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What Type of Insulation Should I Use in my Attic and How Much?  A Guide to Insulation.

What Type of Insulation Should I Use in my Attic and How Much?  A Guide to Insulation.

Figuring out what type of insulation to use can be tricky and as HVAC contractors we are often asked, “what type of insulation should I use in my attic?”  As with many of these questions, the answer depends on where you live so I will tailor this article towards showing you how to figure out what type of insulation and how much of it to use based on your local climate.  Attic roof insulation is as much of an art as it is a science, so we will start with the basics and work our way up.  In this article we will first discuss what insulation is used for, how insulation is rated, and the different types of insulation.  Then we will answer the questions of how much insulation you should have in your attic as well as what type of insulation you should use for your region.

how much insulation should i have in my attic - fiberglass insulationWhat is the Purpose of Insulation?

Your air conditioning and heating system is only a part of maintaining a comfortable and energy efficient home.  Imagine how efficient your furnace would be if you left all of your windows open during a blizzard – the same concept holds true of insulation.  Insulation is a material that is very poor at transferring heat from one side of it to the other, so the worse that a material is at transferring heat, the better of an insulator that it is.

There are several ways that you can make your house more efficient: you can get a more efficient HVAC system, you can set your thermostat to a more efficient temperature (See: What temperature should I set my thermostat to?), you can invest in a better type or more insulation, or ideally you can do all of these.  For more information on choosing an HVAC system that is properly sized for your house, take a look at: What Size Central Air Conditioner Do I Need For My House?

For more information on SEER ratings, energy efficiency and how to reduce your electric bill, here are a couple of other good resources:

SEER vs EER – What Are They and Which Should I Use?

How to Lower Your Electric Bill in the Summer.

How Are Different Types of Insulation Rated?  Introduction to R-values.

If you already know about R-values, then skip to the next section.  Before delving into different types of insulation and how much to use, it is important to first understand how insulation is rated.  Insulation comes in a variety of forms which will be discussed later, but all of these types of insulation are measured the same way.  Insulation is given a rating known as it’s “R-value” where the “R” stands for “resistance to heat flow.”  Simply put, it is a standard means of measuring how easily heat is transferred through a material.  A material with a low R-value such as metal is a poor insulator, whereas several inches of commercial-grade fiberglass insulation would likely have a high R-value.  With R-value, the higher the value, the better of an insulator that a material is.  It is also important to understand that thickness plays a roll in calculating R-value, meaning that if you fill your attic with one inch of insulation that has an R-value of 5, but filled another attic with twelve inches of insulation with a value of 3, then the overall R-value of the second attic would be greater.  That is because when it comes to insulation, thickness matters.

How to Calculate the R-value of Your Attic.

R-value is calculated by multiplying the “factor number” by your insulation’s thickness in inches.  The factor number is simply the R-value per inch of your type of insulation.  For our example, below are the R-values of three of the more common types of insulation.

Cellulose Insulation – 3.7

Fiberglass Batting – 3.2

Loose Fiberglass – 2.5

To calculate the R-value of your attic’s insulation, carefully go into your attic ensuring that you only step on the joists, otherwise you can put your leg through the ceiling!  If you’ve never walked in your attic before, then do a little research first or ask a friend.  Bring your tape measure and find a spot where it is easy to kneel down.  Then stick your tape measure through your insulation until it hits your ceiling ensuring that you do not compress the insulation at all.  Take the measurement in inches and make your calculation.  For instance,  ten inches of cellulose insulation would give your attic an R-value of 37.

Cellulose Insulation has a 3.7 R-value.

3.7 X 10 inches = 37, or a value of R-37.

For more information on R-values, check out this Wiki article: R-value of Insulation.

what type of insulation should i use in my attic - loose fillWhat Type of Insulation Should I Use in my Attic?

You could spend days talking about what type of insulation you should use but we will address a few of the most common types in this section: batts and blankets, loose fill and spray foam.

Batt and Blanket Insulation

The most common form that insulation comes in is a “Batt” or “Blanket.”  These are large rolls of insulation that are easy to cut into usable shapes.  They range from around 1 to 3 inches in thickness and can be used in multiple applications, ranging from walls to attics.

Fiberglass Insulation

Fiberglass is probably the most common type of insulation, but be careful when handling it and make sure you wear gloves, a long-sleeved shirt and don’t touch your eyes.  The fiberglass can be very irritating if it comes into contact with skin.  Fiberglass insulation can vary, but it usually has an R-value of 3.2 per inch.

Rockwool Insulation

Rockwool is a less commonly used type of insulation and can sometimes be hard to come by.  It is slightly better at insulating then fiberglass and doesn’t itch or irritate your skin.  The problem is, if it comes into contact with moisture it can become a serious mold problem so unless you live in a desert, I wouldn’t recommend it.  Rockwool has an R-value of about 4.5.

For more information on mold prevention, take a look at: How Do You Prevent Mold?

Cotton Insulation

Cotton is a great insulator and is easy to work with.  The drawback to using cotton insulation is it’s price as it is three times more expensive than the other types of insulation mentioned so far.  It is a pretty efficient insulator with an R-value of 3.7.

Loose Fill Insulation

Loose fill insulation is a type of insulation that consists of thousands of small granules blown into your walls or attic using a blower.  They are difficult to install yourself but are relatively cheap.  The biggest problem with loose fill insulation is that it has a tendency to settle over time and can shift.

Loose Fill Cellulose

Cellulose is a common type of loose fill and can be a great insulator with an R-value of around 3.7.  The biggest problem is that it has a tendency to settle over time which can cost you 15-20% of its insulating ability.  It is also quite heavy, making it undesirable for many attic installations.

Loose Fill Fiberglass

Loose fill fiberglass is ideal for attic use because it is light and relatively cheap.  It isn’t the best insulator but is decent with an R-value of right around 2.4 per inch.  I’d only recommend this for warmer climates.

For more tips on insulation, check out this article from – Insulation Tips.

Spray Foam Insulation

I’ve saved my favorite type of insulation for last.  If I was going to choose an insulation for my attic, then this would be it.  It is a chemical foam that is sprayed into walls or attics that expands over time and then solidifies.  The biggest advantages are that it is a phenominal insulator and because it is a foam that sticks to your joists and then dries, it actually increases the structural integrity of your attic and removes the necessity for calking.  It also allows moisture to pass through it and evaporate, making it difficult for mold to grow.  The biggest disadvantage is that it is expensive, with a price tag of around four times as expensive as some of the more basic insulations.  There are various types, and their R-values range from 4 to 7.

what type of insulation should i use in my attic - r-value chart by type of insulation

R-value by Type of Insulation.

How Much Insulation Should I Use in my Attic?

The amount of insulation that you use in your attic will vary mostly on the region you live in.  I will tell you that it is very difficult to have too much insulation in your attic, but it is easy to have too little.  Also keep in mind that it is possible to use a hybridized approach by combining multiple types of insulation to save money and gain some of the advantages of each, like using a layer of spray foam insulation topped with several inches of fiberglass blankets.  Efficiency is the name of the game here and in order to figure out how thick the insulation in your attic should be, start by finding the zone number for your location on the map below.

what type of insulation should i use in my attic - insulation amount by region chart

Now, lets look at the insulation chart below and see the R-value range for your region.  Keep in mind that the number on the left is the minimum R-value recommended for your region, but that the more you increase your insulation the less your HVAC system will have to run, increasing it’s longevity.

how much insulation should i have in my attic - chart by region

For example, if you live in our service area of Santa Clarita, California, then you’d be in zone 3.  Zone 3 has a minimum recommended R-value for your attic insulation of R-30, which could be easily attained using ten inches of fiberglass blankets.  But if you lived in Bozeman, Montana, then you would be in zone 6 and would have a minimum recommended R-value for your attic of R-49, which would require nearly two feet of fiberglass blanket insulation.  As such, it would be easier to obtain the required insulation by using a two-pound spray foam.

Figuring out which insulation to use or how thick to make the insulation in your attic will vary greatly depending on what region you live in and your budget.  For more information on this and related topics, visit the ASM Air Conditioning Blog.  If you live in Santa Clarita or Southern California, feel free to give us a call and we would be happy to answer any questions you might have.

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