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walenta 10-25-2007 08:40 AM

is there a payback for higher level insulation?
This may be a bit theoretical, but I am trying to figure out if the additional cost of foam board insulation and higher R value compared to fiberglass is economical and provides a good return on investment over time, ie by figuring savings in heating cost into the equation.

here the facts:

- I am looking to put new insulation into my attic roof
- The attice room will be used as liveable space
- I live in zone 1 as defined by the DOE - recommending R49
- 1 sq foot of R38 in fiberglass cost about 80 cents (before tax)
- the same insulation using foam Boards is about 3$
- the total roof is about 600 Square foot

here is the situation - if I have enough depth in my rafters I would go for fiberglass as it is more economical, however my rafters are only 6 inch deep to the roof

so I am trying to figure out, if I spend more money on the more expensive foam boards, I can achieve an higher R value and insulation, which over the years should save on my heating bill

How can I calculate the savings I will get with each R I increase, ie how much heating bill savings can I expect when I increase insulation by 1 R (assuming the formular is linear, I can calculate my return on investment for different thickness of foam baord as options)

hope this makes sense, I am sure this is not an exact science, but I am looking for some rough estimates to help me with the decisions....

thanks, Chris

Ron6519 10-25-2007 10:44 AM

Use a combination of foam and fiberglass. Build out the rafters so you can get R-19 in there with a air gap and go over it with the foam of whatever thickness you want. The thinner foam will reduce the overall cost.
There is a rule of diminishing returns on the insulation. You will see a payback, but at a certain point it will take longer then your life span. But the way energy prices are going up, maybe not.
If you want a more precise estimation, try your local utility, some of them will do a free energy audit.

NateHanson 10-25-2007 01:07 PM

I'm with Ron. Put in R19 glass, and then fasten an inch or inch and a half of rigid foam over that. It only adds r-5 per inch, but the real world performance is better than fiberglass, when you account for settling, and thermal bridging with all those wooden rafters.

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