ICF for stem wall and basement foundation systems.  This foundation insulation system greatly simplifies this assembly.

R-40 Above Grade Walls.  To get to this level of exterior wall insulation, the wall will be built using one of two methods, either exterior insulation with cavity insulation or some sort of double wall assembly.  I personally prefer the exterior insulation over the double wall, but that’s just my preference.

R-60 ceiling.  The 2021 IRC code has already made the increase in ceiling insulation from R-49 to R-60, though there is a lot of debate on whether this is a good decision.  People against say there is no return on investment for the added insulation, (there is a return, it’s just a very long time period).  People for say the cost is minor if the insulation is added during the build.  Here’s my take, the payback on the investment may not make sense today, but if energy costs continue to rise, that energy cost payback will change.  Add the insulation while the crews are there during the build.

Moving from the code minimums to the suggested metrics will help a home and its occupants on several levels.  Lower energy costs are just one, and in my opinion, comfort and building durability are above energy cost savings.  Decreased air leakage rates with properly designed mechanical ventilation systems will help a home’s durability and can improve indoor air quality.  Better windows will allow a higher level of humidity, which in turn can help both comfort and indoor air quality (many viruses thrive in overly high and low humidity levels).  The increase R-values help building durability, comfort and energy costs, especially when adding exterior insulation.  These changes to the code minimums are easily achieved when building new.  In my opinion, these increases in performance will eventually be the code.

2 Replies to “Construction Design-Performance Metrics”

  1. Randy, you really nailed it in this post – the most important stuff, and why it’s important. Water management is too, although that topic is addressed in detail all over this site.

    I have read the pushback to building better; the common theme is “The bank, the appraiser, and the new homeowner don’t value performance, only size, location and price. Buying a house is tough now, and I get that, but I think we should do more of what you’re recommending anyway. The more people do it, the more likely the next house you move into will be comfortable, durable, and enjoyable to live in. If you can manage your expectations, there are ways to save money.

    1. Thanks Don. You are right, difficulties exist with finance, appraising, some homeowners, and I would add some builders. One thing I am noticing is an uptick in interest of building a better home. I have several projects where I have been contacted by mostly homeowners who want to make improvements to designs or want to know how to improve the home they are currently living in. The interest to build better needs to start with the homeowner, they need the education to know what to ask for. I’m hoping that this blog can be a help in that regard. Thanks for the comment!

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