At What Point Does More Insulation Not Pay Off?
We have an 1879 Victorian house with high ceilings and huge rooms. It was very hard to keep warm when we moved in and also badly in need of renovation as it had last been worked on in the mid 80s by a guy that pinched every penny twice.
So far we have torn out all of the downstairs walls, removed the badly uneven blown in cellulose in the exterior true 2×4 walls , had then spray foamed, and then built and interior “false wall” such that the new stud faces were 5.5” inside of the original stud faces. This gave no thermal bridge between the 2 walls except at the top and bottom of the wall assemblies. This new interior wall was insulated with r19 fiberglass batt after new wiring was installed and the backs of the boxes all great stuffed. The exterior of the house also has 2 layers of .5” blue board with seems offset but not taped under vinyl siding.
ceilings have all been drop 18” with 10” of fiberglass batt
We have been installing ultra low U value tilt and rotate windows as we can afford and using butyl tape to seal around them.
I’m not sure I would have done any different because it all makes the house very quiet but at what point does more insulation not pay off?
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Replies
There is no easy answer to this without running your place through BeOpt.
Around me (zone 5) with cheap natural gas, the general number is an R25 wall and R30 ceiling.
This assumes new construction and low cost insulation, once you go to SPF, there is no ROI on extra insulation.
When it comes to retrofit, the ROI is non existent and it is too labour intensive to insulate. There the main benefit is comfort, insulating and air sealing does make a huge difference for that.
Especially with very old homes, air sealing is the name of the game. 1902 is the newest house I’ve ever remodeled; and based on my experience so far, I’d take a well air sealed house with R-15 on all 6 sides over a leaky house with R-60 all day long.
The PGH movement calls for targets of heating degree days divided by 180 for a wall R value target, and divided by 150 for a ceiling R value target. This certainly is most applicable to very air tight, new construction, but doesn't seem like a bad baseline that's a bit more specific than climate zone based code minimums.
For me in zone 5, with average 6300 HDD, that would be R35 walls and R42 ceiling -- above code minimum in the walls, but below in the ceiling. Any thoughts?
If I only answer the question in the title and not deal with your specific challenges. It is a math problem I use a computer program called BEopt to solve. You build a computer model of your home and can input different insulation R values and their costs per square foot and the program will identify the option with the lowest cost to own.
https://www.nrel.gov/buildings/beopt.html
Generally opening up walls for the sole purpose of adding insulation rarely saves enough money to recover the cost before the home is sold.
Walta