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How to optimize insulation investment - Fiberglass vs Foam

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

Re: How to optimize insulation investment - Fiberglass vs Foam

Hi .
Go by the R-Value ( resistence to heat loss ) .
Spray foam will do a much better job , R-6 per in. thick ,
and it also seals gaps and cracks better . You can spray foam
right on the roof deck with out an air space which
you must have with batt insulation to allow any moisture
to escape , ( any moisture in batt insulation makes it ineffective ) . Because of the density of the foam , if it
is a 2lb foam , moisture will never travel through it and become trapped between the insulation and the roof deck .
The problem with rigid insulation is that you will have alot of gaps between the material and rafters which you will have to foam with a can and it is much more labour intense . This is
one project that you are better off to bring in the

Good luck .
Robin .

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