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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?

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I think it has to do with cost. Blown Cellulose is good enough, and significantly cheaper. –  Chris Cudmore Apr 1 '13 at 19:27
    
Both materials are usually used for 'hot roofs'. Since an attic is a cold roof, there really isn't a reason to spend the extra money. –  DA01 Apr 2 '13 at 17:10
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4 Answers

up vote 15 down vote accepted

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.

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Exactly, there is a point where R-value is just wasted and you should concentrate on other things like thermal breaks, air leakage, etc... –  decker Apr 3 '13 at 19:11
    
You missed the machines you can rent that spray recycled paper or other materials for insulation. That is totally within DIY and doesn't require any PPE at all except maybe a respirator. –  staticx Apr 8 '13 at 18:37
    
@0A0D - Yes, but the questioner was asking about spray foam, not about spray cellulose. There are also DIY PU spray kits available, at least in Canada (these guys, for instance). It still remains more complex and expensive than the batts. –  Compro01 Apr 8 '13 at 19:13
    
@Compro01: In Florida at the Lowes Home Improvement Store, you buy $200 worth of celluose and you get the machine for free. It sprays through a big hose about the size the hoses used for cleaning your car. You just spray.. no skill needed. It's not complex at all and it is cheaper. You can return any unused portion and the cost goes does to $100 or less depending on how much you need. Home Depot has a similar deal. –  staticx Apr 8 '13 at 19:31
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I used spray foam in my attic. Here's why:

  1. I wanted the A/C units in the attic to be in the thermal envelope of the house.

  2. 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.

  3. 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!

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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.

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Early implementations of spray foam insulation had problems. These days, however, it's typically regarded as the best option in a lot of cases (albeit the most expensive option). –  DA01 Apr 8 '13 at 1:45
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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.

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The spray foam is on the underside of the roof deck! not on the floor of the attic. so picture this if your in the attic and lay down on your back. Look up and from eave to eave there is foam insulation but none below you. the attic is inside the thermal envelope and all the mechanicals and electrical are free and clear less a few wires that go out to the eaves and the Vent stacks. Tom –  user13237 May 29 '13 at 12:04
    
@Other Tom... No its not. You spray the floor of your attic to insulate your house. You would spray the underside to insulate your attic. You aren't even the poster... –  Tom May 29 '13 at 14:53
    
it depends if you are doing an "up" spray or a "down" spray, which is determined in part by if you want the attic to be semi-conditioned or not. –  Jeromy French Feb 5 at 17:25
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