I'm looking to upgrade the insulation in my attic bumping it from an R-6 to R-30 or better, and while looking into things I realized I've never seen anyone use rigid or spray foam insulation in an attic.
Is there a particular caveat to these, or is it just that most people prefer fiberglass batts?
Short answer : money.
Longer answer:
Inquiring at Google Contracting Services and doing some back of the envelope math, I came up with the following price ranges for the mentioned insulation types.
Spray foam is about $0.15-0.21/square foot/R
Fibreglass batts are about $0.03-0.07/square foot/R
Rigid foam panels are about $0.10-0.15/square foot/R
As we can see, in terms of insulation value for your money, the batts take a bat to all challengers.
Now someone will look at an insulation value chart and says "But foam has a higher R-value per inch, so it's better insulation since you have non-infinite space to put it in".
Comparing insulation value per thickness, they are correct as shown below:
Spray foam - ~R-6.3-7/inch
Fibreglass batts - ~R-3.1-3.3/inch
Rigid foam - ~R-5-6/inch
Both of the more expensive options achieve more with less thickness, so those options would allow more insulation value to be packed into the same volume.
But, you're going to bang your head into a point of diminishing returns where you'd achieve more energy savings putting the money towards improving things somewhere else, and you'll hit that point sooner than you think, given the way R-values work*, so that increased insulation density doesn't really get you much gain.
Additionally, installing batts (and rigid foam, for that matter) is a pretty simple DIY job and requires minimal PPE (gloves, goggles, dust mask). Spraying in foam indoors means you're going to need a full-body suit, gloves, goggles, respirator, etc. which makes DIYing it outside the skill/comfort level of many, meaning needing to hire a professional to do it for you (and the hassle/expense/loss-of-pride that entails), leading people to just go with the simpler batts.
*expanding, R-value basically means that 1/Rth of the heat (or cold, as the case may be) will escape compared to no insulation. R-6 means 16.6% of the heat is getting out. adding R-24 of insulation and going to R-30 results in only 3.3% getting out, a saving of 13.3%. But add another 24 to get to R-54 only gets us down to 1.85%, a saving of 1.45% for the same investment, and as I said above, you can likely find somewhere else to improve things that will get you more than that small savings for the same money, or at an endpoint, that money will buy you more gas/electricity than the extra insulation will save you over any realistic timeframe.
I used spray foam in my attic. Here's why:
I wanted the A/C units in the attic to be in the thermal envelope of the house.
Placing anything but sprayed open cell or closed cell foam on the underside of the roof deck was the only option for me. I could have used SIP panels if I was replacing the roof.
To get R-value for my area (central FL) I would need 14inches of blown cellulose.
How I did it:
I researched the cost of buying my own foam (DIY) or hiring a contractor. Well guess what it was cheaper to hire a contractor. I found the ones in my area and looked at their work and references. They did it (open cell) all for around $2.20/sf so I spent about $2300. It was well worth it: my A/C's run less = cost less, the house feels better, and the attic stays within around 10 degrees of the house, so my photos stored up there don't ruin.
Just find out what's available in your area and remember you get what you pay for!
If you look on the web, you will read many horror stories regarding sprayed foam: shrinking, cracking and falling off roofs and walls.
To spray, you need the right mix for the time and day, the right temperature on the base you are spraying and the right humidity - getting this right is not easy.
On the other hand, sheet polystyrene is better than fibreglass as a closed cell insulation, does not let heat or air though, you can also (taking care) get a tight fit, which is of the utmost importance. You cannot get fibreglass as an airtight or (more important) water vapor tight fit. The passing wind, warm air and water vapor all pass through fibreglass unchecked.
Keeping in mind that a well insulated home will benefit you, winter and summer for the life of the building. Using polystyrene - fitted between the rafters and below the rafters to avoid heat bridges, is the most cost effective insulation.
It wouldn't even be cost to me. You have tons of crap running through your attic - electric, venting, whatever. Why would you want these things set in stone? What would you do if you needed to properly secure more electric after spraying. Does not make sense to me at all. I have been up in my attic 20-30 times over the past few years moving electric, adding, changing wall configuration... and I cannot imagine dealing with rigid foam.
The main reason is initial cost, but a close second is that most home owners don't know how insulation works and trust R-value, the simplistic measurement on the label. Label information should be enough to make an informed decision, but it is not. R-value is meaningless without further information.
R-value measurement involves near zero air movement; almost comical in its divergence from real-life.
Heat is transferred by convection, conduction, radiation, and phase change. Convection is by far the largest cause of heat loss in homes, but fiberglass and cellulose do very little to slow that movement. Fiberglass is so air permeable, it is used in home air filters!
Even if R-value were a useful measurement on its own, it is not constant. Cellulose settles over time, reducing its R-value and, in wall insulation, leading to uninsulated gaps. R-value of fiberglass, XPS and EPS foam improves as temperature drops. Polyisocyanurate (poly foam)'s declines. The difference isn't huge--around R2 between very cold and very hot days.
In arid areas, one can lay a thin later of closed-cell foam and top it with fiberglass. I usually do not suggest cellulose because its boric acid fire retardant treatment largely dissipates within a few years, and the manufacturer is not required to publish tests of other than new product. In humid areas, this can lead to moisture build-up and is best avoided. In such cases, pure foam is best.
One must still do the math on whether closed-cell foam is worth the cost, but unfortunately doing so is not simple. The more moderate the temperature and lower the air movement in the house, the less benefit closed-cell foam will provide. Some areas, such as Washington and Idaho, have very cheap power and gas, but foam costs are much closer to even across the country. Labor costs may differ greatly as well.
A lot of spray foam companies have popped up lately, I suspect rising heating and cooling bills are driving this trend. Many seem to recommend an open cell (cheaper for them) installation on the backside of the roofing the attic (easier for them, particularly in houses with existing cellulose). As you can see at this link, open cell foam on the roof is going to cause you problems down the road.
http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing:
