Friday, October 1, 2010

Adding Attic Insulation - Part II

Blown-in insulation
In Part I we looked at the basics of attic insulation, including the primary types, pros and cons of each, R-values, and briefly touched on the fact that adding insulation have have a great 'return on investment'.

Before embarking on this project, however, we should look at the "return on investment" aspect with a bit more depth.  For most people, it will be important to have a general idea of how much insulation should be put in, and how much they will save.

Is This Project 'Worth It'?

The good news is that you can actually pretty clearly calculate the affect that adding attic insulation will have.  The National Weather Service calculates climatic numbers that can be used in this calculation.  These numbers, called Heating Degree Days and Cooling Degree Days, measure how extreme your winters and summers are by measuring how often and how much you deviate from 65 degrees. These values directly relate to how much you need to heat and cool a home.

You can look up your HDD and CDD numbers at the weatherdatadepot site.

Next, you need to know how much insulation you have in terms of R-value, and the surface area of your attic.  Combine that with your electricity costs per KwH and your natural gas costs per therm, you can use a calculator to determine savings achieved for different R-values, and even calculate the return on investment.

I've calculate the ROI's for several climates reflective of the USA. The calculations below were assuming a 1125 sq feet attic space, a starting R-value of R-20, electric cooling at 9 cents per KwH, and natural gas heating at 90 cents/therm. The heating and cooling systems were assumed to be typical year 2000 efficiency.

CityHDD/CDDSavings at R-value of 30 Savings at R-Value of 40 Savings at R-value of 49

At current costs, using the 1125 square foot attic space example from above, it would cost $197 to upgrade from R20 to R30; $394 from R20 to R40, and $562 from R20 to R49. The worst-case example from above would require 10 years to payback an R20 to R30 upgrade ASSUMING NO INCREASES in utility costs, where as the best case example would take 5 years. After that, it is pure profit.

How Much Insulation Should I Have?

The best place to start is the Department of Energy site, which has a map.  The most mild climates in the lower 48 are recommended to have R30 to R60, while some of the northeast and northern plains are recommended as much as R49 as a minimum.  From my calculations, R40 seems to be worthwhile in almost every climate, and R49 a must in extreme climates.

As was discussed in part 1, your existing insulation's R-value can be calculated by determining what type of insulation you have.  Simply measure the depth of the insulation in four or five spots in your attic, and take the average depth.  Then, using the table in part I, take the R-value for that type of insulation and multiply it by your average depth.  For example, 8 inches of loose-fill fiberglass would result in an R-value of 20.

Using the Department of Energy site I mentioned above, you can roughly calculate a target amount.  As mentioned above, R40 or R49 are good targets (note that you have diminishing returns as you add more insulation).  If R40 is your target, then you need to purchase another R20 worth of insulation (since you already have R20.

How Much Insulation Should I Buy?

Most stores will help you calculate this so long as you can tell them the rough square footage of your attic, your target R value, and your current R value.  For example, a 45x25 attic space results in 1125 square feet.  Home Depot's GreenFiber Cellulose insulation comes in bags that will cover 58 square feet at a depth of R10.  You'd need two bags for R20 coverage at 58 sq feet, so the math would be:

1125 / 58 * 2 = 39 bags (rounded up)

The calculator below is written with Home Depot's GreenFiber cellulose as the model, assuming a 58 sq foot coverage of R10.

Attic Size  square feet 
Current R-value 
Planned R-value

How Do I Install Blow-in Insulation?

Lowes and Home Depot will lend you a blower machine with a minimum purchase (usually $100 or $200). Installation is a two-person job - one needs to feed the machine, and one will be in the attic blowing. I found that having the feeder feed two bags, then pausing a minute before feeding two more worked well. The pause gave me time to move to a new corner of the attic.  It helps to plan your actions - i.e. where you will start, where you will move to, and where you'll be able to get good footing (or seating).

When working in the attic, be aware of nails and other sharp objects. Wear a particle mask and goggles, and use a head lamp and/or utility lamp for lighting. Long pants and long sleeves are a must, and heavy gloves may be helpful if you are concerned about nails and sharp objects.

Take care to only sit on joists - drywall cannot support an adult's weight. Additionally, in some houses there will be obstacles that should not have insulation touching due to fire hazard. For example, so 'can lights' must have a barrier between the light fixture (which gets quite hot) and the insulation. i strongly recommend to review the Energy Star DIY guide before starting.

If you do have old can lights that require an insulation barrier, you basically have holes in your insulation that are extremely inefficient. You may want to consider upgrading to an air tight fixtureair tight recessed light fixture.

Before you begin it makes sense to look for areas of air infiltration and seal them. Insulation that is stained or particularly discolored is a good sign of infiltration. Caulkcaulk and large gap sealerlarge gap sealer may be necessary. When caulking around light fixture penetrations or other heat sources, be certain to use fire-rated caulk. If you have ducts in your attic, check them for gaps and tears, and fix them as well. Note that most duct tape is not a very good sealer, so be sure to get a product designed for sealing, not just taping.

1 comment:

  1. I'm a big believer in adding attic insulation - did it 10 or 15 years ago (batted fiberglass) and saw immediate drops in the heat bills.