Which Retrofit to Choose? Windows, Insulation, or New HVAC

by ‐ Tags: building science

Let’s say it’s 20 degrees out, and you are outside in nothing but a track suit (I won’t judge your fashion choices) and a pair of sunglasses.  You’re pretty darn cold.  You are given three choices: 1) you can have a new pair of more tightly fitting wraparound sunglasses, 2) you can put on a hat and a sweater under your track jacket or 3) you can eat all the hot chili you want to keep warm from the inside out.  Which do you choose?

They may seem like strange choices, but humor me for a few minutes. People usually consider three major upgrades: windows, insulation, and new mechanical systems.  Unless something is actively failing, it can be difficult to know which to tackle first.  

Windows

That’s where my admittedly odd analogy comes into play.  A lot of people want to replace old windows first.  Between the fact that window manufacturers have promised huge savings (though less so since the FTC started cracking down on this) and the fact that windows are a highly visible part of the building, it seems like a good place to start.  But swapping out windows is like opting for the sunglasses.  They might provide some additional comfort and keep the area around your eyes a little warmer, but it’s not going to make things much better while the rest of you is freezing.  

Replacing windows is similar. They typically do not make up a large proportion of the building’s envelope (a term that refers to the walls, ceilings, and floor of the building). There is also not a lot of efficiency to be gained by going from even old, single pane windows to new ones. A single pane window has an R-value of 1. Adding a storm window brings it to an R-2. Replacing that with a pretty good efficiency new window will bring it up to an R-3 or R-4. There’s a handy chart on the Chicago Conservation Corps blogThat’s not a huge increase in the insulating value of the window, especially given the cost to replace.  

Insulation

Ideally a building is well-insulated all around, including the foundation and slab. In reality, this is rarely the case. In retrofit situations it’s typically exceedingly difficult to insulate the slab and foundation walls, so most insulation upgrades are focused on above grade walls and ceilings. This is the sweater and hat.  If you have uninsulated walls, they are likely somewhere around an R-4.  As a reminder, R-4 the expected R-value from installing nice, new windows. Depending on when your building was built and the size of the framing, you’ll probably be able to get either R-13 or R-19 installed in the walls. The same principles apply for ceilings, where you’ll likely be able to install from R-20 to R-38. This is a lot more insulating value than windows and much less costly.

Mechanical Systems

And finally, the chilli. Replacing your heating system without doing any work to the envelope of your building is like deciding to just eat hot chilli instead of better equipping yourself for the cold.  It helps, but it’s still a losing battle. There are undoubtedly efficiency gains from upgrading to new equipment. To get a rough idea of how much you can expect to save on heating costs, figure out the difference between the efficiency of your new and old equipment. If you’re upgrading to a 95% AFUE boiler from a 80% one, you can expect about 15% fuel savings.  

The best course of action is usually to insulate and air seal the building envelope, thereby reducing heat loss and gain, install new equipment that is sized correctly, and repair, rather than replace windows if they are still in reasonable condition. Of course, if components are failing or have reached the end of their service life, these take priority.