foam vs. fiberglass ROI?

[walenta]

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

[suemarkp]

The equation for heat loss is btu/hr = area * deltaT / Rvalue.

The area of the ceiling is used, and not the pitched roof area (even if insulating the pitched section). Assuming 600 sq ft is the flat ceiling area and a deltaT of 70F (0 outside, 70 inside), and that you can insulate to R20 with fiberglass in a 6" cavity, your heat loss will be 600*70/20 = 2100 Btu/hr. If you can insulate to R40 with foam, the heat loss will be half of that at 1050 Btu/Hr.

So the ROI time is how much heating fuel do you save when using 1050 BTU/Hr less and how long do you need to not use that fuel to add up to the cost of the additional insulation? This depends on your fuel type, furnace efficiency, and cost of the fuel. Here's some BTU equivalents for fuel:
1000 cu ft Gas = 1,028,000 BTU
1 Gal Oil = 139,000 BTU
1 Gal Propane = 91,500 BTU
1 KW-Hr = 3,412 BTU
100,000 BTU = .915 Gal Oil = 119 CF Gas = 29.3 KW-Hr

A little detail to all of this is what is the true R value of your ceiling. You still have wood rafters on lets say 24" centers. If you're insulating between them, you'll have a decent amount of loss through the wood (it is about R1 per inch, so R5 for a 2x6). Most people don't get fiberglass tightly up to each rafter space either (there are gaps). So the roof won't be R20 with R20 insulation. If you were thinking about cutting sheets of foan and putting them between the rafter space, this is even worse as most people can't cut straight and each rafter bay is a slightly different width. But using spray foam can correct for gaps when using rigid foam.

Using foam beneath the rafters, insulates the whole roof. I'd put full sheets of 1" sheet of foam on the living side of the roof if you can. This will add about R5 to the rafters and any insulation you have between the rafters.

Finally, you may have noticed that 1000 or 2000 BTU is not much compared to your furnace size. Once you start getting in to the R20 and R30 territory, other things that are harder to insulate become the heat leaks. Things like windows (typically R4), doors (typically R3 to R6), and air leaks (infultration). These things are small area wise, but a bunch of windows can easily conduct more heat than your ceiling.

[Wgoodrich]

To beef up your fiberglass insulation you are not limited to the width of your ceiling joists. You may install fiberglass no paper between the rafters then lay a second layer the opposite direction over the rafters reaching the desired / required R value. This should also increase sealing affect of bad fit lumber gaps too.

Just remember to install shutes between the soffitt and the attic to allow the required air flow to the upper vents.

Good Luck

Wg

[Shelby]

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



Be very careful of where you use foam insulation. In an attic here in Canada during the summer with very poor roof venting the foam melts and gasses off.
Under roof heat can become hot enough to mumify humans and animals. I have recorded temperatures in excess of 150 degrees.

[Fischer]

The other part of the equation is--using 4 x 8 foam on the underside of your rafter/truss system, more effectively cuts air leaks. There is a greater value in the foam sheating than just the r factor. 1/2" foam is as good as any other thickness for sealing air leaks, and is much easier to work with. All joints should be taped and draped behind side walls, and then sealed to the sidewall poly.

Make sure you detail around the top plate to the rafters. Lots of heat loss in those areas.

[Wgoodrich]

I am with Fischer, why insulate rafters thus heating the whole attic too. Then again what about the rule in the code requiring ventilation in that attic. If you insulate the rafters you have no air chase between that foam and the bottom of the roof decking thus a violation of the Code to my knowledge. How is this being legally done and be within minimum design standards required by Code rules ?

Curious

Wg