When you insulate your house, you’re going to be confronted by a dizzying array of choices. How do you know which one to use? How much do they cost? Are some better than others or is it all hype? In this article, I’m going to do my best to sort through the options and help you make sense of them. Be warned – there’s a LOT of material here, so I’m going to have to break this into several articles.
First, let’s survey the types of insulation that you’re likely to encounter. I’m also going to provide links where appropriate.
- Fiberglass – batts
- Fiberglass – dense batts
- Fiberglass – compressed
- Fiberglass – shredded / loose fill
- Fiberglass – Johns Manville Spider – sprayed in
- Cellulose – loose fill
- Cellulose – damp sprayed in
- Cellulose – dense packed
- Spray foam – open-cell, Icynene
- Spray foam – open-cell, soy based
- Spray foam – high-density closed-cell
- Spray foam – high-density closed-cell, soy based
- Board foam – expanded polystyrene
- Board foam – extruded polystyrene
- Board foam – polyisocyanurate
- Board foam – foil faced polyisocyanurate
- Denim – Bonded Logic batts
This list is long enough. There are other insulation materials that have been used over the years, from vermiculite to horse hair to rice hulls to straw bales, but I’m not going to touch on anything that isn’t commonly available in the United States. Sorry!
Before we go into analyzing all the details of insulation materials, let’s spend a moment defining the basic characteristic of insulation – the R-value.
You’ve undoubtedly heard the term R-value, and if you paid attention, you know that the higher the R-value, the better the insulation. But there’s more than just “higher is better.” What IS R-value? Simply put, R-value is tells you the fraction of heat that can move through the insulation per hour.
if R-1 lets 100 units of heat through per hour then
R-2 lets 50 units through per hour, and
R-4 lets 25 units through, and
R-10 lets 100 divided by 10 = 10 units, and
R-20 lets 100 divided by 20 = 5 units, and
R-50 lets 100 divided by 50 = 2 units
It’s simple, right?
From this, you can see that even R-4 insulation reduces the heat movement a lot – only 25% of the energy moves though. You also see that you have to keep multiplying the amount to make the same relative amount of improvement. So you have to add a lot of insulation to make a smaller and smaller difference. For example, R-20 only lets 5% of the energy through as R-1. But you have to go up to R-50 to go from 5% to 2%. This is called the law of diminishing returns. Where do you stop?
As we’ll see, there’s more to what makes a good insulation material than the R-value. R-values are measured under laboratory conditions. Unfortunately, your home isn’t a laboratory. Insulation will be installed imperfectly. Winds will blow. Moisture will be present…
The first thing you’ll notice is that there are several basic types and forms of insulation, so let’s start by discussing the materials. Once you understand the basic characteristics of the materials, it will be much easier to start looking at the specific versions of each.
- Board insulation
Everybody is familiar with fiberglass insulation. It’s like cotton candy, made with glass. Soft and fluffy and used in most of the homes in the United States. But why?
Fiberglass is relatively inexpensive to manufacture. It’s light and easy to handle. And you can buy it anywhere. This makes it really popular with contractors. They can just put it in and most people will not question the choice. Most forms irritate skin and the respiratory system, so it should be handled with gloves and a respirator.
Fiberglass insulates by reducing air currents, so compressing it usually reduces it’s insulating ability (called R-value). Though some forms are naturally more dense.
Because it’s glass, water doesn’t damage it. However, if it gets wet, it loses most of it’s R-value until it dries out. And, of course it’s glass, so it doesn’t burn!
Most forms of fiberglass do nothing to impede the flow of air or moisture. But the recent version, Spider, can be sprayed in dense enough to greatly impede the flow of air.
Fiberglass in batts is very inexpensive.
In recent years, cellulose has received much acclaim as a “green” insulation because it’s made almost entirely of recycled newspaper, making it’s carbon footprint quite low.
Cellulose can be used in loose format, were it is pretty dusty and messy. If you’ve ever been in an attic filled with cellulose insulation, you know that you wouldn’t want to go in often. It’s like walking through a snowstorm.
Some forms of cellulose contain a glue that activates with a fine mist of water when applied. This allows it to be sprayed into wall cavities at high densities. It can also be applied onto attic floors, with the glue preventing it from being so dusty and messy. Finally, it can be injected into walls at a high density where it works very effectively as an air barrier.
The downside of cellulose is that it’s newspaper. If it wasn’t sprayed with fire retardant, it would be a huge fire hazard. But all cellulose is treated, so it will not burn. If anybody tells you otherwise, they probably just work for a contractor that sells some other type of insulation!
Cellulose, in general, is relatively inexpensive.
There are a variety of different types of spray foams, each with unique properties. All professionally applied spray foams come in two parts that are mixed in specific proportions at a controlled temperature. Get the mixture or the temperature wrong, and the foam will not work properly. For example, it might shrink and pull away from the rafters or it might not even harden.
When done properly, all spray foams work as an effective air barrier. I’ve mentioned elsewhere on these pages that stopping air movement is as important or more important than the R-value of the material. So an equal R-value of spray foam vs. fiberglass or cell foam will almost always result in a better insulated home. This is a very important consideration.
There are two main classes of spray foam: open cell and closed cell. Open cell is spongier. Close cell is hard. Water and moisture will move through open cell foam. Closed cell foam is an excellent water and moisture barrier. Open cell foam typically has 1/2 to 2/3 the R-value of closed cell foam for a given thickness. Closed cell foam has about twice the R-value of fiberglass or cellulose.
Spray foams are required in most places to have a secondary fire retardant applied if they will be left exposed. Usually, you have it sealed in a wall or attic, so this isn’t an issue. However, if used in a basement, this is an important consideration.
Spray foam is relatively costly to install. Typically one to two dollars per square foot per inch of thickness. Normally you would install about three inches in a wall and five to seven inches in a ceiling.
Recycled denim has become a popular insulation product in the last few years. I’ve used it and it’s a pleasure to work with because there’s no mess and no itchiness.
The material comes in batts, like fiberglass, making it easy to install in attics and walls. However it is hard to cut because of all the string in it. so it’s a mixed bag, but overall, if I were installing batts, I would use this stuff.
Denim batts are quite dense, so they’re pretty effective at reducing air movement, though not as much as dense packed cellulose or the Spider fiberglass.
Denim is moderately expensive to install, costing in between fiberglass and spray foam.
Sheet foam is sold in 4’x8′ sheets at most home centers. It comes in a few formulations, ranging from expanded polystyrene (the same stuff your white foam cooler is made of) to extruded polystyrene (the hard, blue or pink foam) to polyisocyanurate (which has the highest R-value of any commonly available insulation product).
Because this is rigid insulation, it is not used in the same way as other insulations. Typically, it would be installed as the house is built, attached to the wall studs or the foundation.
When installed properly, board insulation products will provide a near perfect air barrier. However, this requires that all seams are properly sealed.
The cost of board insulation is moderate. Typically a 2″ thick sheet costs about one dollar per square foot.
In the next installment, we’ll delve into greater detail about different insulations, which work best where, and the variety of characteristics for each form.
Colorado Energy Center – Table of R-values of common materials
Applegate Insulation – One of many manufacturers of cellulose insulation
Bonded Logic Ultratouch – recycled denim insulation
Johns Manville Spider – high density blown in fiberglass insulation