Weather Proofing Your Home in R. I.

Insulation Types

The most common insulation types in recent construction is cellulose, fiberglass, or in some cases, fiberglass  batt.  Often you can tell by simply looking at the insulation, but before we proceed, one word of caution.  According to the EPA, houses built between 1930 and 1950 may have had asbestos insulation.  Asbestos can be dangerous in certain situations, and its best not to work in areas with asbestos.   However, there is much more to the story of asbestos - if you have a concern check out the EPA's Asbestos page.

Cellulose insulation is often made of recycled newspapers and wood that has been treated to be fire resistant and mold/fungus resistant.  This insulation is thought to be safer to handle for the general public, and my home built in 2006 was installed with Cellulose insulation.  Cellulose is generally a grayish or off-white color, and depending on the vendor, you may actually be able to discern some newsprint on it.

Fiberglass Batt

Fiberglass insulation is literally tiny strands of glass.  It is lighter than cellulose, but it can be a major irritant to eyes, nose, and throats of those performing the install or working around it.  Of course, once installed it should be out of sight and no issue, but when installing it you should wear long pants and long sleeves, as well as goggles and a mask.  In fact, that is really a 'best practice' for any insulation installation.  Fiberglass is usually yellow or pink colored.

R-Values

The R-value is the measurement of the thermal resistance of an insulating material.  Thermal resistance is simply how much the material resists heat or cold from penetrating through it.

You can get an idea for how efficient an insulating material is by its R-value at 1 inch thickness.  For example, cellulose's R-value at 1 inch depth is around 3.5.  Fiberglass can range from 2.5 to 3.5.  Polystyrene boards can achieve 4.0 to even 5.0, though rigid polystyrene does not make for a suitable attic insulation.

Thus, if you want an R-45 attic and plan to use cellulose, you'd want to fill to a depth of 45/3.5, or approximately 13 inches.  Using fiberglass, that would be about 15 inches.

Which Insulation Should I Use?
The good news is that it is OK to mix insulation types.  In other words, if your attic has fiberglass insulation, but you want to augment with cellulose, there is no adverse affect.  However, there is more to the choice of fiberglass vs cellulose.

I've summarized the pros and cons of the two types in the table below.

Type	R-Value	Pros	Cons
Cellulose	3.5-3.7	Less irritating to skin, throat, and eyes. Readily available. Good R-value per inch.  Dampens sound better than fiberglass.	Relatively heavy. Settles more than fiberglass.
Loose-fill Fiberglass	2.5	Light weight, naturally fire and mold resistant, doesn't settle as much as cellulose.	Very irritating to skin, eyes, and throat. Slightly worse R-value than cellulose.
Fiberglass batt	3.5	Light weight, naturally fire and mold resistant, doesn't settle as much as cellulose. Batting makes it less messy	Irritating to skin, eyes, and throat. Fits between joists, but may be tough to make fit where you need it due to chimneys, wiring, pipes, and other obstructions

Cellulose Insulation Package

For many, the choice may be relatively easy.  If you already have loose-fill (blow-in) insulation in the attic, then fiberglass batt is not really an option.  Installation of loose-fill cellulose can be the easier replacement. 

we have looked at the basics of attic insulation, including the primary types, pros and cons of each, R-values, and briefly touch on the fact that adding insulation does have a great return on investment just in the value of fuel saved and in the comfort added.

Is This Project 'Worth It'?

The good news is that you can actually 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.

How Much Insulation Should I Have?

The best place to start is the Department of Energy site, which has a map.  The northeast 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, we 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 You Buy?

We can help you calculate this we we know the rough square footage of your attic, your target R value, and your current R value.  For example, a 45 x 25 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.

What other factors should I concider?

In some houses there will be obstacles that should not have insulation touching due to fire hazard. For example, electrical 'can lights' must have a barrier between the light fixture (which gets quite hot) and the insulation.

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 fixture.

Before we 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. Caulk and large gap sealer may be necessary. When caulking around light fixture penetrations or other heat sources, we use fire-rated caulk. If you have ducts in your attic, we check them for gaps and tears, and fix them as well. Note that most duct tape is not a very good sealer, so we will suggest a product designed for sealing, not just taping.