Attic Insulation – How to Evaluate the Types and Amount You Should Add
Adding insulation to your attic is a great way to save energy, but there are a number of choices that you need to sort through. What is the best type for your situation? What is the best location for this added insulation? How much should you add?
The first thing that you need to determine is how much insulation you already have. Measure the depth and determine the type. Then multiply the depth by the appropriate value: Cellulose – 3.1, Fiberglass Batt or Blown – 3.7, Closed-Cell Foam – 6.2, Open-Cell Foam – 3.6.
Before you move any further to insulate your attic, you should make sure that you have any knob and tube wiring in your attic that would be covered by insulation replaced. The wires in this type of system need to be in the open air in order to dissipate heat. If they are surrounded by insulation this will trap the heat and cause a fire hazard.
Once you have calculated the R-value of your insulation, you need to determine how much more you should add. Energy Starr has developed recommended levels of attic insulation for every Zone throughout the US. See map and chart below. However, what it boils down to is most houses don’t have enough insulation. R-30 is the bare minimum and unless you live in south Florida, the recommended amount is twice that.
Recommended insulation levels for retrofitting existing wood-framed buildings
|
Zone |
Attic Insulation |
|
1 |
R30 to R49 |
|
2 |
R30 to R60 |
|
3 |
R30 to R60 |
|
4 |
R38 to R60 |
|
5 to 8 |
R49 to R60 |
Source: Energy Star
Your next step is to evaluate the various types of insulation to see which suits your situation the best:
Cellulose: is the blow-in insulation of choice for most do-it-yourselfers. It’s usually the most cost effective solution; it easily fills in around pipes, wires, framing and ducts. Cellulose insulation is basically recycled ground up newsprint that has been mixed with chemicals that deters mold, fire and vermin. It has the lowest R-value per inch, so it should only be used where space is not an issue since you will need 19 inches of it to get R-60.
Fiberglass Fill: performs very much like cellulose, it also has a high recycled content but it will only take 16 inches to achieve R-60. It’s a little more expensive than cellulose, and it is very itchy.
Fiberglass Batt: can be used to insulate the rafters, as long as adequate venting is maintained. In other words, keep a continuous air space between the insulation from the eave all the way to the ridge. You might consider insulating the rafters and the floor of the attic, in this way you can have 8 inches at each location and create a storage space in your attic that won’t get too hot in the summer or too cold in the winter.
Open-Cell Foam: can be used in the rafters too. However, it can be sprayed directly on the underside of the roof sheathing, effectively sealing your attic from any air leaks to the outside. This eliminates any venting of the attic, but since heat does not build up in the attic it isn’t a problem. Open cell foam is significantly more expensive than fiberglass and needs to be covered by gypsum board or other thermal barrier, and since it’s R-value is effectively the same as fiberglass you may not want to use it unless you have 2×12 rafters or you are going to insulate the attic floor and build a platform on top of it.
Closed-Cell Foam: is similar to open-cell foam in that it can be sprayed on the roof sheathing and it must be covered by a thermal barrier. However, it provides almost twice the R-value per inch and is not that much more expensive than open-cell foam. Other benefits of foam insulation are that they provide one of the best air barriers possible and they completely fill in around all obstructions and fill in all voids providing a higher effective R-value. Foam insulation is now available in do-it-yourself kits, but this should only be attempted by experienced D-I-Yers.
Radiant Barriers: work differently than thermal insulation. They reflect the heat away from the 
thermal insulation, instead of slowing the heat transfer through it. This effectively increases the performance of your thermal insulation in your attic by 50% to 70%. Radiant barriers work best if they are installed between the attic insulation and the roof sheathing, and when they are sloped so that dust will not accumulate on it as quickly. Furthermore, radiant barriers are fairly inexpensive and easy to install.
Your last step is to determine if you are going to add thermal insulation, a radiant barrier, or both. If your house already has what is the code minimum in many locations of R-30, adding a radiant barrier should boost its effectiveness to 45 or even 50. Or if you have an old house with 5 ½” of fiberglass or cellulose (R-17 to R-20) and you add another R-19 and a radiant barrier, you effective insulation value should be between 54 and 65.
Dan Bossenbroek
November 15, 2009
Tags: Building Envelope, Energy Conservation, Insulation Posted in: Energy Conservation
2 Responses
Twitted by JustGreenHomes - November 23, 2009
[...] This post was Twitted by JustGreenHomes [...]
Great post. I liked the R-Value map. I believe the most efficient, healthy attic is comprised from several factors. These three are the correct level of fiberglass insulation (easily substituted by closed-cell foam if affordable), radiant barrier use, and correct ventilation.
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