Attic Insulation Cost Savings Calculator

Wanted to post a mini-rant about evaluating the return on investment (ROI) for attic insulation. 

Most online calculators and buyers focus on cost savings per month, but this may not be nearly the right approach, especially if you find your HVAC is running for hours in the daytime without stopping or loses the ability to maintain the set temperature until the sun sets. If that's you then you may be using the wrong math to figure out whether you should upgrade your attic insulation. 

Instead of looking at monthly savings, you should be looking at the total cost to make your home comfortable.  Let me explain how that worked out for me.

Fact: In a very hot or very cold climate the difference between poor and very good attic insulation can be a 50% difference in the heat pump size!  That's worth a lot more to me than $40/month in the summer. 

In addition to reducing the absolute BTUs needed to heat/cool the top floor of your home the attic insulation will also extend the lifetime of your heat pump and air handler by reducing the amount of time it runs per day.  Basically, less time on = less wear and tear = longer lifespan. 

Let me share my own numbers.  I just paid $5k to replace a 2-3 year old heat pump for the upstairs zone.  The warranty sold to the previous owners who replaced the unit was not transferable so if I paid to just repair it ($2k) I wouldn't get a longer warranty on the rest of the unit. OK, fine, so pay for a new unit with a full 10 year warranty.  Unfortunately, after replacement the unit is still unable to adequately cool the upstairs when the home is in full sunlight. This means my upstairs heat pump stays on for 6 hours or longer during the day if the outside temp reaches even just 85 degrees, while the downstairs unit runs almost exactly half as much.  We're absolutely going to hit 90 degrees on a regular basis and I can plan on a few days of 110 degrees or more this summer. 

Here you can see what happened Wednesday on a clear day vs. the rest of the week with overcast skies. From noon until after 8 PM the heat pump never turned off.  I know cause I was carefully observing it during this period that this data is accurate.  

Effectively I'll be unable to use the upstairs, where my home office and bedroom are, during the day.

As an aside, it is quite possible that running excessively long for each summer day caused my unit to fail early.

So my choices are:

1. New heat pump and air handler that is 50% larger: $8k + $100 / month
2. New mini-split for the home office:  $4k  + $100 / month
   
3. Attic air sealing + insulation:  $3k (minus) $50 / month
   

In both of the first cases I have to pay more per month due to the additional cooling capacity costing more to run. In the last case I (hopefully) pay less for installation AND save some money every month. 

If we did the math naively, sure, I'll pay $3k for insulation and not get that money back for 5 years, but you can see that's not the whole story here.

So based on this calculation, I'm not saving $50 a month, I'm saving $5k up front and $150 a month compared to my other two options.  This takes into account the most important part:  What is it going to take to get to comfortable?

So please be sure to add in these factors when considering whether you should get additional attic insulation, especially if you find your heat pump works OK for spring and fall but not winter or summer.