Deciphering Remodel U-Factor Requirements
On a 4,500 square foot remodel in Belmont, Massachusetts we are replacing and adding several doors and windows. Per the definitions in the current IRC both of these are considered to be “fenestration.” The local inspector was there to sign off on our insulation when he noticed one of our new french doors. A beautiful, and very expensive, full lite pair of patio doors, complete with a full lite glazing unit (aka a fixed glass panel) on either side.
“You’re gonna have a problem with those.”
I froze–like I said. Expensive.
“U-factor required in Massachusetts is .30 for all fenestration, those are .31.”
He was right, the doors and units were all labeled .31 U-factor, which is the rate that a door loses heat. The lower the number, the more energy efficient the door is. I told the inspector to leave it with me and began to dig into my sales orders to see what had happened.
I discovered that our initial quote for the doors, as a single mulled unit (a pair of three foot glass doors flanked by two three foot glazing units) had a U-factor of .30. It had complied, but in order to decrease the header size (a type of beam that supports the load above a door), I had asked for the units to be separated so I could put a post on either side of the door. In doing this, we had limited the manufacturers ability to get to the lower U-factor number.
I dug deeper to try and understand the specific code requirements in this situation. We are remodeling an existing building, so I looked at the 2018 IECC, chapter 5 “Existing Buildings.” Here we find R501.5 “Except as otherwise permitted by this code, materials permitted by the applicable code for new construction shall be used.” So for our remodel, we must comply with the codes listed in chapter 4 of the IECC and amended by the Commonwealth of Massachusetts. So what are those requirements? At a first glance things don’t look so good for our poor doors. Table R402.1.2 of the 2018 IECC is quite clear, for climate zone 5, the U-factor for fenestration is .30. With the code, you always have to dig a little deeper; there are exceptions everywhere. So we look to R302.3.1:
An area-weighted average of fenestration products shall be permitted to satisfy the U-factor requirements.
Now we are talking. Not only are we replacing the two doors and glazing units, but we are also replacing all the windows in the basement, and a few others throughout the house. And all of these had very low U-factors. We also have further assistance from R402.3.3:
Not greater than 15 square feet of glazed fenestration per dwelling unit shall be exempt for the U-factor and SHGC requirements in Section R402.1.2.
We can actually exempt 15 square feet of glazing from our calculation; that will really bring the average down! We also find help from R402.3.4:
One side-hinged opaque door assembly of not greater than 24 square feet shall be exempt for the U-factor and SHGC requirements in Section R402.1.4.
Now in our particular case, we are not replacing any opaque doors, but this is definitely a code exemption to remember. The IECC defines an opaque door as a door that is not less than 50% opaque, meaning a door that has less than 50% of its surface glazed. This might commonly include a new front door.
I presented all my research to the local official, with citations of the relevant code and a table showing my weighted area average calculation. For each window or door, I simply calculated the area in square inches of each window type, and multiplied that by the number of windows. Then I converted this to feet. I multiplied this number by the U-factor of that window to arrive at my “Area U-Factor.” I added all these up. On the table that number is 35.39. Then I added up my total area in feet and subtracted the 15 square feet allowed in R402.3.3, arriving at 118.81 for my Gross Glazing Area. Dividing 35.39 by 118.81 gives me a U-factor weighted average of .298. We comply!
The inspector appreciated the detail and agreed with the methodology. He admitted that exemption was new to him as well. The codes in Massachusetts and elsewhere are a tangled web: IRC, IBC, IECC, IEBC, the Amendments in 780 CMR. It’s very easy to get lost and follow only the prescriptive codes, but by digging into the text and seeking out the exemptions, you can find ways around what might seem impossible.