So... quick update on this!
We insulated the rimjoists this summer along with basement wall with R10 XPS.
I slealed everything with foam gun and with SIVA tape - to those that knwo its an amazing tape specifically for XPS. I didnt want to use normal contractor tape since after looking for reviews and testing most tapes peeled off after 1 year or so.
Well, my first serious heating bill is in. Granted last year in November we still didn't have the attic insulation at R60 however savings are at 50% or more.
I calculated last year on December/January during the winter spell and with attic already insulated at 4.5 BTUs/HSF ... Now I used my living space of around 1200 sq ft... however I am not sure if I should use the basement as well since we have as many ducts in the basement as upstairs even though it's unfinished.
This november is in.
740 HDD vs 670 last year - about 11% colder.
Therefore, we spent 19 therms this year vs 42 last year (last year our setbacks were also more agressive. Nowadays I don't keep the house colder than 60F because of... wife)
When calculating BTUs per HDD per sq ft should i do 2300 sq ft or just 1150? Not sure what to use ... we have 5 ducts downstairs so in essence it's a heated basement of sorts - even though we only have a very leaky cellar door. Good knows how much is leaving the house that way.
This year, our house has a heating need of 2.23 BTUs/HDD/sqf
If I include the basement, then it's 1.11 BTU/HDD/sqft
Both seem super low... The average new construction on a TIGHT house says its about 5 BTUs ... Although I tried making our house as tight as possible, this a 1950s house where nothing was done to walls nor sheahing ... Can the pros chime in, maybe I did the calculations wrong? But I think they are right ...
19 therms * 100,066 = 1 901 254 BTUs
Divide this by 740 HDDs = 2569 BTUs/HDD
Divide it by sq footage ...
Acording to EIA, a newer house from 2005, has an energy intensity (I guess that's the proper name of the calculations above) is 3.6 ...
Well...I am at worst case scenario 2.23 ... best case scenario (if I include basement as conditioned space at 1.11 !!! )
If we do the reverse for the full winter and make calculations of how much I am going to spend this winter...
2.23 as base load x sq footage x HDD per season
I come out at 150 therms (yes I know that the bigger the delta heat transfers faster so these are just rough estimates). This would compare to the 300 therms I used last year AFTER attic was already R60. So, I am assuming with these calcs, that insulating the basement PLUS rim joist can have an impact of almost 50%. Heck, Ill be happy with even 30%-40%. Which is crazy!
If indeed I end up with 150 therms for a 5900 HDD winter, that's like 140 dollars or so of heating costs!
My full consumption for the year would come out at around 190-200 therms (heating + base load). So about 180$ in NG ... I pay 17$ month for access so my bill would be 384 dollars per year where 204$ would be just Customer charge.
The customer charge would be enough to heat this house if I were to change heating to electricity by using mini splits. At 150 therms, that's about 4400 kWh. Using a minisplit with COP = 4 it means 1100 kwh of usage @ 9 cents per kWh .... = total heating costs of $100. A savings of 280-300$ per year.