Replies

1. 
   kbentley57 | Sep 19, 2022 06:20am | #1

   Just tossing around a few numbers, assuming a 2000 sq. ft. house, with a few assumptions about a 9' ceiling height, and 40 x 50 dimension.

   1. There's 40 x 50 = 2,000 sq. ft of attic, and floor space each
   2. There's 9*2(40 + 50) = 1,620 ft of wall area
   3. The total area is 2,000 + 2,000 + 1,620 = 5,620 sq. ft.

   If your attic and half of the floor are insulated to some degree, you have, as a fraction of the total area insulated

   4. (2,000 + 1,000) / 5,620 = 0.53

   If you finish insulating the floor, you'd have

   5. (2,000 + 2,000)/5,620 = 0.71, or 71% of your total area insulated.

   Looking at the coverage numbers alone isn't quite enough, because the heat loss through the floor is into a space that undoubtedly stays warmer than the air outside, so that the exchange counts for less than say heat loss through the attic or walls. There are also considerations for air leakage, which probably accounts for as much of what you're feeling than thermal transfer through the materials you have.

   On the other hand, R19 fiberglass insulation is about $1/sq ft all in. So if you needed another 1k of insulation coverage, it would cost you $1,000 and a weekend of crawling around beneath your house. It would give you a 33% increase in area covered by insulation, but in the 'least useful' spot. It would be hard to say by how much it would reduce your heating costs, and comfort level. Maybe 5-10%? Any number is really just a guess.

   On the other hand, retrofitting the walls with blown in cellulose would require about 1200 lbs of material, roughly $1,000, and whatever labor rate to install it / repair finishes costs in your area. It probably starts around $5,000, though I really don't have a good idea for the costs out there. It may be more worthwhile to save the $1,000 you would have spent on the fiberglass, and put it towards insulating the walls.

1. 
   DC_Contrarian_ | Sep 19, 2022 01:52pm | #2

   I think you're answering the wrong question.

   The question isn't whether insulating the crawl space is the best use of time and money. The question is whether insulating the crawl space is worth it. And the second question is independent of the first, the amount of energy lost through one surface of a house is independent of the amount lost through the others.

   So the question is whether the cost of insulating the crawl space is justified by the reduction in energy usage and increase in comfort. The first part, energy usage, is rather easy to calculate. Comfort is harder, because it's not something that is typically modeled and is somewhat subjective. It's also something that is hard to put a price tag on.

   I recommend insulating for comfort even if it doesn't necessarily make pure economic sense. Cold floors are usually caused by cold air getting in and sinking, it's not necessarily coming through the crawl space. But first step is to make sure your floors are as air sealed as possible. Blow-in insulation usually does a pretty good job on the ceiling of air sealing.

   I recommend this article on doing crawl spaces right:

   https://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces

2. 
   RinOaky | Sep 19, 2022 02:03pm | #3

   @kbently57, thank you for the comprehensive reply. I left various details out of my post as I’m already a bit verbose, but some additional notes below to clarify on some of your points.
   @DC_Contrariarn, thank you as well. I only just saw your message as I was about to send the below, let me get back to you on the article you sent.

   - House is 1,600 s/f
   - My estimation that 50% of the crawl space is insulated wasn’t quite accurate: Two rooms have finished spaces below them, so I’m assuming they put insulation in the ceilings. As such, really…only around 500 s/f of the crawl space still needs to be insulated

   I know, 500 s/f isn't much and it's only $400 — so money isn’t the issue — rather it’ll be incredibly miserable work…it really ain’t fun down there.

   I understand wall insulation might be more beneficial, but oddly…I don’t have tons of wall, at least on the front (attaching a pic). And I “think” someone may have already done this (I sometimes see spots in the stucco that look like insulation holes), but I’m not positive. I also don’t know if that’s recommended with knob & tube wiring(?), although it can’t be worse than whoever already blew in insulation in the attic over the K&T :/

   I still imagine those 14 huge old single-paned windows are my downfall…maybe I need to simply get some curtains. The windows on the rear of the house are smaller, and they’ve already been replaced with double-paned.

   
1. 
   kbentley57 | Sep 19, 2022 02:07pm | #4

   That's a great looking house!

   Come on now, roll up those sleeves and get after it. That's only 5 packs worth of insulation. You'll be done by lunch!

3. 
   RinOaky | Sep 19, 2022 08:41pm | #5

   @kbently57, thanks for the house compliment. But let me tell you, this would not be a done-before-lunch job. I’ve logged a lot of hours down there — building a gas line and updating electrical up to code (the best was the romex someone ran through a heater duct!!) — and it’s just brutal: hillside, diminishing clearance, razor sharp rocks in your knees, etc. Plus lots of obstacles, like insulating above the furnace. I’ll still consider it, but more because I’m OCD and hate seeing things done halfway :/

   @DC_Contrarian, I brewed a pot of coffee so my brain could understand that article ;) I don’t know if the moisture issue would be as bad here in Oakland as the Washington example (or maybe it’s irrelevant, dew point is about temperature differences), but he seems to be arguing against fiberglass…and instead recommending rigid, foil-faced insulation or possibly spray foam. (I also wondered if mineral wool would suffer the same as fiberglass, but I think I'm remembering its ability to still perform even when wet.)

   In any case, there's already around 500 s/f of R19 down there (vapor barrier facing the wrong way, of course), so any issues fiberglass may produce are technically already present. If you told me my home would feel warmer if I insulated the rest of the area -- and I wouldn't have moisture damage -- I'd definitly do it. But hard to count on that. When I moved in I really, really wanted to install radiant in the joists down there, but same thing, there was no way to truly predict if it would be effective in my space.

   I can’t deny I’m thinking window curtains again…just to combat all the heat loss from those windows. In truth, curtains would probably do more than crawlspace insulation :/