But that is the pit, what’s the whole story here? I’ve been planning external insulation of the foundation since I bought the place. It was built in the days when you didn’t insulate the slab or basement, except on the inside when you finished it for living, and that lets lots of heat out through the floor and walls of the basement. Not too bad, since the earth helps keep temperature swings smaller – the deep ground temperature in Anchorage is about 36 F year round, instead of -20F to 80F outside air temp, but bad enough. I like external insulation on a foundation wall for a number of reasons, the main being that it keeps the masonry inside the insulation layer, letting it serve as thermal mass in the heated building. My favorite technique is passive annual heat storage, but I can’t really do that full blown here.
So in May I finally got my home energy audit for the state’s rebate program, and not surprisingly, foundation insulation is by far the most cost effective measure I can take to save heating energy at home – more than insulating the upper walls and attic more, WAY more than replacing doors or windows. My computer-printed recommendations include adding R-14 to the basement walls, then going out 2 feet around the perimeter of the slab. I am doing a slightly buffer version of this – I am going down 4 feet below grade and also doing the 3 feet of concrete block wall above grade, then going out with the insulation 4 feet once I am down 4 feet. I am using 4 inches of blueboard for approximately R-20 added. This should trap quite a bit of the heat of the basement in. The state program’s computer printout indicated that the installed materials cost should be about $2300, partly because they have me covering less square footage with less R-value insulation, but also I think they have underestimated. The break-even point is calculated at $7000, and that is about what it is going to cost me – ~$4700 for 102 sheets of 2″x4’x8′ Dow blueboard (extruded polystyrene) delivered from Lowes, a bit over $1000 for excavator rental, ~$700 for hired friends’ labor, and misc. for adhesive, stucco, flashing, waterproofing membrane/tar if needed, etc (probably a bit more than $7000 in the end), but I will have achieved a higher insulation value than that assumed in the end too. I will get money back from the state program for this. That amount is dependent on how many energy points I go up for all the work I do in the next year or so (18 months from my first energy audit), how much I spend, and is capped – no more than $10,000 total, but quite likely less. My auditor said I am at a cusp and it will be fairly easy to recover $4000 for anything I do.
I did a quick calculation given the heating degree days in Anchorage, an assumed current insulation value of about R-10 on the basement walls, an upgrade to R-30, the local cost of gas (grossly about a buck a therm), a boiler efficiency of 82% – and got a payback of about 35 years, or a savings of about 2.5% of the installed cost every year in gas bills. I may have made some errors, as it really should be more than this I believe. One obvious error is assuming that the cost of gas is not going to go up. I’ll try to ferret that out a bit better, but the real test will be real utility bills. The interplay of thermal mass and thermal resistance of the ground messes with the straight-forwardness of an R-value thermal conductance static problem.
This graphic (unreadable – I know) came from Grist.org today and was from a report on efficiency being likely underestimated as a negative cost (i.e. it pays you) way to achieve meaningful carbon cuts. Basement insulation is on there – about smack dab in the middle.
Other recent endeavors have included painting the entry door and area and making drunken tiramisu (‘drunken’ describing both the human state while making it, and the alcohol content of the final dish – probably interrelated) at a wonderful impromptu dinner party at Dallas’.