Does closed cell spray foam justify it's cost in an attic?
I currently have a 24x40' bi-level home, both levels finished, side walls insulated with batts, double pane windows (1000sqft per level). It was built in 2001 and when I moved in in 2009 I increased the attic cellulose from it's 9" R30 to 18" R60. I don't believe I saw any appreciable decrease in heat loss... however the home doesn't lose much BTU, with an ambient temp of 30f and an interior temp of 70f I consume 15,000btu/h or less per hour.
Now, this brings me to the question of foam attic insulation, around here it runs about $3 per square foot of attic space... but i'm told for instance with my existing house, if I were to pull out the 18" of cellulose and replace it with 6" of spray foam my BTU usage would reduce by HALF for the entire house... is that in any way possible or true? I was told this by a foam installer (of course) as well as an energy auditor whom makes no money or recommendation on who to perform the work.
I don't think i'll retrofit this home, but i'm looking at 0-5 year old ranch homes with 4000sqft / 2000sqft main level of space and if at all possible i'd like to make some decisions that allow me to consume a similar amount of BTU to heat those structures.
Wrong and wrong.
Whomever is telling you that 6" of foam will work better than 18" of cellulose is a liar or just plain dumb and I would tell that energy auditor to pound sand.
What the spray foam will do is help lessen air loss through into the attic. With that said, the 18" of cellulose has probably settled out to a pretty good air barrier at this point.
The R-Value is not a linear relationship so it is quite possible that R-30 to R-60 would not show that much improvement.
If you are really interested, you can air seal and insulate for a tighter envelope.
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